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Title: Hawkins Electrical Guide, Number One - Questions, Answers, & Illustrations, A Progressive Course - of Study for Engineers, Electricians, Students and Those - Desiring to acquire a Working Knowledge of Electricity and - its Applications
Author: Hawkins, Nehemiah
Language: English
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[Illustration]
[Illustration]
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
HAWKINS
ELECTRICAL GUIDE
NUMBER
ONE
QUESTIONS
ANSWERS
&
ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
PREFACE
_The word “guide” is defined as:_
=_One who leads another in any path or direction; a person who shows or
points out the way, especially by accompanying or going before; more
particularly, one who shows strangers or tourists about; a conductor;
leader, as “let us follow our guide.”_=
_This book, or =“Guide,”= is so called because it =leads= or =points out
the way= to the acquirement of a theoretical and practical knowledge of
Electricity._
_There are several guides, each covering in detail a certain phase of the
broad subject of Electricity and leading the reader progressively, and in
such a way, that he easily grasps, not only the simple fundamental facts,
but the more complex problems, encountered in the study of Electricity.
This is accomplished by the aid of a =very large number of illustrations=,
together with specific explanations, worded in =concise and simple
language=._
_The Guides are written partly in the question and answer form, as this
style of presentation has met with hearty approval, not only from those of
limited education, but also from the better informed._
_Where recourse is had to the question and answer form, the special aim of
the author has been to give short and direct answers, in such plain
language as to preclude a misconception of the meaning. With this in view,
=the answer gives= simply =the information sought by the question=._
_=The answer is limited to one paragraph= so that the reader may
concentrate upon the fact or facts demanded by the question._
_Any enlargement of the answer or specific explanations of items contained
therein, are presented in separate paragraphs printed, in smaller type._
_With this plan of =separating the answer=, as it were, from items of
secondary importance, and making it short and simple, its content is more
forcibly impressed upon the mind of the reader._
_In a text book, it is necessary to illustrate and explain the various
species of commercial apparatus met with in practice, and in this
connection the Publishers desire to call attention to the manner in which
the author has treated what may be classed as the =“descriptive matter.”=
Contrary to the usual custom of giving descriptions of commercial machines
in the main text, where they would occupy considerable space, to the
exclusion of the more important matter, all such descriptions are placed
in small type directly under the illustrations, leaving space for an
adequate presentation of the underlying =principles=, =theories=, and for
the large amount of =practical information= that is essential to obtain a
general knowledge of Electricity and its numerous applications._
_Credit is largely due to Frank D. Graham, B.S., M.S. (Princeton
University), and M.E. (Stevens Institute), practical engineer, for the
authorship of the =Guides=, and for original sketches illustrating
electrical principles and construction._
TABLE OF CONTENTS GUIDE NO. 1.
=INTRODUCTORY CHAPTER=
=SIGNS AND SYMBOLS=
=ELECTRICITY= =1 to 4=
Nature and source--=kinds of electricity:= static, current,
dynamic, radiated, positive, negative, atmospheric, frictional,
resinous, vitreous.
=STATIC ELECTRICITY= =5 to 26=
Electrical =attraction and repulsion=--the charge--distribution
of the charge--free and bound electricity--conductors and
insulators--=electroscopes=--gold leaf electroscope--electric
screens--electrification by induction--nature of the induced
charge--the electrophorus--=condensers=; Leyden jar--=electric
machines=--action of Toepler-Holtz machine--Wimshurst machines.
=THE ELECTRIC CURRENT= =27 to 34=
Volt--ampere--ohm--=Ohm’s law=--production of the electric
current--current strength--voltage drop in an electric current.
=PRIMARY CELLS= =35 to 67=
=The word “battery”=--action of cell--chemical changes;
polarization--effects of polarization--methods of depolarization--
depolarizers--depolarizer bag--Volta’s contact law--contact
series of metals--=laws of chemical action in cell=--
requirements of a good cell--=single and two fluid cells=--
the Leclanche cell--Fuller bichromate cell--the Edison
cell--Grenet bichromate cell--Daniell cell--directions for
making a Daniell cell--gravity cells--Daniell gravity
cell--so-called “dry” cells--=points relating to dry cells=--
care of cells--cleanliness--separating the elements--creeping--
amalgamated zinc--battery connections.
=CONDUCTORS AND INSULATORS= =68 to 74=
The so-called “non-conductors”--=table of conductors and
insulators=--mode of transmission--effect of heat--heating
effect of the current--=insulators=--impregnating compounds--
water as a conductor.
=RESISTANCE AND CONDUCTIVITY= =75 to 82=
=Standard of resistance=--conductivity of metals and
liquids--effect of heat--=laws of electrical resistance=--
conductivity--specific conductivity--=divided circuits=.
=ELECTRICAL AND MECHANICAL ENERGY= =83 to 92=
=Definitions:= energy, matter, molecule, work, foot-pound,
volt-coulomb, ampere-hour, power, horse power, =watt=, kilowatt,
watt-hour--mechanical equivalent of heat--=British thermal
unit=--=electrical horse power=--the farad.
=EFFECTS OF THE CURRENT= =93 to 104=
=Thermal effect=--use of heat from the current--magnetic
effect--chemical effect--=electrolysis=--electro-chemical
series--electric osmose--electric distillation--muscular
contractions--=electroplating=--electrotyping.
=MAGNETISM= =105 to 124=
Two kinds of magnetism--nature of each--=poles=--magnetic
field--magnetic force--=magnetic circuit=--magnetic flux--the
Maxwell--the Gauss--=magnetic effect of the current=--corkscrew
rule--solenoids--permeability--magnetic saturation--magnetomotive
force--=reluctance=--analogy between electric and magnetic
circuits--=hystereses=--residual magnetism.
=ELECTROMAGNETIC INDUCTION= =125 to 136=
=Faraday’s discovery=--Faraday’s machine--Faraday’s principle--
line of force--induction of current--laws of electromagnetic
induction--rules for direction of induced current--Fleming’s
rule--Ampere’s rule--the palm rule--=self-induction=.
=INDUCTION COILS= =137 to 154=
Self-induction--mutual induction--=primary induction coils=--
=secondary induction coils=--plain secondary induction
coils--secondary induction coils with vibrator and condenser;
=cycle of action=--magnetic vibrators--vibrator adjustment--
table of induction coil dimensions--table of sparking distances
in air--=points relating to induction coils=--wiring diagram.
=THE DYNAMO= =155 to 160=
Operation--=essential parts=--field magnets--armature--
=construction of dynamos=--parts; bed plate, field magnets,
armature, commutator, brushes.
=THE DYNAMO: BASIC PRINCIPLES= =161 to 170=
=Definitions=--essential parts--=elementary alternator=--
operation--direction of induced current--application of
Fleming’s rule--cycle of operation--=the sine curve=; its
construction and application.
=THE DYNAMO: CURRENT COMMUTATION= =171 to 180=
How the current is produced--=how direct current is obtained=--
the commutator--inductors--“continuous current”--action of four
coil elementary dynamo--=conditions for steadiness of the current=.
=CLASSES OF DYNAMO= =181 to 198=
Classification--bipolar and multi-polar dynamos--difference
between dynamo and magneto--self-exciting dynamo--=the series
dynamo=--regulation of series dynamo; difficulties experienced--
=the shunt dynamo=--adaptation--operation--characteristic--
regulation--=the compound dynamo=--service intended for--
regulation--over compounding--usual degree of over compounding--
short shunt--long shunt--voltage of short and long shunt machines
--separately excited dynamos--=Dobrowolski three wire dynamo.=
=FIELD MAGNETS= =199 to 220=
=Object=--essential parts--=classes of field magnet=--multi-polar
field magnets--construction--choice of materials--design--=pole
pieces=--eddy current--=laminated fields=--construction to reduce
reluctance of the magnetic circuit--magnetizing coils--=methods of
winding=--coil ends--insulation--attachment of coils--coil
connections--=heating=--ventilation.
INTRODUCTORY CHAPTER
The subject matter of this work relates to one of the secrets of creation
which appears to have been intended at the very beginning to be “sought
out.” This idea is expressed in a certain saying copied three or four
thousand years ago by the men of Hezekiah, King of Judah: from Solomon’s
proverbs: “_It is the glory of God to conceal a thing: But the glory of
Kings_ (i.e., _wise men_), _to search out a matter._”
In all that may be said hereafter through the work, it is admitted that
the results recorded are the determinations of experiments performed by an
incredible number of searchers extending through many ages. These
inquiries have been pursued with a generous rivalry which has permitted
discovery to be added to discovery, until the sum total has been wrought
into such exactness that it has been thoughtlessly stated that there is
nothing more, save its application.
It may be well, however, to state a few fundamental facts relating to
electricity: 1, Electricity and magnetism are one and the same thing; 2,
what is really known about it has come as a discovery and not as an
invention. Thus, we say the intrepid explorer discovered the pole, not
that he invented it. So with electricity it has been a subject of
discovery while its many applications to useful purposes have been
veritable inventions; 3, the earth itself is a magnet.
This last is shown by the fact that the earth affects a magnet just as one
magnet affects another. Magnets are bodies, either natural or artificial,
which have the property of attracting iron, and the power, when freely
suspended, of taking a direction toward the poles of the earth. The
natural magnet is sometimes called the _loadstone_. This word is said to
be derived from _loedan_, a Saxon word which signifies to guide. It is an
oxide of iron of a peculiar character, found occasionally in beds of iron
ore. Though commonly met with in irregular masses only a few inches in
diameter, however, loadstones of larger sizes are sometimes found.
By means of simple experiments it may be ascertained that the magnet has
the following general properties, viz: 1, power of attraction; 2, power of
repulsion; 3, power of communicating magnetism to iron or steel; 4,
polarity, or the power of taking a direction toward the poles of the
earth; 5, power of inclining itself toward a point below the horizon.
Speaking generally we may say, that magnetism is a department of
electrical science which treats of the properties and effects of the
magnet. The same terms are also used to denote the unknown cause of
magnetic phenomena, as when we speak of magnetism as excited, imparted,
and so on.
Lightning and the Northern Lights are displays of electricity on a grand
scale. Electricity is a term derived from the Greek word for _amber_, that
being the substance in which a property of the agent now denominated
electricity was first observed.
The ancient Greek philosophers were acquainted with the fact that amber,
when rubbed, acquired the property of attracting light bodies; hence the
effect was denominated electrical and in later times, the term electricity
has been used to denote the unknown cause of electrical phenomena, and
broadly the science which treats of electrical phenomena and their causes.
Electricity, whatever it may prove to be, is not _matter_ nor is it
_energy_; it is however a means or medium of transmitting energy.
If electricity is to transmit or convey energy along a wire,
this energy must be imparted to the electricity from some
external source, that is to say, before electricity can perform
any work it must be set in motion, against more or less
resistance. This involves that pressure must be applied, and to
obtain this pressure, energy must be expended from some external
source.
Accordingly, in electrical engineering, the first principle to be grasped
is that of _energy_. Without the expenditure of energy no useful work can
be accomplished.
Energy may be defined as _the capacity for performing work_.
Although electricity is not energy, electricity under pressure is a form
of energy spoken of as electrical energy.
In an expenditure of energy in this form, the electricity acts simply as a
transmission agent or medium to transmit the energy imparted to it in
causing it to flow.
In a similar manner, steam acts as a transmission agent or
medium to transmit the heat energy of the coal to the steam
engine, where it is converted into mechanical energy.
As just stated, electricity under pressure is a form of energy, and its
generation is simply a transformation of energy from one form into
another. Usually, mechanical energy is converted into electrical energy,
and a dynamo is employed for effecting the transformation.
In transforming the mechanical energy of waterfalls into
electric energy, this natural power of water due to its weight
and motion is first converted into rotary motion by a turbine or
water wheel, and then converted into electric energy by a
dynamo, or an alternator.
All dynamos are but machines for converting into electric energy
the energy which is given to them by some prime mover, as a
steam engine, a gas engine, by hydraulic or even by wind power.
All electric motors are merely machines for reconverting the
electric energy which they receive by means of the conducting
wires or mains, into mechanical energy.
All electric lamps are contrivances for converting into luminous
energy a percentage of the electric energy that is supplied
through the mains.
=Potential and Kinetic Energy.=--_Potential energy_ is the capacity for
performing work which a body possesses _by virtue of its position_.
_Kinetic energy_ is the capacity for performing work which a body
possesses _by virtue of its motion_.
It must be evident that position or motion given to a body
enables it to perform work. In the first instance, for example,
a heavy weight at the top of a high tower possesses potential
energy. A ten pound weight supported one foot above a plane has
ten foot pounds of potential energy.
The flywheel of a steam engine in motion is an example of a body
possessing kinetic energy. Some of this kinetic energy which was
stored up in the fly wheel during the working stroke is expended
in moving the engine over the “dead center,” and any other point
where no torque is produced by the pressure on the piston.
=Chemical Energy= can be converted into electric energy to a limited
extent by means of the electric battery, but the cost of this energy is so
high that it is commercially feasible only where small quantities are
required, and the cost of production is secondary to the convenience of
generation, as for signalling purposes, the operation of bells and
annunciators, etc.
The chemical energy of coal and other fuels cannot be directly
converted into electric energy. For power producing purposes,
the chemical energy of a fuel is first converted into heat by
combustion, and the heat thus obtained converted into mechanical
energy by some form of heat engine, and the mechanical energy
subsequently transformed into electric energy in an electric
generator.
_Energy cannot be created or destroyed._ This is the law known as the
=conservation of energy= which has been built up by Helmholtz, Thomson,
Joule and others. It teaches further, that energy can be transmitted from
one body to another or transformed in its manifestations.
Energy may be dissipated, that is, converted into a form from
which it cannot be recovered, as is the case with the great
percentage of heat escaping from the exhaust nozzle of a
locomotive or in the circulating water of a steamship, but the
total amount of energy in the universe, it is argued, remains
constant and invariable.
Following this law comes the doctrine of the =conservation of electricity=
as announced by Lippman, being undoubtedly the outcome of the ideas of
Maxwell and of Faraday as to the nature of electricity. According to their
doctrine, electricity _cannot be created or destroyed, although its
distribution may be altered_.
Lippman states that every charge of electricity has an opposite
and equal charge somewhere in the universe more or less
distributed; that is, the sum of positive charges is always
equal to the sum of negative charges.
In _altering the distribution of electricity_, we may cause =more= to
appear at one place and =less= at another, or may change it from the
condition of rest to that of motion, or may cause it to spin round in
whirlpools or vortices, which themselves can attract or repel other
vortices. According to this view all our electrical machines and batteries
are merely instruments for altering the _distribution_ of electricity by
moving some of it from one place to another, or for causing electricity,
when accumulated or heaped, together in one place, to do work in
returning to its former distribution.
Electrical engineering has developed largely and widely within a very
short time and its many applications has created so great a demand for
various kinds of electrical apparatus, that their manufacture forms one of
the leading industries.
Electricity is very valuable as a medium for the transmission of energy,
especially to long distances; it is also used to great advantage in
lighting, being free from the disagreeable properties of gas or oil.
Again, electricity finds various applications, in extracting gold from the
ore, pumping and ventilation of mines, traction, telephone, telegraph,
electroplating, therapeutics, etc.
These few, of its many applications will perhaps serve to indicate the far
reaching interest and importance of electricity, and possibly help to
kindle in the student something of the eagerness in his work and
enthusiasm without which he will fail to do justice either to his calling
or to himself.
SIGNS AND SYMBOLS
The following signs, symbols and abbreviations are almost universally
employed in descriptive and technical works on electrical subjects.
Although, in the arrangement of the Guides, the direct current and
alternating current matter has been kept separate, it is perhaps advisable
in the case of signs and symbols, to combine those relating to the
alternating current with the direct current and other symbols, making a
single table, rather than have them scattered throughout the work.
=1. Fundamental.=
_l_, Length. cm. = centimeter; in., or ″ = inch, ft. or
′ = foot.
M, Mass. gr. = mass of 1 gramme kg. = 1 kilogramme.
T, _t_, Time, _s_ = second.
=2. Derived Geometric.=
S, _s_, Surface.
E, Volume.
α, β, Angle.
=3. Derived Mechanical.=
_v_, Velocity.
ω, Angular velocity.
_r_, Momentum.
_a_, Acceleration.
_g_, Acceleration due to gravity = 32.2 feet per second.
F, _f_, Force.
W, Work.
P, Power.
δ, Dyne.
ε, Ergs,
ft. lb., Foot pound.
H.P., h.p. Horse power.
I.H.P., Indicated horse power.
B.H.P., Brake horse power.
E.H.P., Electrical horse power.
J, Joule’s equivalent.
_p_, Pressure.
K, Moment of inertia.
=4. Derived Electrostatic.=
_e_, Pressure difference.
_i_, Current.
_r_, Resistance.
_q_, Quantity.
_c_, Capacity.
_sc_, Specific inductive capacity.
=5. Derived Magnetic.=
_m_, Strength of pole.
[Script J], Intensity of magnetization.
[Script M], Magnetic moment.
[Script H], Horizontal intensity of earth’s magnetism.
[Script H], Field intensity.
φ, Magnetic flux.
[Script B], Magnetic flux density or magnetic induction.
[Script H], Magnetizing force.
[Script F], Magnetomotive force.
[Script R], Reluctance, magnetic resistance.
μ, Magnetic permeability.
κ, Magnetic susceptibility.
ν, Reluctivity (specific magnetic resistance).
=6. Derived Electromagnetic.=
R, Resistance, ohm.
O, do, megohm.
E, Volt, pressure.
E_im Impressed pressure.
E_a, E_o Active pressure; ohmic drop.
E_v Virtual pressure.
E_max Maximum pressure.
E_av Average pressure.
E_ef Effective pressure.
E_i Inductance pressure.
E_c Capacity pressure.
U, Difference of pressure, volt.
I, Intensity of current, ampere.
I_im Impressed current.
I_a Active current.
I_v Virtual current.
I_max Maximum current.
I_av Average current.
I_ef Effective current.
Q, Quantity of electricity, ampere-hour; coulomb.
C, Capacity, farad.
W, Electric energy, watt-hour; Joule.
P, Electric power, watt; kilowatt.
_p_, Resistivity (specific resistance) ohm centimeter.
G, Conductance, mho.
γ, Conductivity (specific conductivity).
Y, Admittance, mho.
Z, Impedance, ohm.
X, Reactance, ohm.
X_i Inductance reactance.
X_c Capacity reactance.
B, Susceptance, mho.
L, Inductance (coefficient of Induction), henry.
_v_, Ratio of electromagnetic to electrostatic unit of quantity
=3×10^{10} centimeters per second approximately.
=7. Symbols in general use.=
D, Diameter.
_r_, Radius.
_t_, Temperature.
θ, Deflection of galvanometer needle.
N, _n_, Number of anything.
π, Circumference ÷ diameter = 3.141592.
ω, 2π_f_ = 6.2831 × frequency, in alternating current.
~, _f_, Frequency, periodicity, cycles per second.
φ, Phase angle.
G, Galvanometer.
S, Shunt.
N, n, North pole of a magnet.
S, s, South pole of a magnet.
A.C. Alternating current.
D.C. Direct current.
P.D. Pressure difference.
P.F. Power factor.
C.G.S. Centimeter, Gramme, Second system.
B.&S. Brown & Sharpe wire gauge.
B.W.G. Birmingham wire gauge.
R.p.m. Revolutions per minute.
C.P. Candle power.
[--o--], Incandescent lamp.
[--X--], Arc lamp.
[Symbol: Condenser] OR [Symbol: Condenser] Condenser.
[Symbol: Battery of cells] Battery of cells.
[Symbol: Dynamo] Dynamo, or direct current motor.
[Symbol: Alternator] Alternator, or alternating current motor.
[Symbol: Converter] Converter.
[Symbol: Static transformer] Static transformer.
[Symbol: Inductive resistance] Inductive resistance.
[Symbol: Non-inductive resistance] Non-inductive resistance.
CHAPTER I
ELECTRICITY
=Nature and Source of Electricity.=--What is electricity? This is a
question that is frequently asked, but has not yet been satisfactorily
answered. It is a force, subject to control under well known laws.
While the nature and source of electricity still remain a mystery, many
things about it have become known, thus, it is positively assured that
electricity never manifests itself except when there is some mechanical
disturbance in ordinary matter.
The true nature of electricity has not yet been discovered. Many think it
a quality inherent in nearly all the substances, and accompanied by a
peculiar movement or arrangement of the molecules. Some assume that the
phenomena of electricity are due to a peculiar state of strain or tension
in the ether which is present everywhere, even in and between the atoms of
the most solid bodies. If the latter theory be the true one, and if the
atmosphere of the earth be surrounded by the same ether, it may be
possible to establish these assumptions as facts.
The most modern supposition regarding this matter, by Maxwell, is that
light itself is founded on electricity, and that _light waves_ are merely
_electromagnetic waves_. The theory “that electricity is related to, or
identical with, the luminiferous ether,” has been accepted by the most
prominent scientists.
But while electricity is still a mystery, much is known about the laws
governing its phenomena. Man has mastered this mighty force and made it
his powerful servant; he can produce it and use it.
Electricity, it is also conceded, is without weight, and, while it is
without doubt, one and the same, it is for convenience sometimes
classified according to its motion, as:
1. Static electricity, or electricity _at rest_;
2. Current electricity, or electricity _in motion_;
3. Magnetism, or electricity _in rotation_;
4. Electricity _in vibration_ (radiation).
Other useful divisions are:
1. Positive;
2. Negative electricity;
3. Static;
4. Dynamic electricity.
=Static Electricity.=--This is a term employed to define electricity
produced by friction. It is properly employed in the sense of a static
charge which shows itself by the attraction or repulsion between charged
bodies.
When static electricity is discharged, it causes more or less of a
current, which shows itself by the passage of sparks or a brush discharge;
by a peculiar prickling sensation; by a peculiar smell due to its chemical
effects; by heating the air or other substances in its path; and sometimes
in other ways.[1]
=Current Electricity.=--This may be defined as the quantity of electricity
which passes through a conductor in a given time--or, electricity in the
act of being discharged, or electricity in motion.
An electric current manifests itself by heating the wire or conductor; by
causing a magnetic field around the conductor and by causing chemical
changes in a liquid through which it may pass.
=Dynamic Electricity.=--This term is used to define current electricity to
distinguish it from static electricity.
=Radiated Electricity.=--Electricity in vibration. Where the current
oscillates or vibrates back and forth with extreme rapidity, it takes the
form of waves which are similar to waves of light.
=Positive electricity.=--This term expresses the condition of the point of
an electrified body having the higher energy from which it flows to a
lower level. The sign which denotes this phase of electric excitement is
+; all electricity is either positive or negative.
=Negative Electricity.=--This is the reverse condition to the above and is
expressed by the sign or symbol -. These two terms are used in the same
sense as _hot_ and _cold_.
=Atmospheric Electricity= is the free electricity of the air which is
almost always present in the atmosphere. Its exact cause is unknown.
The phenomena of atmospheric electricity are of two kinds; there are the
well known manifestations of thunderstorms; and there are the phenomena of
continual slight electrification in the air, best observed when the
weather is fine; the Aurora constitutes a third branch of the subject.
[Illustration: FIG. 1.--The electric eel. There are several species
inhabiting the water, and which have the power of producing electric
discharges by certain portions of their organism. The best known of these
are the _Torpedo_, the _Gymnotus_, and the _Silurus_, found in the Nile
and the Tiger. The Electric Ray, of which there are three species
inhabiting the Mediterranean and Atlantic is provided with an electric
organ on the back of its head, as shown in the illustration. This organ
consists of laminæ composed of polygonal cells to the number of 800 or
1000, or more, supplied with four large bundles of nerve fibres; the under
surface of the fish is -, the upper +. In the Surinam eel, the electric
organ goes the whole length of the body along both sides. It is able to
give a very severe shock, and is a formidable antagonist when it has
attained its full length of 5 or 6 feet.]
=Frictional Electricity= is that produced by the friction of one substance
against another.
=Resinous Electricity.=--The kind of electricity produced upon a resinous
substances such as sealing wax, resin, shellac, rubber or amber when
rubbed with wool or fur. Resinous electricity is _negative electricity_.
=Vitreous Electricity.=--A term applied to the positive electricity
developed in a glass rod by rubbing it with silk. This electric charge
will attract to itself bits of pith or paper which have been repelled from
a rod of sealing wax or other resinous substance which had been rubbed
with wool or fur.
CHAPTER II
STATIC ELECTRICITY
Static electricity may be defined simply as _electricity at rest_; the
term properly applies to an isolated charge of electricity produced by
friction. The presence of static electricity manifests itself by
_attraction_ or _repulsion_.
=Electrical Attraction and Repulsion.=--When a glass rod, or a stick of
sealing wax or shellac is held in the hand and rubbed with a piece of
flannel or cat skin, the parts will be found to have the property of
attracting bodies, such as pieces of silk, wool, feathers, gold leaf,
etc.; they are then said to be _electrified_. In order to ascertain
whether bodies are electrified or not, instruments called _electroscopes_
are used.
There are two opposite kinds of electrification:
1. Positive;
2. Negative.
Franklin called the electricity excited upon glass by rubbing it with silk
_positive_ electricity, and that produced on resinous bodies by friction
with wool or fur, _negative_ electricity.
The electricity developed on a body by friction depends on the rubber as
well as the body rubbed. Thus glass becomes negatively electrified when
rubbed with catskin, but positively electrified when rubbed with silk.
[Illustration: FIGS. 2 and 3.--Pith ball pendulum or electroscope; the
figures illustrate also electrical attraction and repulsion.]
The nature of the electricity set free by friction depends on the degree
of polish, the direction of the friction, and the temperature. If two
glass discs of different degrees of polish be rubbed against each other,
that which is most polished is positively, and that which is least
polished is negatively electrified. If two silk ribbons of the same kind
be rubbed across each other, that which is transversely rubbed is
negatively and the other positively electrified. If two bodies of the same
substance, of the same polish, but of different temperatures, be rubbed
together, that which is most heated is negatively electrified. Generally
speaking, the particles which are most readily displaced are negatively
electrified.
In the following list, which is mainly due to Faraday, the substances are
arranged in such order that each becomes positively electrified when
rubbed with any of the bodies following, but negatively when rubbed with
any of those which precede it:
1. Catskin.
2. Flannel.
3. Ivory.
4. Rock crystal.
5. Glass.
6. Cotton.
7. Silk.
8. The hand.
9. Wood.
10. Metals.
11. Caoutchouc.
12. Sealing wax.
13. Resin.
14. Sulphur.
15. Gutta-percha.
16. Gun cotton.
=The Charge.=--The quantity of electrification of either kind produced by
friction or other means upon the surface of a body is spoken of as a
charge, and a body when electrified is said to be _charged_. It is clear
that there may be charges of different values as well as of either kind.
When the charge of electricity is removed from a charged body it is said
to be _discharged_. Good conductors of electricity are instantaneously
discharged if touched by the hand or by any conductor in contact with the
ground, the charge thus finding a means of escaping to earth. A body that
is not a good conductor may be readily discharged by passing it rapidly
through the flame of a lamp or candle; for the flame instantly carries off
the electricity and dissipates it in the air.
=Distribution of the Charge.=--When an insulated sphere of conducting
material is charged with electricity, the latter passes to the surface of
the sphere, and forms there an extremely thin layer. The distribution of
the charge then, depends on the _extent_ of the surface and not on the
mass.
Boit proved that the charge resides on the surface by the following
experiment:
A copper ball was electrified and insulated. Two hollow
hemispheres of copper of a larger size, provided with glass
handles, were then placed near the sphere, as in fig. 4. So long
as they did not touch the sphere, the charge remained on the
latter, but if the hemispheres touched the inner sphere, the
whole of the electricity passed to the exterior, and when the
hemispheres were separated and removed the inner globe was found
to be completely discharged.
The distribution of a charge over an insulated sphere of conducting
material is uniform, provided the sphere is remote from all other
conductors and electrified bodies.
[Illustration: FIG. 4.--Boit’s experiment which proved that the _charge
resides on the surface_.]
Figs. 5 to 8 show, by the dotted lines, the distribution of a charge for
bodies of various shapes. Fig. 6 shows that for elongated bodies, the
charge collects at the ends.
The effects of points is illustrated in fig. 9; when a charged body is
provided with a point as here shown, the current accumulates at the point
to such a high degree of density that it passes off into the air, and if a
lighted candle be held in front of the point, the flame will be visibly
blown aside.
Fig. 10 shows an _electric windmill_ or experimental device for
illustrating the escape of electricity from points. It consists of a vane
of several pointed wires bent at the tips in the same direction, radiating
from a center which rests upon a pivot. When mounted upon the conductor of
an electrostatic machine, the vane rotates in a direction opposite that of
the points. The movement of the vane is due to the repulsion of the
electrified air particles near the points and the electricity on the
points themselves. The motion of the air is called _electric wind_. This
device is also called _electric flyer_, and _electric whirl_.
[Illustration: FIGS. 5 to 8.--Illustrating the distribution of the charge
on conductors of various shapes.]
=“Free” and “Bound” Electricity.=--These terms may be defined as follows:
The expression _free electricity_ relates to the ordinary state of
electricity upon a charged conductor, not in the presence of a charge of
the opposite kind. A free charge will flow away to the earth if a
conducting path be provided.
A charge of electricity upon a conductor is said to be _bound_, when it is
attracted by the presence of a neighboring charge of the opposite kind.
=Conductors and Insulators.=--The term _conductors_ is applied to those
bodies which readily allow electricity to flow through them, in
distinction from _insulators_ or so-called _non-conductors_, which
practically allow no flow of electricity.
Strictly speaking, there is no substance which will prevent the passage of
electricity, hence, the term non-conductors, though extensively used, is
not correct.
[Illustration: FIG. 9.--Experiment to illustrate the effect of pointed
conductors.]
[Illustration: FIG. 10.--Electric windmill which operates by the reaction
due to the escape of the electric charge from the points.]
=Electroscopes.=--These are instruments for detecting whether a body be
electrified or not, and indicating also whether the electrification be
positive or negative. The earliest electroscope devised consisted of a
stiff straw balanced lightly upon a sharp point; a thin strip of brass or
wood, or even a goose quill, balanced upon a sewing needle will serve
equally well. Another form of electroscope is the pith ball pendulum,
shown in figs. 2 and 3. When an electrified body is held near the
electroscope it is attracted or repelled thus indicating the presence and
nature of the charge.
=Gold Leaf Electroscope.=--This form of electroscope, which is very
sensitive, was invented by Bennet. Its operation depends on the fact that
_like charges repel each other_.
[Illustration: FIG. 11.--Gold leaf electroscope; it consists of two strips
of gold foil suspended from a brass rod within a glass jar. Used to detect
the presence and sign of an electric charge.]
The gold leaf electroscope as shown in fig. 11, is conveniently made by
suspending the two narrow strips of gold leaf within a wide mouthed glass
jar, which both serves to protect them from draughts of air and to support
them from contact with the ground. A piece of varnished glass tube is
pushed through the cork, which should be varnished with shellac or with
paraffin wax. Through this passes a stiff brass wire, the lower end of
which is bent at a right angle to receive the two strips of gold leaf,
while the upper end is attached to a flat plate of metal, or may be
furnished with a brass knob.
When kept dry and free from dust it will indicate excessively small
quantities of electricity. A rubbed glass rod, even while two or three
feet from the instrument, will cause the leaves to repel one another. If
the knob be brushed with only a small camel’s hair brush, the slight
friction produces a perceptible effect. With this instrument all kinds of
friction can be shown to produce electrification.
The gold leaf electroscope can be further used to indicate the _kind_ of
electricity on an excited body. Thus, if a piece of brown paper be rubbed
with a piece of india rubber, the nature of the charge is determined as
follows:
[Illustration: FIG. 12.--Distribution of electrification on a charged
hollow sphere. If an insulated conductor _d_ be inserted through the
opening in the sphere and brought in contact with the interior surface and
afterwards carefully removed, it will be found, by testing with the gold
leaf electroscope, that it has received no charge. If touched to the
outside, however, the conductor will receive part of the charge.]
First charge the gold leaves of the electroscope by touching the
knob with a glass rod rubbed on silk. The leaves diverge, being
electrified with positive electrification. When they are thus
charged the approach of a body which is positively electrified
will cause them to diverge still more widely; while, on the
approach of one negatively electrified, they will tend to close
together. If now the brown paper be brought near the electroscope,
the leaves will be seen to diverge more, proving the electrification
of the paper to be of the same kind as that with which the
electroscope is charged.
The gold leaf electroscope will also indicate roughly the amount
of electricity on a body placed in contact with it, for the gold
leaves open out more widely when the quantity of electricity
thus imparted to them is greater.
[Illustration: FIGS. 13 and 14.--Electrification produced by rubbing
dissimilar bodies together and then separating them. If the insulated
glass and leather discs A and B be rubbed together, _but not separated_,
no signs of electrification can be detected; but if the discs be drawn
apart a little distance the space between them is found to be an electric
field, and as they separate farther and farther, electric forces will be
found to exist in more and more of the surrounding space, the
electrification being indicated by “lines of force.” It should be noted
that _work has to be done_ in separating the charged discs to overcome the
attraction which tends to hold them together. The stress indicated by the
lines of force consists of a tension or pull in the direction of their
length and a pressure or thrust at right angles to that direction.]
=Electric Screens.=--That the charge on the outside of a conductor always
distributes itself in such a way that there is no electric force within
the conductor was first proved experimentally by Faraday. He covered a
large box with tin foil and went inside with the most delicate
electroscopes obtainable. Faraday found that the outside of the box could
be charged so strongly that long sparks would fly from it without any
electrical effects being observable anywhere inside the box.
To repeat the experiment in modified form, let an electroscope
be placed beneath a bird cage or wire netting, as in fig. 15.
Let charged rods or other powerfully charged bodies be brought
near the electroscope outside the cage. The leaves will be found
to remain undisturbed.
[Illustration: FIG. 15.--The electric screen. A screen of wire gauze
surrounding a delicate electrical instrument will protect it from external
electrostatic induction.]
=Electrification by Induction.=--An insulated conductor, charged with
either kind of electricity, acts on bodies in a neutral state placed near
it in a manner analogous to that of the action of a magnet on soft iron;
that is, it decomposes the neutral electricity, attracting the opposite
and repelling the like kind of electricity. The action thus exerted is
said to take place by _influence_ or _induction_.
The phenomenon of electrification by induction may be demonstrated by the
following experiment:
In fig. 16, let the ebonite rod be electrified by friction and
slowly brought toward the knob of the gold leaf electroscope.
The leaves will be seen to diverge, even though the rod does not
approach to within a foot of the electroscope.
[Illustration: FIG. 16.--Experiment to illustrate electrostatic induction.
The leaves will diverge, even though the charged ebonite rod does not
approach to within a foot of the electroscope.]
This experiment shows that the mere _influence_ which an electric charge
exerts upon a conductor placed in its vicinity is able to produce
electrification in that conductor. This method of producing
electrification is called _electrostatic induction_.
As soon as the charged rod is removed the leaves will collapse, indicating
that this form of electrification is only a temporary phenomenon which is
due simply to the presence of the charged body in the neighborhood.
=Nature of the Induced Charge.=--This is shown by the experiment
illustrated in fig. 17.
Let a metal ball A be charged by rubbing it with a charged rod,
and let it then be brought near an insulated metal cylinder B
which is provided with pith balls on strips of paper C, D, E, as
shown.
The divergence of C and E will show that the ends of B have
received electrical charges because of the presence of A, while
the failure of D to diverge will show that the middle of B is
uncharged. Further, the rod which charged A will be found to
repel C but to attract E.
[Illustration: FIG. 17.--Experiment illustrating the nature of an induced
charge. The apparatus consists of a metal ball and cylinder, both mounted
on insulated stands, pith balls being placed on the cylinder at points C,
D, and E.]
From these experiments, the conclusion is that when a conductor is brought
near a charged body, the end away from the inducing charge is electrified
with the same kind of electricity as that on the inducing body, while the
end toward the inducing body receives electricity of opposite sign.
=The Electrophorus.=--This is a simple and ingenious instrument, invented
by Volta in 1775 for the purpose of procuring, by the principle of
induction, _an unlimited number of charges of electricity from one single
charge_.
It consists of two parts, as shown in fig. 19, a round cake of resinous
material B, cast in a metal dish or “sole” about one foot in diameter, and
a round disc A, of slightly smaller diameter made of metal or of wood
covered with tinfoil, and provided with a glass handle. Shellac, or
sealing wax, or a mixture of resin shellac and Venice turpentine, may be
used to make the cake.
[Illustration: FIGS. 18 and 19.--The electrophorus and method of using.
Charge B; place A in contact with B, and touch A (fig. 18). The disc is
now charged by _induction_ and will yield a spark when touched by the
hand, as in fig. 19.]
To use the electrophorus, the resinous cake B must be first beaten or
rubbed with fur or a woolen cloth, the disc A is then placed on the cake,
touched with the finger and then lifted by the handle. The disc will now
be found to be charged and will yield a spark when touched with the hand,
as in fig. 19.
The “cover” may be replaced, touched, and once more removed, and will thus
yield any number of sparks, the original charge on the resinous plate
meanwhile remaining practically as strong as before.
The theory of the electrophorus is very simple, provided the student has
clearly grasped the principle of induction.
[Illustration: FIGS. 20 to 23.--Illustrating “how the electrophorus
works.”]
When the resinous cake is first beaten with the cat’s skin its surface is
negatively electrified, as indicated in fig. 20. Again, when the metal
disc is placed down upon it, it rests really only on three or four points
of the surface, and may be regarded as an insulated conductor in the
presence of an electrified body. The negative electrification of the cake
therefore acts inductively on the metallic disc or “cover,” attracting a
positive charge to its under side, and repelling a negative charge to its
upper surface, as shown in fig. 21.
If, now, the cover be touched for an instant with the finger, the negative
charge of the upper surface (which is upon the upper surface being
repelled by the negative charge on the cake) will be neutralized by
electricity flowing in from the earth through the hand and body of the
experimenter. The attracted positive charge will, however remain being
bound as it were by its attraction towards the negative charge on the
cake.
[Illustration: FIG. 24.--Lines of force of a charged sphere and a
conductor under induction. The negative electrification on the end _a_ of
the cylinder indicates that a certain number of lines end there, while the
positive electrification on the end _b_ similarly indicates that an
_equal_ number of lines set out from that end. It is one of the
fundamental properties of a conductor that it yields instantly to the
smallest electric force, and that no electric force can be permanently
maintained within the substance of a conductor in which no current is
passing. There can, therefore, be no electrostatic strain and no lines of
force within the material of a conductor where the electric field has
become steady. Hence the lines starting from _b_ are entirely distinct
from those ending at _a_. The two sets are equal in number because no
charge has been given to the cylinder, either positive or negative, and
therefore the sum of all the positive electrifications (or lines starting
from _b_) must be equal to the sum of all the negative electrifications
(or the lines ending at _a_). In all nine lines have been drawn at each
end of the cylinder, leaving the thirteen lines emanating from the sphere
which do not run on to the cylinder. If the cylinder be withdrawn to a
distance from K, it (the cylinder) will be found to show no signs of
electrification.]
Fig. 22 shows the result after the cover has been touched. If, finally,
the cover be lifted by its handle, the remaining positive charge will no
longer be “bound” on the lower surface by attraction, but will distribute
itself on both sides of the cover, and may be used to give a spark. It is
clear that no part of the original charge has been consumed in the
process, which may be repeated as often as desired. As a matter of fact,
the charge on the cake slowly dissipates--especially if the air be damp.
Hence it is needful sometimes to renew the original charge by again
beating the cake with the cat’s skin.
[Illustration: FIG. 25.--Faraday’s ice-pail experiment. An ice-pail P
connected with the gold leaves of an electroscope C, is placed on an
insulating stand S. A charged conductor K, carried by a silk thread, is
lowered into the pail, and finally touches it at the bottom. While it is
being lowered the leaves of the electroscope diverge farther and farther,
until K is well within the pail, after which they diverge no more, even
when K touches the pail or is afterwards withdrawn by the insulating
thread. After withdrawal, K is found to be completely discharged.]
The labor of touching the cover with the finger at each operation may be
saved by having a pin of brass or a strip of tinfoil projecting from the
metallic “sole” on to the top of the cake, so that it touches the plate
each time, and thus neutralizes the negative charge by allowing
electricity to flow in from the earth.
[Illustration: FIGS. 26 to 29.--Explanation of Faraday’s ice pail
experiment. For simplicity the electroscope, insulating stand and silk
thread have been omitted. Only the three principal conductors K, P, and
the earth E are shown. In fig. 26 the ball K is sufficiently close to P to
act inductively on it; six lines are shown as falling on P, and the other
six as passing to E by different paths. Corresponding to the six lines
falling on P from K, six others pass to E from the lower surfaces. In fig.
27 where K is just entering the pail, two lines only pass from K to E
through the dielectric; the remaining ten fall on P, and ten others
starting from the distant parts of P pass to E. In fig. 28, K is so far
within P that none of its lines can reach E through the dielectric; they
all fall on P and from the outside of P an equal number start and pass
through the dielectric to E. It is evident that in this position K can be
moved about within P, without affecting the outside distribution in the
slightest, and that even when K touches P as shown in fig. 29, and when,
therefore, all lines between them disappear, the lines in the dielectric
outside remain just as they are in fig. 28. K is now completely
discharged, since lines no longer emanate from it, hence it can be removed
by the silk cord without disturbing the electrification of P. If K be
again charged and introduced into P it will be again discharged, for the
fact that P is already charged will have no effect on the final result,
provided when K touches P it is well _under cover_.]
Since the electricity thus yielded by the electrophorus is not obtained at
the expense of any part of the original charge, it is a matter of some
interest to inquire whence is the source from which the energy of this
apparently unlimited supply is drawn; for it cannot be called into
existence without the expenditure of some other form of energy. The fact
is, _more work is done in lifting the cover when it is charged_ with the
positive electricity than when it is not charged; for when charged, there
is the force of the electric attraction to be overcome as well as the
force of gravity; this excess force is the real origin of the energy
stored up in the separate charges.
[Illustration: FIGS. 30 and 31.--The Leyden jar and discharger. Its
discovery is attributed to the attempt of Musschenbrock and his pupil
Cuneus to collect the supposed electric “fluid” in a bottle half filled
with water. The bottle was held in the hand and was provided with a nail
to lead the “fluid” down through the cork to the water from the electric
machine. The invention of the Leyden jar is also claimed by Kleist, Bishop
of Pomerania.]
=Condensers; Leyden Jar.=--A _condenser_ is an apparatus for condensing a
large quantity of electricity on a comparatively small surface. The form
may vary considerably, but in all cases it _consists essentially of two
insulated conductors, separated by an insulator and the working depends on
the action of induction_.
A form of condenser generally used in making experiments on static
electricity is the Leyden jar, so named from the town of Leyden where it
was invented. It consists of a glass jar coated inside and out to a
certain height with tinfoil, having a brass rod terminating in a knob
passed through a wooden stopper, and connected to the inner coat by a
loose chain, as shown in fig. 30.
The jar may be charged by repeatedly touching the knob with the charged
plate of the electrophorus or by connecting the inner coating to one knob
of an electrical machine and the outer coating to the other knob.
The discharge of a condenser is effected by connecting the plates having
an opposite charge. This may be done by use of a wire or a discharger, as
shown in fig. 31; the connection is made between the outer coat and the
knob.
When the knob of the discharger is sufficiently close to the knob of the
jar, a bright spark will be observed between the knobs. This discharge
occurs whenever the difference of potential between the coats is great
enough to overcome the resistance of the air between the knobs.
Let a charged jar be placed on a glass plate so as to insulate
the outer coat. Let the knob be touched with the finger. No
appreciable discharge will be noticed. Let the outer coat be in
turn touched with the finger. Again no appreciable discharge
will appear. But if the inner and outer coatings be connected
with the discharger, a powerful spark will pass.
=Electric Machines.=--Various machines have been devised for producing
electric charges such as have been described. The ordinary “static” or
electric machine, is nothing but a continuously acting electrophorus.
Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of
the stationary plate E, are pasted paper sectors, beneath which are strips
of tinfoil AB and CD called _inductors_.
In front of E is a revolving glass plate carrying discs _l_, _m_, _n_,
_o_, _p_ and _q_, called _carriers_.
To the inductors _AB_ and _CD_ are fastened metal arms _t_ and _u_, which
bring _B_ and _C_ into electrical contact with the discs _l_, _m_, _n_,
_o_, _p_ and _q_, when these discs pass beneath the tinsel brushes carried
by _t_ and _u_.
A stationary metallic rod _rs_ carries at its ends stationary brushes as
well as sharp pointed metallic combs.
The two knobs _R_ and _S_ have their capacity increased by the Leyden jars
_L_ and _L′_.
[Illustration: FIG. 32.--The Toepler-Holtz electric machine.]
[Illustration: FIG. 33.--Principle of Toepler-Holtz electric machine.]
=Action of the Toepler-Holtz Machine.=--The action of the machine
described above is best understood from the diagram of fig. 33. Suppose
that a small + charge is originally placed on the inductor _CD_. Induction
takes place in the metallic system consisting of the discs _l_ and _o_ and
the rod _rs_, _l_ becoming negatively charged and _o_ positively charged.
As the plate carrying _l_, _m_, _n_, _o_, _p_, _q_ rotates in the
direction of the arrow the negative charge on _l_ is carried over to the
position _m_, where a part of it passes over to the inductor _AB_, thus
charging it negatively.
When _l_ reaches the position _n_ the remainder of its charge, being
repelled by the negative electricity which is now on _AB_, passes over
into the Leyden jar _L_.
When _l_ reaches the position _o_ it again becomes charged by induction,
this time positively, and more strongly than at first, since now the
negative charge on _AB_, as well as the positive charge on _CD_, is acting
inductively upon the rod _rs_.
When _l_ reaches the position _u_, a part of its now strong positive
charge passes to _CD_, thus increasing the positive charge upon this
inductor.
In the position _v_ the remainder of the positive charge on _l_ passes
over to _L′_. This completes the cycle for _l_. Thus as the rotation
continues _AB_ and _CD_ acquire stronger and stronger charges, the
inductive action upon _rs_ becomes more and more intense, and positive and
negative charges are continuously imparted to _L′_ and _L_ until a
discharge takes place between the knobs _R_ and _S_.
There is usually sufficient charge on one of the inductors to start the
machine, but in damp weather it will often be found necessary to apply a
charge to one of the inductors by means of the ebonite or glass rod before
the machine will work.
=The Wimshurst Machine.=--The essential parts of an ordinary Wimshurst
machine, as shown in fig. 34, are two insulating plates or drums. On each
plate are fixed a large number of strips of conducting material, which are
equal in size and are equally spaced--radially if on a plate, and
circumferentially if on a drum. The plates, or drums, are made to rotate
in opposite directions. The capacity of the inductors therefore varies
from a maximum when each strip on one plate is facing a strip on the
other, to a minimum when the conducting strips on each plate are facing
blank or insulating portions of the other plate.
There are three pairs of contact brushes, the members of two of
the pairs being at opposite ends of diametrical conducting rods
placed at right angles to one another; the third pair are
insulated from one another and form the principal collectors,
the one giving positive and the other negative electricity.
The plates are revolving in opposite directions; thus if there
be a charge on one of the conducting segments of one plate and
an opposite charge on one of the conducting segments on the
other plate near it, their potential will be raised as the
rotation of the plates separates them.[2]
[Illustration: FIG. 34.--The Wimshurst Electric Machine.]
CHAPTER III
THE ELECTRIC CURRENT
The ordinary statement that an electric current is flowing along a wire is
only a conventional way of expressing the fact that the wire and the space
around the wire are in a different state from that in which they are when
no electric current is said to be flowing.
In order to make laymen understand the action of this so called current,
it is generally compared with the flow of water.
In comparing hydraulics and electricity, it must be borne in mind,
however, that there is really no such thing as an “electric fluid,” and
that water in pipes has mass and weight, while electricity has none. It
should be noted, however, that electricity is conveniently spoken of as
having weight in explaining some of the ways in which it manifests itself.
All electrical machines and batteries are merely instruments for moving
electricity from one place to another, or for causing electricity, when
accumulated in one place, to do work in returning to its former level of
distribution.
The _head_ or _pressure_ in a standpipe is what causes water to move
through the pipes which offer _resistance_ to the _flow_.
Similarly, the conductors, along which the electric current is said to
flow, offer more or less _resistance_ to the flow, depending on the
material. Copper wire is generally used as it offers little resistance.
The current must have pressure to overcome the resistance of the conductor
and flow along its surface. This pressure is called _voltage_ caused by
what is known as _difference of potential_ between the source and
terminal.
[Illustration: FIG. 35.--Analogy of the flow of water to the electric
current. The water in the reservoirs A and B stands at different heights.
As long as this difference of level is maintained, water from B will flow
through the pipe R to A. If by means of a pump P the level in B be kept
constant, flow through R will also be maintained. Here, by means of the
work expended on the pump, the level in the reservoir is kept constant;
and in the corresponding case of the electric current, by the conversion
of chemical energy a constant difference of potential is maintained.]
The pressure under which a current flows is measured in _volts_ and the
quantity that passes in _amperes_. The resistance with which the current
meets in flowing along a conductor is measured in _ohms_.
=Ques. What is a volt?=
Ans. A volt is that electromotive force (E. M. F.) which produces a
current of one ampere against a resistance of one ohm.
=Ques. What is an ampere?=
Ans. An ampere is the current produced by an E. M. F. of one volt in a
circuit having a resistance of one ohm. It is that quantity of electricity
which will deposit .005084 grain of copper per second.
=Ques. What is an ohm?=
Ans. An ohm is equal to the resistance offered to an unvarying electric
current by a column of mercury at 32° Fahr., 14.4521 grams in mass, of a
constant cross sectional area, and of the length of 106.3 centimeters.
=Ohm’s Law.=--_In a given circuit, the amount of current in amperes is
equal to the E. M. F. in volts divided by the resistance in ohms; that
is:_
current = pressure / resistance = volts / ohms
expressed as a formula:
I = E/R (1)
in which
I = current strength in amperes;
E = electromotive force in volts;
R = resistance in ohms.
From (1) is derived the following:
E = IR (2)
R = E/I (3)
From (1) it is seen that the flow of the current is proportional to the
voltage and inversely proportional to the resistance; the latter depends
upon the material, length and diameter of the conductor.
Since the current will always flow along the path of least resistance; it
must be so guarded that there will be no leakage. Hence, to prevent
leakage, wires are _insulated_, that is, covered by wrapping them with
cotton or silk thread or other insulating material. If the insulation be
not effective, the current may leak, and so return to the source without
doing its work. This is known as a _short circuit_.
The conductor which receives the current from the source is called the
_lead_, and the one by which it flows back, the _return_.
When wires are used for both lead and return, it is called a _metallic
circuit_: when the ground is used for the return, it is called a _ground
circuit_. An electric current is said to be:
1. _Direct_, when it is of unvarying direction;
2. _Alternating_, when it flows rapidly to and fro in opposite
directions;
3. _Primary_, when it comes directly from the source;
4. _Secondary_, when the voltage and amperage of a primary current have
been changed by an _induction coil_;
5. _Low tension_, when its voltage is low;
6. _High tension_, when its voltage is high.
A high tension current is capable of forcing its way against
considerable resistance, whereas, a low tension current must
have its path made easy.
=Production of the Electric Current.=--To produce a steady flow of water
in a pipe two conditions are necessary. There must first be available a
hydraulic pressure, or, as it is technically called, a “_head_” of water
produced by a pump, or a difference of level or otherwise.
In addition to the pressure there must also be a suitable path or channel
provided for the water to flow through, or there will be no flow, however
great the “head,” until something breaks down under the strain. In the
case just cited, although there is full pressure in the water in the pipe,
there is no current of water as long as the tap remains closed. The
opening of the tap completes the necessary _path_ (the greater part of
which was already in existence) and the water flows.
[Illustration: FIG. 36.--Hydraulic analogy of the electric current. If,
say 10 gallons of water flow in every second into a system of vessels and
pipes of any shape, whether simple or more complicated as shown in the
figure, and 10 gallons flow out again per second, it is evident that
through every cross section of any vessel or pipe of the system 10 gallons
of water pass every second. This follows from the fact that water is an
uncompressible liquid and must be practically of the same density
throughout the system. The water moves slowly where the section is large
and quickly where it is small, and thus the quantity of water that flows
through any part of the system is independent of the cross section of that
part. The same condition holds good for the electric current; if in a
closed circuit a constant current circulates, the same amount of
electricity will pass every cross section per second. Hence the following
law: _The magnitude of a constant current in any circuit is equal in all
parts of the circuit._]
For the production of a steady electric current two very similar
conditions are necessary. There must be a steadily maintained electric
pressure, known under different aspects as “electromotive force,”
“potential difference,” or “voltage.” This alone, however, is not
sufficient. In addition, a suitable conducting path is necessary. Any
break in this path occupied by unsuitable material acts like the closed
tap in the analogous case above mentioned, and it is only when all such
breaks have been properly bridged by suitable material, that is, by
conductors, that the effects which denote the flow of the current will
begin to be manifested.
The necessary electromotive force or voltage required to cause the current
to flow may be obtained:
1. Chemically;
2. Mechanically;
3. Thermally.
In the first method, two dissimilar metals such as copper and zinc called
_elements_, are immersed in an exciting fluid or _electrolyte_.
[Illustration: FIG. 37.--Volta’s “Crown of Cups.” The metallic elements C
and Z each consisted of two metals, the plate C being of copper and the
plate Z of zinc. They were placed, as shown, in the glass vessels, which
contained salt water and ordinary water or lye. Into each vessel, except
the two end ones, the copper end of one arc and the zinc end of the next
were introduced, the series, however long, ending with copper dipping into
the terminal vessel at one end and zinc into that at the other. The
arrangement is almost exactly that of a modern one-fluid primary battery.]
When the elements are connected at their terminals by a wire or conductor
a chemical action takes place, producing a current which flows from the
copper to the zinc. This device is called a _cell_, and the combination of
two or more of them connected so as to form a unit is known as a
_battery_. _The word battery is frequently used incorrectly for a single
cell._ That terminal of the element from which the current flows is called
the _plus_ or _positive pole_, and the terminal of the other element the
_negative pole_.
Cells are said to be _primary_ or _secondary_ according as they
generate a current of themselves, or first require to be charged
from an external source, storing up a current supply which is
afterwards yielded in the reverse direction to that of the
charging current.
An electric current is generated mechanically by a _dynamo_. In either
case _no electricity is produced, but part of the supply already existing
is simply set in motion by creating an electric pressure_.
An electric current, according to the third method, is generated directly
from heat energy, as will be later explained; the current thus obtained is
very feeble.
[Illustration: FIG. 38.--Hydrostatic analogy of fall of potential in an
electrical circuit.]
[Illustration: FIG. 39.--Showing method of connecting voltmeter to find
potential difference between any two points as _m_ and _n_ on an
electrical circuit.]
=Strength of Current.=--It is important that the reader have a clear
conception of this term, which is so often used. The exact definition of
the strength of a current is as follows:
_The strength of a current is the quantity of electricity which flows past
any point of the circuit in one second._
_Example._--If, during 10 seconds, 25 coulombs of electricity
flow through a circuit, then the average strength of the current
during that time is 2-1/2 coulombs per second, or 2-1/2
amperes.
=Voltage Drop in an Electric Circuit.=--A difference of potential exists
between any two points on a conductor through which a current is flowing
on account of the resistance offered to the current by the conductor.
For instance, in the electrical circuit shown in fig. 39, the potential at
the point _a_ is higher than that at _m_, that at _m_ higher than that at
_n_, etc., just as in the water circuit, shown in fig. 38, the hydrostatic
pressure at _a_ is greater than that at _m′_, that at _m′_ greater than
that at _n′_, etc. The fall in the water pressure between _m′_ and _n′_
(fig. 38) is measured by the water head _n’s_.
In order to measure the fall in electrical potential between _m_ and _n_,
(fig. 39), the terminals of a volt meter are placed in contact with these
points as shown. Its reading will give the difference of potential between
_m_ and _n_, in volts, provided that its own current carrying capacity is
so small that it does not appreciably lower the potential difference
between the points _m_ and _n_ by being touched across them; that is,
provided the current which flows through it is negligible in comparison
with that which flows through the conductor which already joins the points
_m_ and _n_.
CHAPTER IV
PRIMARY CELLS
The word “battery” is a much abused word, being often used incorrectly for
“cell,” as in fig. 40. Hence, careful distinction should be made between
the two terms.
_A battery consists of two or more cells joined together so as to form a
single unit._
There are numerous forms of primary cell; they may be classified as
follows:
1. According to the service for which they are designed;
2. According to the chemical features.
With respect to the first method cells are classified as:
1. Open circuit cells;
Used for _intermittent work_, where the cell is in service for
short periods of time, such as in electric bells, signaling
work, and electric gas lighting. If kept in continuous service
for any length of time the cell soon polarizes or “runs down,”
but will recuperate after remaining on open circuit for some
little time.
2. Closed circuit cells.
This type of cell is adapted to furnishing current continuously,
as in telegraphy, etc.
With respect to the second method, cells are classified as:
1. One fluid;
2. Two fluid;
=Ques. Describe a primary cell.=
Ans. A primary cell consists of a vessel containing a liquid in which two
dissimilar metal plates are immersed.
In _one fluid_ cells both metal plates are immersed in the same
solution. In _two fluid_ cells each metal plate is immersed in a
separate solution, one of which is contained in a porous cup
which is immersed in the other liquid.
=Ques. What name is given to the metal plates?=
Ans. They are called _elements_.
=Ques. What is the fluid called?=
Ans. The _electrolyte_ or _exciting fluid_.
The term “electropoion” is a trade name for the electrolyte
employed in the Fuller cell.
=Action of a Primary Cell.=--The fundamental fact on which the
electro-chemical generation of current depends is, that if a plate of
metal be placed in a liquid there is a difference of electrical condition
produced between them of such sort that the metal either takes a lower or
higher electrical potential than the liquid, according to the nature of
the metal and the liquid. If two different metals be placed in one
electrolytic liquid, then there is a difference of state produced between
them, so that, if joined by wire outside the liquid, a current of
electricity will traverse the wire. This current proceeds in the liquid
from the metal which is most acted upon chemically to that which is least
acted upon.
Referring to fig. 41, the construction and action of a simple primary cell
may be briefly described as follows:
Place in a glass jar some water having a little sulphuric or other acid
added to it. Place in it separately two clean strips, one of zinc, Z, and
one of copper, C. This cell is capable of supplying a continuous flow of
electricity through a wire whose ends are brought into connection with
the two strips. When the current flows, the zinc strip is observed to
waste away, its consumption in fact furnishing the energy or electromotive
force required to drive the current through the cell and the connecting
wire. The cell may therefore be regarded as a kind of chemical furnace in
which the fuel is the zinc.
[Illustration: FIG. 40.--Simple primary cell. It consists of two
dissimilar metal plates (such as copper and zinc which are called the
_elements_), immersed in the _electrolyte_ or exciting fluid contained in
the glass jar.]
=Ques. How are the positive and negative elements of a primary cell
distinguished?=
Ans. The plate attacked by the electrolyte is the negative element, and
the one unattacked the positive element.
=Chemical Changes; Polarization.=--The chemical changes which take place
in a simple cell, consisting of zinc and copper elements in an electrolyte
of dilute sulphuric acid, may be briefly described as follows: When the
two elements are connected and the current commences to flow, the
sulphuric acid acts on the surface of the zinc plate and forms sulphate of
zinc. The formation of this new substance necessitates the liberation of
some of the hydrogen contained in the sulphuric acid, and it will be found
that bubbles of free hydrogen gas speedily appear on the surface of the
negative element, that is, on the copper plate.
While the zinc is being dissolved to form zinc sulphate, hydrogen gas is
liberated from the sulphuric acid.
[Illustration: FIG. 41.--Simple primary cell with circuit closed, showing
direction of the current.]
Some bubbles of the gas rise to the surface of the electrolyte and so
escape into the air, _but much of it clings to the surface of the copper
element which thus gradually becomes covered with a thin film of
hydrogen_.
Partly on account of the decreased area of copper plate in contact with
the electrolyte, and partly because the hydrogen tends to produce a
current in the opposite direction, the useful electrical output becomes
considerably diminished and the cell is said to be _polarized_. This state
of affairs may be rectified by stirring up the electrolyte, or by shaking
the cell, so as to assist the hydrogen bubbles to detach themselves from
the surface of the copper plate and make their way to the atmosphere
through the electrolyte. This, however, is only a temporary remedy, as the
polarized condition will soon be reached again, and a further agitation of
the cell will be necessary. Hence, a simple cell of this kind is not
desirable for practical work, and it must be modified to adapt it to
constant use.
When the sulphuric acid in a cell acts in the zinc element and produces
sulphate of zinc, a certain amount of work is done which is manifested
partly in the form of useful electric energy, and partly as heat which
warms the electrolyte and which is thereby lost for all practical
purposes.
=Ques. If the zinc and copper electrodes of a simple cell be not connected
externally what changes take place within the cell?=
Ans. The zinc plate immediately becomes strongly charged with negative
electricity, and the copper plate weakly so. As long as the plates remain
unconnected, and the zinc is pure, no further action takes place.
=Ques. If the electrodes be connected externally what happens?=
Ans. If the plates be connected by a wire outside the electrolyte, the
tendency which dissimilar electrical charges have to neutralize one
another causes a flow of negative electricity through the wire from zinc
to copper, and a positive flow in the opposite direction. The “static”
charge being thus disposed of, a fresh charge is given to the plates by
the action of the acid, which commences to dissolve the zinc. As long as
the wire connects the copper and zinc plates, the acid will continue its
action on the zinc until either acid or zinc is exhausted.
The reader may ask: how can there be a positive flow when both
plates are negatively electrified?
An analogy is the best way to make this point clear: Imagine two
equal vessels, from each of which the air has been partially
exhausted, but from one (A) 10 times as much air has been taken
as from the other (B). Connect A and B by a tube. Now, although
both vessels have less than the atmospheric pressure, that is,
both have “negative” pressures, yet a current of air will flow
from B to A until the pressures in each are equalized; that is,
until both have equal “negative charges” of air.
There is a second important effect of the acid solution or electrolyte in
a cell. If pure sulphuric acid were used, the first action or production
of an electrical charge on the zinc plate would be the same, but when the
plates were joined by the wire the current would soon cease. The reason
for this lies in the fact that the sulphate of zinc, which is the compound
produced by the acid plus the zinc, being insoluble in pure undiluted
sulphuric acid, remains on the surface of the zinc plate. The coating of
sulphate of zinc thus formed also operates as a protective agent, and no
further electrical charge can be induced until it is removed. The addition
of water to the acid has the effect of allowing the sulphate of zinc to
dissolve, and the zinc plate is left free for further action.
=Ques. What governs the rate of current flow of a primary cell?=
Ans. The size of the elements and their proximity.
=Effects of Polarization.=--The film of hydrogen bubbles affects the
strength of the current of the cell in two ways:
1. It weakens the current by the increased _resistance_ which it offers to
the flow, for bubbles of gas are bad conductors;
2. It weakens the current by setting up an opposing _electromotive
force_.
Hydrogen is almost as oxidizable a substance as zinc, especially
when freshly deposited (in the “nascent” state), and is
electro-positive; hence, the hydrogen itself produces a
difference of potential, which would tend to start a current in
the opposite direction to the true zinc-to-copper current. It is
therefore an important matter to abolish this polarization,
otherwise the currents furnished by batteries would not be
constant.
=Methods of Depolarizing.=--One of the chief aims in the arrangement of
the numerous cells which have been devised is to avoid polarization. The
following are the methods usually employed:
1. Chemical methods;
_a_. Oxidation of the hydrogen by potassium bichromate and by
nitric acid.
_b_. Substitution of the hydrogen by some other substance which
does not give a counter electromotive force of polarization; for
instance, in the Daniell cell by replacement of the copper in
copper sulphate by the hydrogen, the copper being deposited on
the positive pole.
2. Electro-chemical means;
It is possible by employing double cells, to secure such action
that some solid metal, such as copper, shall be liberated
instead of hydrogen bubbles, at the point where the current
leaves the liquid. This electro-chemical exchange obviates
polarization.
3. Mechanical methods.
_a_. Agitation of the liquid or of the positive electrode, in
order to prevent the accumulation of hydrogen thereon.
_b_. Corrugating or roughing the positive electrode, as in the
Smee cell. This causes the hydrogen gas to form in large bubbles
which rise to the surface more rapidly than the small bubbles
which form on a smooth electrode.
In the simplest form of cell, as zinc, copper, and dilute sulphuric acid,
no attempt has been made to prevent the evil of polarization, hence, it
will quickly polarize when the current is closed for any length of time,
and may be classified as an open circuit cell.
When polarization is remedied by chemical means, the chemical added is one
that has a strong affinity for hydrogen and will combine with it, thus
preventing the covering of the negative plate with the hydrogen gas.
[Illustration: FIGS. 42 and 43.--Carbon cell and carbon cylinder. Carbon
possesses a natural power to prevent a limited amount of polarization by
absorbing the hydrogen gas coming from the zinc rod; hence it is used in
various shapes for open circuit cells, which gives rise to as many
different names, such as _Samson_, _Hercules_, _Law_, _National_,
_Standard_, etc. In all these types of cell, sal-ammoniac and zinc are
used, and by corrugating the carbon, fluting it, or making concentric
cylinders, special merits are obtained in each case. The carbon element is
usually made in the form of a porous cup, filled with oxide of manganese
to prevent polarization, and then sealed. The zinc rod is inserted through
a porcelain insulator. About 4 to 6 ounces of sal-ammoniac are generally
used for cells of ordinary size. The salt is placed in the jar, water
poured in until it is about two-thirds full, and then stirred till all the
salt is dissolved. When the carbon cylinder is inserted, the solution
should be within 1-1/2 inches of the top of the jar. The electromotive
force is from 1.0 to 1.4 volts for the different forms of carbon cell.]
=Ques. What is a depolarizer?=
Ans. A substance employed in some types of cell to combine with the
hydrogen which would otherwise be set free at the positive electrode and
cause polarization.
The chemical used for this purpose may be either in a _solid_ or
_liquid_ form, which gives rise to several types of cell, such
as cells with a single fluid, containing both the acid and the
depolarizer, cells with a single exciting fluid and a solid
depolarizer, and cells with two separate fluids.
In the two fluid cell, the zinc is immersed in the liquid
(frequently dilute sulphuric acid) to be decomposed by the
action upon it, and the negative plate is surrounded by the
liquid depolarizer, which will be decomposed by the hydrogen gas
it arrests, thereby preventing polarization.
In _open circuit cells_ polarization does not have much opportunity to
occur, since the circuit is closed for such a short period of time; hence,
these cells are always ready to deliver a strong current when used
intermittently.
In _closed circuit cells_ polarization is prevented by chemical action, so
that the current will be constant and steady till the energy of the
chemicals is expended.
=Ques. What is a depolarizer bag?=
Ans. A cylinder of hemp or other fabric used in place of a porous pot in
some forms of Leclanche cell, and also as a support for the depolarizing
mass in some forms of dry cell where the electrolyte is of a thin
gelatinous nature.
=Volta’s Contact Law.=--When metals differing from each other are brought
into contact, different results are obtained, both as to the kind of
electrification as well as the difference of potentials.
Volta found that iron, when in contact with zinc, becomes negatively
electrified; the same takes place, but somewhat weaker, when iron is
touched with lead or tin. When, however, iron is touched by copper or
silver, it becomes positively electrified. Volta, Seebeck, Pfaff, and
others have investigated the behavior of many metals and alloys when in
contact with each other.
The following lists are so arranged that those metals first in each list
become positively electrified when touched by any taking rank after them:
CONTACT SERIES OF METALS
_According to Volta_. _According to Pfaff._
+ zinc + zinc
lead cadmium
tin tin
iron lead
copper tungsten
silver iron
gold bismuth
graphite antimony
- manganese ore copper
silver
gold
uranium
tellurium
platinum
- palladium
Volta laid down a law regarding the position of the metals in his table
which may be stated as follows:
_The difference of potential between any two metals is equal to the sum of
the differences of potentials of all the intermediate members of the
series._
Hence, it is immaterial for the total effect whether the first and the
last are brought into contact directly, or whether the contact is brought
about by means of all or any of the intermediate metals.
Volta’s law further asserts that when any number of metals are
brought into contact with each other, but so that the chain
closes with the metal with which it was begun, the total
difference must be zero.
=Laws of Chemical Action in the Cell.=--There are two simple laws of
chemical action in the cell:
1. _The amount of chemical action in a cell is proportional to the
quantity of electricity that passes through it._
One coulomb of electricity in passing through the cell liberates
.000010352 of a gramme of hydrogen, and causes .00063344 of a
gramme of zinc to dissolve in the acid.
2. _The amount of chemical action is equal in each cell of a battery
connected in series._
=Requirements of a Good Cell.--=The several conditions which should be
fulfilled by a good cell are as follows:
1. Its electromotive force should be high and constant;
2. Its internal resistance should be small;
3. It should be perfectly quiescent when the circuit is open;
4. It should give a constant current, and therefore must be free from
polarization, and not liable to rapid exhaustion;
5. It should be easily cared for, and if possible, should not emit
corrosive fumes;
6. It should be cheap and of durable materials.
=Single and Two Fluid Cells.=--The distinction between a single and a two
fluid cell has already been given. The single fluid cell of Volta with its
zinc and copper plates represents the simplest form of primary cell.
In the _two fluid cell_, the positive (zinc) plate is immersed in the
exciting liquid (usually dilute sulphuric acid) and is decomposed by the
action upon it, while the negative plate is placed in the liquid
depolarizer which is decomposed by the hydrogen arrested by it, thus
preventing polarization.
In some forms of cell, the two liquids are separated by a porous partition
of unglazed earthenware, which, while it prevents the liquids mixing
except very slowly, does not prevent the passage of hydrogen and
electricity.
[Illustration: FIGS. 44 and 45.--Leclanche cell and porous cup. This very
common form of cell is an example of the single fluid type, with a solid
depolarizer surrounding the negative element; the latter is generally
carbon, the positive element being zinc. The liquid used is a strong
solution of ammonium chloride, commonly known as sal-ammoniac, and which
resembles table salt. In the porous cup type of cell, a carbon slab is
placed in the porous cup, and is surrounded by a mixture of small pieces
of carbon and manganese dioxide, the top being covered by means of pitch,
leaving one or two small holes for air and gas to pass through. The
depolarizer will take care of a limited amount of the hydrogen produced
when the cell is on closed circuit, but if the circuit be closed for any
length of time polarization occurs. The cell is thus of the open circuit
class, and will furnish a good current where it is required only
intermittently. Zinc is dissolved only when the cell is being used. This
type of cell, or its modification, is used for gas lighting and bell work.
The cell requires very little attention. Water must be added as the
solution evaporates, and the zinc rod replenished when necessary. The
electromotive force is about 1.48 volts and the internal resistance about
4 ohms.]
Complete depolarization is usually obtained also in single fluid cells,
having in addition a depolarizing solid body, such as oxide of manganese,
oxide of copper, or peroxide of lead, in contact with the carbon pole.
Such cells really do not belong to the single fluid cells, and are
considered in the two fluid class.
A few examples of single and double fluid primary cells will now be
described.
=The Leclanche Cell.=--This cell was invented by Leclanche, a French
electrician, and was the first cell in which sal-ammoniac was used. This
form of cell, as shown in fig. 45, is in general use for electric bells,
its great recommendation being that, once charged, it retains its power
without attention for considerable time.
Two jars are employed in its construction; the outer one is of glass,
contains a zinc rod, and is charged with a solution of ammonium chloride,
called sal-ammoniac.
The inner jar is of porous earthenware, containing a carbon plate, and is
filled with a mixture of manganese peroxide and broken gas carbon. When
the carbon plate and the zinc rod are connected, a steady current of
electricity is set up, the chemical action which takes place being as
follows: _the zinc becomes oxidized by the oxygen from the manganese
peroxide, and is subsequently converted into zinc chloride by the action
of the sal-ammoniac_.
After the battery has been in continuous use for some hours, the manganese
becomes exhausted of oxygen, and the force of the electrical current is
greatly diminished; but if the battery be allowed to rest for a short
time, the manganese obtains a fresh supply of oxygen from the atmosphere,
and is again fit for use.
After about 18 months work, the glass cell will probably require
recharging with sal-ammoniac, and the zinc rod may also need renewing; but
should the porous cell get out of order, it is better to get a new one
than to attempt to recharge it.
The directions for setting up a Leclanche cell are as follows:
1. Place in the glass jar six ounces of sal-ammoniac, and pour
in water until the jar is one-third full, then stir thoroughly.
2. Place the porous cup in the solution, and if necessary add
water until it rises to within 1-1/2 inches of the top of the
porous cup.
3. Put the zinc rod in place and set the cell away (not
connected up), for about 12 hours, so as to allow the liquid to
thoroughly soak into the porous cup. This will lower the level
of the liquid to about one-third the height of the jar. The cell
will then be ready for use. As the level of the liquid is
lowered by evaporation, it should be maintained at the stated
height by adding water.
The Leclanche cell is adapted to open circuit work, being extensively used
for ringing electric bells.
The objections to the Leclanche cell are:
1. Rapid polarization;
2. High internal resistance due to porous pot;
3. Restricted space for electrolyte causing rapid lowering of level of
liquid by evaporation;
4. Eating away of the zinc rod at the surface of the liquid, rendering
the rod useless before the lower part is consumed.
=Fuller Bichromate Cell.=--In the bichromate cells or the chromic acid
cells, bichromate of soda, or bichromate of potassium, is used for the
depolarizer, water and sulphuric acid being added for attacking the zinc.
The Fuller cell is of the two fluid type. A pyramidal block of zinc at the
end of a metallic rod covered with gutta-percha is placed in the bottom of
a porous cup containing an ounce of mercury. The cup is then filled with a
very dilute solution of sulphuric acid or water and placed in a jar of
glass or earthenware containing the bichromate solution and the carbon
plate. The diffusion of the acid through the porous cup is sufficiently
rapid to attack the zinc, which being well amalgamated, prevents local
action; while the hydrogen passes through the porous cup and combines with
the oxygen in the bichromate of potassium. This type of cell has an
electromotive force of 2.14 volts, and is suited to open circuit, or
semi-closed circuit work. The directions for setting up a Fuller cell are
as follows:
1. To make the “electropoion” fluid, mix together one gallon of
sulphuric acid and three gallons of water, and in a separate
vessel, dissolve six pounds of bichromate of potash in two
gallons of boiling water; then thoroughly mix together the two
solutions.
2. Immerse the zinc in a solution of dilute sulphuric acid, and
then in a bath of mercury, and rub it with a brush or cloth so
as to reach all parts of the surface.
3. Pour into the porous cell one ounce (a tablespoonful) of
mercury, and fill the porous cell with water up to within two
inches of the top.
4. Place the porous cell and the carbon plate in the glass jar,
as in fig. 46, and fill glass jar to within about three inches
of the top with a mixture of three parts of electropoion fluid
to two parts of water.
[Illustration: FIGS. 46 and 47.--The telephone standard and compound forms
of the Fuller cell. The type shown in fig. 46 is especially adapted to
long distance telephoning, and that shown in fig. 47 to incandescent
lamps, motors, nickel and other electroplating. The Fuller cell is a
double fluid variety and has the advantage over the Grenet type, in that
the zinc is always kept well amalgamated and does not require removal from
the solution. The Fuller cell is suitable for open and semi-closed circuit
work; its electromotive force is about 2.14 volts.]
5. The zinc should be lifted out occasionally and the sulphate
washed off.
6. The supply of mercury in the porous cell should be
maintained, so as to have the zinc always well amalgamated.
7. To renew, clean all deposits from carbon plate and zinc, and
set up with fresh solution.
=The Edison Cell.=--This is a single fluid cell with a solid depolarizer,
as shown in fig. 48, and is well adapted for use on closed circuits.
[Illustration: FIG. 48.--Edison cell, type R R. The electrolyte used is
caustic soda, the positive element zinc, and the negative element copper
oxide. The Edison cell is suitable for large stationary gas engine
ignition, railroad crossing signals, electroplating, fire alarms,
telephone circuits, etc.]
The positive element is zinc, and the negative element black oxide of
copper. The exciting fluid is a solution of caustic potash. The black
oxide of copper plates are suspended from the cover of the jar by a light
framework of copper, one end of which forms the positive pole of the
battery. A zinc plate is suspended on each side of the copper oxide
element and kept from coming in contact with the latter by means of
vulcanite buttons.
When the cell is in action, the water is decomposed, and the oxygen thus
liberated combines with the zinc and forms oxide of zinc, which combines
with the potash to form a double salt of zinc and potash. The last
combination dissolves as rapidly as it is formed. The hydrogen liberated
by the decomposition of the water reduces the copper oxide to pure
metallic copper. It is highly important that the copper oxide plates be
completely submerged in the solution of caustic potash, and that heavy
paraffin oil be poured on top of the solution to the depth of about 1/4 of
an inch to exclude the air. If oil be not used, the formation of creeping
salts will reduce the life of the battery fully two-thirds. The battery
has a low electromotive force, about 0.7 of a volt, but as the internal
resistance is also very low, quite a large current can be drawn from the
cell.
The _Bunsen Cell_, shown in figs. 49 and 50, is a two fluid cell
constructed with zinc and carbon electrodes. The negative plate is carbon,
the positive plate amalgamated zinc. The excitant is a dilute solution of
sulphuric acid. The top part of the carbon is sometimes impregnated with
paraffin (to keep the acid from creeping up).
The force of the Bunsen cell increases after setting up for about an hour,
and the full effect is not attained until the acid soaks through the
porous cell. Carbons are not affected and last any length of time. The
zinc is slowly consumed through the mercury coating.
=Grenet Bichromate Cell.=--In this cell, as shown in figs. 49 and 50, the
positive element is zinc and the negative element carbon. The electrolyte
is a solution of bichromate of potash in a mixture of sulphuric acid and
water.
[Illustration: FIGS. 49 and 50.--American and French forms of Grenet cell.
The elements are zinc and carbon. In the Grenet cell, a zinc plate is
suspended by a rod between two carbon plates, so that it does not touch
them, and when the cell is not in use the zinc is withdrawn from the
solution by raising and fastening the rod by means of a set screw, as the
acid attacks the zinc when the cell is on open circuit. This cell has an
electromotive force of over 2 volts at first, and gives a strong current
for a short time, but the liquid soon becomes exhausted, as will be noted
by the change in the color of the solution from an orange to a dark red,
and must be replenished. The zinc should be kept well amalgamated and out
of the solution except when in use. It is a good type of cell for
experimental work. To make the electrolyte take 3 ounces of finely
powdered bichromate of potash and 1 pint of boiling water; stir with a
glass rod and after it is cool, add slowly, stirring all the time, 3
ounces of sulphuric acid. The electrolyte may also be prepared as follows:
take 4 ounces of bichromate of soda, 1-1/4 pints of boiling water, and 3
ounces of sulphuric acid.]
The cell consists of a glass bottle containing the electrolyte and fitted
with a lid from which the elements are supported. There is a zinc plate in
the center and a carbon plate on each side. The two carbon plates are
connected to the same terminal, thus forming a large negative surface, and
the zinc plate to a terminal on the top of the brass rod to which it is
attached. This rod slides through a hole in the lid so that the zinc
plate can be lifted out of the electrolyte when the cell is not at work,
thus preventing wasteful consumption of zinc and of the electrolyte.
Bichromate cells give a strong current, the electromotive force of a
single cell being 2 volts.
[Illustration: FIG. 51.--The Bunsen cell. This is a two fluid cell and has
a bar of carbon immersed in strong nitric acid contained in a porous cup.
This cup is then placed in another vessel, containing dilute sulphuric
acid, and immersed in the same liquid, is a hollow cylindrical plate of
zinc, which nearly surrounds the porous cup. The hydrogen, starting at the
zinc, traverses by composition and recomposition, the sulphuric acid; it
then passes through the porous partition, and enters into chemical action
with the nitric acid, so that none of it reaches the carbon. Water is
produced by this action, which in time dilutes the acid, and orange
colored poisonous fumes of nitric oxide rise from the battery. If the
nitric acid first be saturated with nitrate of ammonia, the acid will last
longer and the fumes be prevented. Strong sulphuric acid cannot be used in
any battery; one part of sulphuric acid is generally added to 12 parts by
weight, or 20 by volume, of water. _Grove_ used a strip of _platinum
instead of carbon_ in his cell. A solution of bichromate of potassium is
frequently substituted for the nitric acid in the porous cup, thereby
avoiding disagreeable fumes. Bunsen’s and Grove’s cells produce powerful
and constant currents, and are well adapted for experiments, but they
require frequent attention, and are expensive, so that they are little
used for work of long duration. The electromotive force of these cells is
from 1.75 to 9.51 volts.]
=Daniell Cell.=--This is one of the best known and most widely used forms
of primary cell. It is a double fluid cell, composed of an inner porous
vessel containing an electrolyte of either dilute sulphuric acid or
dilute zinc sulphate solution, and an outer vessel containing a saturated
solution of copper sulphate.
A zinc rod is placed in the inner electrolyte, and a thin plate of sheet
copper in the outer electrolyte. Sometimes this arrangement of the
elements is modified, the outer vessel being made of copper and serving as
the copper plate. This would then contain the copper sulphate solution,
while the zinc sulphate and the zinc rod would be contained in the porous
pot as before.
The chemical reactions which take place in a Daniell cell are as follows:
The zinc dissolves in the dilute acid, thus producing zinc
sulphate, and liberating hydrogen gas. The free hydrogen passes
through the walls of the porous pot, but when it reaches the
copper sulphate solution it displaces some of the copper
therefrom, and combines with this solution, forming sulphuric
acid. The copper, which is thus set free, is deposited on the
surface of the copper plate. In this way polarization is
avoided, and a practically constant current is obtained.
When the zinc sulphate solution is employed in place of dilute
acid, a similar series of chemical reactions occur, except that
the zinc is liberated instead of hydrogen.
Daniell cells are used especially for electroplating, electrotyping and
telegraphic work. The electromotive force of a single cell is 1.079 volts.
=Directions for Making a Daniell Cell.=--The simple Daniell cell shown in
fig. 52 may be easily made as follows: The outer vessel A, consists of a
glass jar (an ordinary glass jam jar will do) containing a solution of
sulphuric acid (1 part in 12 to 20 parts of water), and a zinc rod B.
Inside the jar is placed a porous pot C containing a strip of thin sheet
copper D, and a saturated solution of sulphate of copper (also called
“blue stone” and “blue vitrol”).
The zinc is preferably of the Leclanche form, which will be found to be
cleaner, more durable, and cheaper than a zinc sheet. The porous pot
should be dipped in melted paraffin wax, both top and bottom, to prevent
the solution mingling too freely and “creeping.” A few crystals of copper
sulphate are placed in the pot as shown.
[Illustration: FIG. 52.--Simple Daniell cell for closed circuit work. To
maintain a constant current for an indefinite time, it is only necessary
to maintain the supply of copper crystals and zinc. The cell as shown in
the figure is easily made by following the direction given in the
accompanying text.]
In mixing the sulphuric acid and water, _the acid should be added to the
water--never the reverse._ Zinc sulphate is sometimes used instead, as it
reduces the wasteful consumption of the zinc, but it should be pure.
With care the cell will last for weeks. When it weakens or “runs down,” an
addition of sulphuric acid to the outer jar and a few more crystals
placed in the porous pot will put the cell in good condition.
[Illustration: FIG. 53.--Daniell gravity cell, “crowfoot” pattern. This is
a two fluid cell in which gravity instead of a porous cup is depended upon
to keep the liquids separate. The two solutions consist of copper sulphate
and dilute sulphuric acid, the elements being made of zinc and copper.]
=Gravity Cells.=--In a two liquid cell, instead of employing a porous cell
to keep the two liquids separate, it is possible, where one of the liquids
is heavier than the other, to arrange that the heavier liquid shall form a
stratum at the bottom of the cell, the lighter floating upon it. Such
arrangements are called _gravity cells_; but the separation is never
perfect, the heavy liquid slowly diffusing upwards.
=Daniell Gravity Cell.=--In this cell, shown in fig. 53, the same elements
are used as in the ordinary Daniell cell, but the porous pot is dispensed
with, the two solutions being separated by the action of gravity as
explained in the preceding paragraph.
[Illustration: FIG. 54.--Partz acid gravity cell. In this form of cell,
the electrolyte which surrounds the zinc is either magnesium sulphate or
common salt. The depolarizer is a bichromate solution which surrounds the
perforated carbon plate located in the bottom of the jar. A vertical
carbon rod fits snugly into the tapered hole in the carbon plate, and
extends through the cover forming the positive pole. The depolarizer,
being heavier than the electrolyte, remains at the bottom of the jar, and
the two liquids are thus kept separate. This depolarizer is placed on the
market in the form of crystals, known as sulpho-chromic salt, made by the
action of sulphuric acid upon chromic acid. When dissolved, its action is
similar to that of the chromic acid solution. After the cell has been set
up with everything else in place, the crystals are introduced into the
solution, near the bottom of the jar, through the vertical glass tube
shown, and slowly dissolve and diffuse over the surface of the carbon
plate. When the cell current weakens a few tablespoonfuls of the salt
introduced through the tube will restore the current to its normal value.
The cell should remain undisturbed to prevent the solution from mixing.
Its electromotive force is from 1.9 to 2 volts, and the 6 in. × 8 in. size
has an internal resistance of about .5 ohm. Since the depolarizer is quite
effective, the cell may be used on open or closed circuit work.]
The copper sulphate solution, being the heavier of the two, rests at the
bottom of the battery jar, while the dilute sulphuric acid remains at the
top. To suit this arrangement the copper and zinc elements are located as
shown, the copper elements being at the bottom, and the zinc element,
shaped like a crow’s foot (hence the name “crowfoot cell”) is suspended at
the top.
The absence of the porous pot decreases the internal resistance, but the
electromotive force is the same as in the ordinary type of Daniell cell.
[Illustration: FIG. 55.--Wheelock cell; the elements are carbon and zinc.
To set up, place the grid in the bottom of the jar and fill its two
troughs each about half full of mercury. Place the porous cell in position
on the grid so that it sits perfectly upright, resting in the recess of
the latter. The zincs stand with lower ends resting in mercury in the
troughs of the grid. Into the porous cell, to a height of _only
two-thirds_ full, pour solution consisting of equal parts water and
sulphuric acid, by measure. Add to this 1/2 pound nitrate soda, 1 ounce
chromic acid. This solution may be made up in the above proportion and
kept in covered receptacle in any desired quantity, ready for use. In the
outer jar for 6 × 8 size, 2-1/2 pints of water, and 1/2 gill sulphuric
acid, 1 part sulphuric acid to 20 parts water, or as much sulphuric acid
as it will take without boiling. When a charge becomes exhausted it may be
renewed by adding sulphuric acid and salts in the proportions given above,
after drawing out with syringe enough of the old solutions to make room
for the additions, but the best action is obtained with entirely new
solutions. Zincs must be kept thoroughly amalgamated by keeping a good
supply of mercury in the troughs.]
When a current is produced by a Daniell cell:
1. Copper is deposited on the copper plate;
2. Copper sulphate is consumed;
3. The sulphuric acid remains unchanged in quantity;
4. Zinc sulphate is formed;
5. Zinc is consumed.
If, however, the copper sulphate solution be too weak, the water
is decomposed instead of the copper sulphate, and hydrogen is
deposited on the copper plate. This deposit of hydrogen lowers
the voltage, hence care should be taken to maintain an adequate
supply of copper sulphate.
The voltage of a Daniell cell varies from about 1.07 volt to 1.14 volt,
according to the density of the copper sulphate solution and the amount of
zinc sulphate present in the dilute sulphuric acid.
=“Dry” Cells.=--It is often necessary to use cells in places where there
is considerable jarring or motion, as for automobile or marine ignition.
The ordinary cell is not well adapted to this service on account of the
liability of spilling the electrolyte, hence, the introduction of the
so-called dry cell.
A dry cell is composed of two elements, usually zinc and carbon, and a
liquid electrolyte. A zinc cup closed at the bottom and open at the top
forms the negative electrode; this is lined with several layers of
blotting paper or other absorbing material.
The positive electrode consists of a carbon rod placed in the center of
the cup; the space between is filled with carbon--ground coke and dioxide
of manganese mixed with an absorbent material. This filling is moistened
with a liquid, generally sal-ammoniac. The top of the cell is closed with
pitch to prevent leakage and evaporation. A binding post for holding the
wire connections is attached to each electrode and each cell is placed in
a paper box to protect the zincs of adjacent cells from coming into
contact with each other when finally connected together to form a
battery.
=Points Relating to Dry Cells.=--The following instructions on the care
and operation of dry cells should be carefully noted and followed to get
the best results:
[Illustration: FIGS. 56 and 57.--Round and rectangular types of the
so-called “dry” cell.]
1. In renewing dry cells (or any other kind of cell), a greater
number should never be put in series than was originally
required to do the work, because the additional cells increase
the voltage beyond that required, which causes more current than
is necessary to flow through the coil. This increased current
flow shortens the life of the battery.
2. In connecting dry cells in places where there is vibration,
heavy copper wire should not be used, because vibration will
cause it to break.
3. Water should not be allowed to come in contact with the paper
covers of the cells because they form the insulation, hence,
when moist, current will leak across from one cell to another,
resulting in running down the battery.
4. Dry cells will deteriorate when not in use, making it
necessary to renew them about every sixty days. The reason dry
cells deteriorate is because the moisture evaporates. Freezing,
exposure to heat, and vibration which loosens the sealing,
causes the evaporation.
5. Weak cells can be strengthened somewhat by removing the paper
jacket, punching the metal cup full of small holes, and then
placing in a weak solution of sal-ammoniac, allowing the cells
to absorb all they will take up. This is only to be recommended
in cases of emergency when they are hard to get.
[Illustration: FIGS. 58 to 63.--Various zincs; fig. 58 Fuller; fig. 59
Daniell; fig. 60 Leclanche square; fig. 61 Leclanche round; fig. 62
Sampson; fig. 63, bottle.]
6. The average voltage of a dry cell when new is one and
one-half volts, while the amperage ranges from about twenty-five
to fifty amperes according to size.
7. A dry cell when fresh should show from =20= to =25= amperes
when tested; the date of manufacture should also be noted as
fresh cells are most efficient.
8. Dry cells should be tested with an ammeter, care being taken
to do it quickly as the ammeter being of a very low resistance
short circuits the cell. A volt meter is not used in testing
because, while the cells are not giving out current, their
voltage remains practically the same, and a cell that is very
weak will show nearly full voltage. When no ammeter is at hand,
the battery current may be tested by disconnecting the end of
one of the terminal wires and snapping it across the binding
post of the other terminal; the intensity of the spark produced
will indicate the condition of the battery.
=Points Relating to the Care of Cells.=--To get the best results from
primary cells, they should receive proper attention and be maintained in
good condition. The instructions here given should be carefully followed.
[Illustration: FIGS. 64 to 66: Various carbons; fig. 61 Cylindrical form;
fig. 65 Calland star; fig. 66, wheel.]
=Cleanliness.=--In the care of batteries, cleanliness is essential in
order to secure best results. Zincs and coppers should be thoroughly
cleaned every time a cell is taken out of use. The zinc, after being
thoroughly cleaned, should be rubbed with a little mercury. This prevents
local action. Porous cups should be soaked in clean water four or five
hours and then wiped dry.
The terminals of each cell should be thoroughly cleansed and scraped
bright so as to get good contact of the connecting wires and thus avoid
extra resistance in the circuit.
=Separating the Elements.=--Obviously the positive and negative elements
of a cell must not be in contact within the exciting fluid; they should be
separated by a space of 3/8 to 1/2 inch. In the case of cells without
porous cups, periodic attention must be given to ensure this condition
being maintained.
[Illustration: FIGS. 67 to 69.--Various zincs: fig. 67 Crowfoot; fig. 68
Lockwood; fig. 69 fire alarm.]
=Creeping.=--As evaporation of the electrolyte takes place in a cell, it
increases in strength, and crystals are left on the sides of the jar
previously wetted by the solution, the action being very marked when the
solution is a saturated one. The space between these crystals and the side
of the jar acts as a number of capillary tubes, and draws up more liquid,
which itself evaporates and deposits crystals above the former ones. So
that finally the film of crystals passes over the edge of the jar and
forms on the outside, thus making a kind of syphon which draws off the
liquid. This action may, to a great extent, be prevented by warming the
edges of the glass, or stoneware, jars, and of the porous pots, before the
cells are made up, and dipping them while warm into some paraffin wax
melted in warm oil, a precaution that should always be carried out when a
dense solution of zinc sulphate is employed in the cell.
=Amalgamated Zinc.=--To “amalgamate” a piece of zinc, dip it into dilute
sulphuric acid to clean its surface, then rub a little mercury over it by
means of a piece of rag tied on to the end of a stick, and lastly, leave
the zinc standing for a short time in a dish to catch the surplus mercury
as it drains off.
[Illustration: FIGS. 70 and 71.--Two forms of copper element: fig. 70,
regular form for crowfoot cell; fig. 71, signal pan bottom copper.]
The action of the amalgamated zinc is not well understood; by some it is
considered that amalgamating the zinc prevents local currents by the
amalgam _mechanically_ covering up the impurities on the surface of the
zinc and preventing their coming into contact with the liquid. By others
it is thought that amalgamating the zinc protects it from local action by
causing a film of hydrogen gas to adhere to it. This theory is based on
the fact that while no action takes place when amalgamated zinc is placed
in dilute sulphuric acid at ordinary atmospheric pressure, the creation of
a vacuum above the liquid causes a rapid evolution of hydrogen, which,
however, stops on the readmission of the air.
Amalgamating a zinc causes it to act as a somewhat more positive substance
than before, therefore the voltage of a cell containing amalgamated zinc
is slightly higher than that of a cell constructed with unamalgamated
zinc.
[Illustration: FIG. 72.--Diagram of a series battery connection: four
cells are shown connected by this method. If the cell voltage be one and
one-half volts, the pressure between the (+) and (-) terminals of the
_battery_ is equal to the product of the voltage of a single cell
multiplied by the number of cells. For four cells it is equal to six
volts.]
The addition of a very small amount of zinc to mercury causes the mercury
to act as if it were zinc alone, arising perhaps from the amalgam having
the effect of bringing the zinc to the surface.
=Battery Connections.=--There are three methods of connecting cells to
form a battery; they may be connected:
1. In series;
2. In parallel;
3. In series multiple.
A series connection consists in joining the positive pole of one cell to
the negative pole of the other, as shown in fig. 72; this adds the voltage
of each cell.
Thus, connecting in series four cells of one and one-half volts
each will give a total of six volts.
Fig. 73 illustrates a parallel or multiple connection; this is made by
connecting the positive terminal of one cell with the positive terminal of
another cell and the negative terminal of the first cell with the negative
terminal of the second cell.
[Illustration: FIG. 73.--Diagram of a multiple or parallel connection.
When connected in this manner the voltage of the battery is the same as
that of a single cell, but the current is equal to the amperage of a
single cell multiplied by the number of cells. Thus with 1-1/2 volt 15
ampere dry cells, the combination or battery connected as shown would give
4 × 15 = 60 amperes at a pressure of 1-1/2 volts.]
[Illustration: FIG. 74.--Diagram of a series multiple connection. Two sets
of cells are connected in series and the two batteries thus formed,
connected in parallel. The pressure equals the voltage of one cell,
multiplied by the number of cells in one battery, and the amperage, that
of one cell multiplied by the number of batteries. This form of connection
is objectionable unless all the cells be of equal strength. If old cells
be placed on one side and new cells on the other, current will flow (as in
fig. 75) from the stronger through the weaker until the pressure of all
the cells thus becomes equal. This process therefore wastes some of the
energy of the strong cells.]
_A paralleled or multiple connection adds the amperage of each cell; that
is, the amperage of the battery will equal the sum of the amperage of each
cell._
For instance, four cells of twenty-five amperes each would give
a total of one hundred amperes when connected in parallel.
A series multiple connection, fig. 74, consists of two series sets of
cells connected in parallel. In series multiple connections the voltage of
each set of cells or battery must be equal, or the batteries will be
weakened, hence each battery of a series multiple connection should
contain the same number of cells.
_The voltage of a series multiple connection is equal to the voltage of
one cell multiplied by the number of cells in one battery, and the
amperage is equal to the amperage of one cell multiplied by the number of
batteries._
[Illustration: FIG. 75.--Diagram to illustrate incorrect wiring. The
current pressure of the six cell battery being greater than that of the
smaller unit, current will flow from the former through the latter until
the pressure of the six cells is equal to that of the four cells.]
Fig. 75 shows an incorrect method of wiring in series multiple connection.
If the circuit be open, the six cells, on account of having more
electromotive force than the four cells, will overpower them and cause a
current to flow in the direction indicated by the arrows until the
pressure of the six cells has dropped to that of the four. This will use
up the energy of the six cells, but will not weaken the four cell battery.
This action can be corrected by placing a two-way switch in the circuit at
the junction of the two negative terminals so that only one battery can be
used at a time.
CHAPTER V
CONDUCTORS AND INSULATORS
Bodies differ from each other in a striking manner in the freedom with
which the electric current moves upon them. If the electric current be
imparted to a certain portion of the surface of glass or wax, it will be
confined strictly to that portion of the surface which originally receives
it, by contact with the source of electricity; but if it be in like manner
imparted to a portion of the surface of a metallic body, it will
instantaneously diffuse itself uniformly over the entire extent of such
metallic surface, exactly as water would spread itself uniformly over a
level surface on which it is poured.[3]
Bodies in which the electric current moves freely are called _conductors_,
and those in which it does not move freely are called _insulators_. There
is, however, no substance so good a conductor as to be devoid of
resistance, and no substance of such high resistance as to be a
_non-conductor_.
_Mention should be made here of the misuse of the word non-conductor; the
so-called “non-conductors” are properly termed insulators._
The bodies named in the following series possess conducting power in
different degrees in the order in which they stand, the most efficient
conductor being first, and the most efficient insulator being last in the
list.
=TABLE OF CONDUCTORS AND INSULATORS=
{ Silver
{ Copper
{ Aluminum
{ Zinc
{ Brass (according to composition)
{ Platinum
{ Iron
Good conductors[4] { Nickel
(metals and alloys) { Tin
{ Lead
{ German silver (copper 2 parts, zinc 1,
nickel 1)
{ Platinoid (German silver 49 parts, tungsten 1
part)
{ Antimony
{ Mercury
{ Bismuth.
{ Charcoal and coke
{ Carbon
{ Plumbago
{ Acid solutions
Fair conductors { Sea water
{ Saline solutions
{ Metallic ores
{ Living vegetable substances
{ Moist earth.
{ Water
{ The body
{ Flame
{ Linen
{ Cotton
Partial conductors { Mahogany }
{ Pine } Dry woods
{ Rosewood }
{ Lignum Vitæ }
{ Teak
{ Marble.
{ Slate
{ Oils
{ Porcelain
{ Dry leather
{ Dry paper
{ Wool
{ Silk
{ Sealing wax
{ Sulphur
Insulators, or { Resin
so-called { Gutta-percha
non-conductors. { Shellac
{ Ebonite
{ Mica
{ Jet
{ Amber
{ Paraffin wax
{ Glass (varies with quality)
{ _Dry_ air.
[Illustration: FIGS. 76 to 78.--Various covered wires. fig. 76, single;
fig. 77, duplex; fig. 78, automobile high tension cable.]
The earth is a good conductor; much difficulty is frequently experienced
by the wires making contact with some substance that will conduct the
electricity to the earth. This is called “grounding.”
[Illustration: FIGS. 79 to 81.--Standard porcelain insulators. Fig. 79,
tube type; figs. 80 and 81, grooved insulators.]
=Mode of Transmission.=--The exact nature of electricity is not known, yet
the laws governing its action, under various conditions are well
understood, just as the laws of gravitation are known, although the
constitution of gravity cannot be defined. Electricity, though not a
substance, can be associated with matter, and its transmission requires
energy. While it is neither a gas nor a liquid, its behavior sometimes is
similar to that of a fluid so that it is said to “flow” through a
conductor. This expression of flowing does not really mean that there is
an actual movement in the wire, similar to the flow of water in a pipe,
but is a convenient expression for the phenomena involved.
=Effect of Heat.=--The conducting power of bodies is affected in different
ways by their temperature. In the metals it is diminished by elevation of
temperature; but in all other bodies, and especially in liquids, it is
augmented. Some substances which are insulators in the solid state, become
conductors when fused.
Sir H. Davy found that glass raised to a red heat became a
conductor; and that sealing wax, pitch, amber, shellac, sulphur,
and wax, became conductors when liquefied by heat.
=Heating Effect of the Current.=--If a current of electricity pass over a
conductor, no change in the heat condition of the conductor will be
observed as long as its transverse section is so considerable as to leave
sufficient space for the free passage of the current. But, if this
thickness be diminished, or the quantity of electricity passing over it be
augmented, or, in general, if the ratio of the electricity to the
magnitude of the space afforded to it be increased, the conductor will be
found to undergo an elevation of temperature, which will be greater, the
greater the quantity of the electricity and the less the space supplied
for its passage.
[Illustration: FIG. 82.--Standard two wire porcelain cleat.]
These heat effects are manifested in different degrees in different
metals, according to their varying conducting powers.
The poorest conductors, such as platinum and iron, suffer much greater
changes of temperature by the same charge than the best conductors, such
as gold and copper.
The charge of electricity, which only elevates the temperature of one
conductor a small amount, will sometimes render another incandescent, and
will vaporize a third.
=Insulators=.--The term insulator is used in two ways: 1, as in insulating
substance or medium, and 2, as a specially formed piece of some insulating
material, such as glass, porcelain, etc. No substance has the power of
absolutely preventing the passage of electric currents between conductors
but many have sufficient insulating power for practical purposes. The
properties to be desired in a good insulating material are:
1. Permanence;
2. High power of resisting breakdown;
3. Mechanical strength;
4. Fairly high dielectric or insulation resistance;
5. Special qualities for the use to which the material is to be put.
Permanence is the most important quality, and is the one least easily
attained. The power of resisting breakdown is a complex quality, for it is
not solely dependent on mere puncturing pressure, but also on mechanical
goodness, and to a certain extent on the insulation resistance. It cannot
be easily determined by a simple laboratory test, but must be found by
experience of actual service conditions.
=Impregnating Compounds.=--These are used for the treatment of fibrous
materials. They increase the insulating properties of the fibrous
materials, render them moisture proof and able to withstand the effect of
heat with less rapid deterioration.
When wires or cables are to be used under water, they must be made
impervious, and great care must be taken to prevent the water penetrating
and thus injuring the insulation.
=Water as a Conductor.=--Water, whether in the liquid or vaporous form, is
a conductor, though of an order greatly inferior to the metals. This fact
is of great importance in electrical phenomena. The atmosphere contains,
suspended in it, always more or less aqueous vapor, the presence of which
impairs its insulating property.
The best insulators become less efficient if their surface be moist, the
electricity passing by the conducting power of the moisture. This
circumstance also shows why it is necessary to dry previously the bodies
on which it is desired to develop electricity by friction.
CHAPTER VI
RESISTANCE AND CONDUCTIVITY
_Resistance is that property of a substance that opposes the flow of an
electric current through it._
The practical electrician has to measure electrical resistance,
electromotive forces, and the capacities of condensers. Each of these
several quantities is measured by comparison with ascertained standards,
the particular methods of comparison varying, however, to meet the
circumstances of the case.
Ohm’s law states that the strength of a current due to an electromotive
force falls off in proportion as the resistance in the circuit increases.
It is therefore possible to compare two resistances with one another by
finding out in what proportion each will cause the current of a constant
battery to fall off.[5]
_Silver is taken as the standard, with the percentage of 100, and the
conductivity of all other metals is expressed in hundredths of the
conductivity of silver._
=Conductivity of Metals and Liquids.=--The metals in general, conduct
well, hence their resistance is small, but metal wires must not be too
thin or too long, or they will resist too much, and permit only a feeble
current to pass through them. The liquids in the battery do not conduct
nearly so well as the metals, and different liquids have different
resistances. Pure water will hardly conduct at all, unless the voltage be
very high.
Salt and saltpetre dissolved in water are good conductors, and so are
dilute acids, though strong sulphuric acid is a bad conductor. Gases are
bad conductors.
=Effect of Heat.=--Another very important fact concerning the resistance
of conductors is that the resistance in general increases with the
temperature. While this fact is true regarding metals, it does not apply
to non-metals. The resistance of different metals does not increase in the
same proportion. Iron at 100 degrees C, has lost 39 per cent. of the
conducting power it possessed at zero, while silver loses but 23 per cent.
=Laws of Electrical Resistance.=--Resistances in a circuit may be of two
kinds:
1. Resistance of the conductors;
2. Resistance due to imperfect contact.
The latter kind of resistance is affected by pressure, for when
the surfaces of two conductors are brought into more intimate
contact the current passes more freely from one conductor to the
other.
The following are the laws of the resistance of conductors:
1. _The resistance of a conducting wire is proportional to its length._
If the resistance of a mile of telegraph wire be 13 ohms, that
of fifty miles will be 50 × 13 = 650 ohms.
2. _The resistance of a conducting wire is inversely proportional to the
area of its cross section_, and therefore in the usual round wires _is
inversely proportional to the square of its diameter._
Ordinary telegraph wire is about 1/6th of an inch thick; a wire
twice as thick would conduct four times as well, having four
times the area of cross section; hence an equal length of it
would have only 1/4th the resistance.
3. _The resistance of a conducting wire of given length and thickness
depends upon_ the material of which it is made--that is, upon the specific
resistance _of the material_.
=Conductivity.=--This is _the inverse of resistance_. The term expresses
the capability of a substance to conduct the electric current.
If the symbol Y represent the conductivity of a substance, and I the
current then:
I/Y = its resistance;
and if R represent the resistance of a substance, then
I/R = its conductivity.
Good conductors of heat are also good conductors of electricity.
=Specific Conductivity.=--The figure which indicates the relation between
one substance and another as to their capacity to conduct electricity is
called _specific_ or _relative conductivity_. Taking the specific
conductivity of silver as 100, that of pure copper is 96.
The specific resistance of a substance is the reverse of its relative
conductivity. The specific resistance of a metal is generally expressed in
millionths of an ohm as the resistance of a centimeter cube of that metal
between opposite sides.
The following table gives the data for a few metals:
Specific Resistance Specific
Substance. in Microhms. Conductivity.
Silver 1.609 100.
Copper 1.642 96.
Gold 2.154 74.
Iron (soft) 9.827 16.
Lead 19.847 8.
German Silver 21.470 7.5
Mercury (liquid) 96.146 1.6
The specific resistance of copper is therefore:
1.642 / 1,000,000 ohms, or 1.642 microhms.[6]
=Divided Circuits.=--If a circuit be divided, as in fig. 83, into two
branches at A, uniting again at B, the current will also be divided, part
flowing through one branch and part through the other.
_The relative strength of current in the two branches will be proportional
to their conductivities._
This law will hold good for any number of branch resistances connected
between A and B. Conductivity is, as shown before, the reciprocal of
resistance.
EXAMPLE--If, in fig 83, the resistance of R = 10 ohms, and R′ =
20 ohms, the current through R will be to the current through R′
as 1/10 to 1/20; or, as 2:1, or, in other words, 2/3 of the
total current will pass through R and 1/3 through R′. The joint
resistance of the two branches between A and B will be less than
the resistance of either branch singly, because the current has
increased facilities for travel. In fact, the joint conductivity
will be the sum of the two separate conductivities.
Taking again the resistance of R = 10 ohms and R′ = 20 ohms, the
joint conductivity is
1/10 + 1/20 = 3/20
and the joint resistance is equal to the reciprocal[7] of 3/20
or 6-2/3
[Illustration: FIG. 83.--Divided circuit with two conductors _in
parallel_.]
In most cases the resistance of the different branches will be alike. This
simplifies the calculations considerably. Take, for instance, two branches
of 100 ohms resistance each and find the joint resistance.
SOLUTION: 1/100 + 1/100 = 2/100; the reciprocal is 100/2 = 50
ohms, or, in other words, the joint resistance is one-half of
the resistance of a single branch, and each branch, of course,
will carry one-half of the total current in amperes.
With three branches of equal resistance, the joint resistance
will be 1/3; with four branches 1/4; with 100 branches 1/100 of
the resistance of a single branch.
[Illustration: FIG. 84.--Hydraulic analogy for divided circuits. In the
system of pipes shown, water flows from A B to C D through the six
vertical pipes 1 to 6, the greatest amount going through the one which
offers the least resistance. If pipes 1 to 6 all have the same dimensions,
equal quantities of water will flow through them. It follows that the
resistance which the water encounters diminishes with the increase in the
number of pipes between A B and C D. The electrical circuit presents the
same conditions: the greater the number of parallel connections
(corresponding to the pipes 1 to 6) the less is the resistance encountered
by the current.]
If, for instance, the resistance of an incandescent lamp hot be 180 ohms,
the joint resistance of 100 such lamps connected in multiple is
180/100 = 1.8 ohms.
If the electromotive force of the system is to be, say 110 volts, then,
according to Ohm’s law, the current for 100 lamps is:
110/1.8 = 61.11 amperes.
giving for each lamp a current of
110/180 = .61 ampere.
In the case of two branches only, the following rule may be applied also:
_Multiply the two resistances and divide the product by their sum._
Written as a formula:
Joint resistance = (R × R′)/(R + R′)
Again, assuming that R = 10 ohms and R′ = 20 ohms:
Joint resistance (10 × 20)/(10 + 20) = 200/30 = 6-2/3 ohms.
This rule _cannot_ be employed for more than two branches at a time.
EXAMPLE--A current of 42 amperes flows through three conductors
in _parallel_ of 5, 10 and 20 ohms resistance respectively. Find
the current in each conductor.
SOLUTION: Joint Conductance = 1/5 + 1/10 + 1/20 = 7/20.
Supposing the current to be divided into 7 parts, 4 of these
parts would flow in the first conductor 2 in the second and 1 in
the third.
The whole current is 42 amperes.
4/7 of 42 = 24.
2/7 of 42 = 12.
1/7 of 42 = 6.
Current in first conductor = 24 amperes. }
“ “ second “ = 12 “ } Ans.
“ “ third “ = 6 “ }
CHAPTER VII
ELECTRICAL AND MECHANICAL ENERGY
The production of electricity is simply a transformation of energy from
one form into another, usually mechanical energy is changed into
electrical energy and a dynamo is simply a device for effecting the
transformation.
Prof. Fessenden truly remarks there are two independent
properties of matter--gravity and inertia--and these give two
ways of defining force and energy.
It should always be remembered that electricity is something real,
although not easily defined. And then, too, while it is not matter and not
energy, yet under proper conditions (it having the power of doing work) it
is convenient to speak of its performances as electric energy. The
following questions and answers, although few in number, may present the
subject with clearness.
=Ques. What is energy?=
Ans. Energy is the capacity for doing work.
Steam under pressure is an example, a spring bent ready to be
released is another form, again, water stored in an elevated
tank has capacity for doing work. These examples illustrate
_potential energy_, as distinguished from _kinetic energy_.
Potential energy may be defined as _energy due to position_, and
kinetic energy, as _energy due to momentum_.
=Ques. What is matter?=
Ans. Matter is anything occupying space, and which prevents other matter
occupying the same space at the same time.
=Ques. What name is given the smallest quantity of matter which can
exist?=
Ans. The atom.
An atom means that which cannot be cut, scratched, or changed
in form and that cannot be affected by heat or cold or any known
force; although inconceivably small, atoms possess a definite
size and mass.
=Ques. What is a molecule?=
Ans. A molecule is composed of two or more atoms.
=Ques. What is the behaviour of these minute bodies?=
Ans. They are perpetually in motion, vibrating with incredible velocities.
=Ques. Why at this point are definitions of energy and of matter most
useful?=
Ans. Because, as stated, all electric action is an exhibition of energy,
and energy must act through matter as its medium.
=Ques. What is the difference between electricity and magnetism?=[8]
Ans. The ultimate nature of neither is known. There are, however, some
differences. To sustain a current of electricity requires energy. To
sustain magnetism requires no energy. A current of electricity is always
accompanied by a magnetic field of peculiar form. Magnetism alone cannot
produce electricity. Electricity can do work; but magnetism cannot in the
same sense--and alike with electricity, neither can it exist without
contact with matter.
=Ques. How is energy transmitted from one part of a material substance to
another?=
Ans. Gradually and successively. It requires a medium and also time.
=Ques. What is the principal use or function in mechanics of electricity?=
Ans. It is purely that of transmission. It corresponds to ropes, shafts
and fluids as a medium of conveying and translating power or work.
=Ques. What is work?=
Ans. Work is the overcoming of resistance through a certain distance.
As a quantity of water moving from a higher to a lower level
will do work, so also will a quantity of electricity falling
through a difference of potential.
=Ques. How is work measured?=
Ans. In foot pounds.
=Ques. What is a foot pound?=
Ans. The amount of work done in raising a weight of one pound one foot or
the equivalent, overcoming a pressure of one pound through a distance of
one foot.
=Ques. What is the electrical unit of work?=
Ans. The _volt-coulomb_.
A volt-coulomb of work is performed when one ampere of current
flows for one second in a circuit whose resistance is one ohm,
when the pressure is one volt.
=The Ampere-Hour.=--A gallon of water may be drawn from a hydrant in a
minute, or in an hour; it is still one gallon. So in electricity, a given
amount of the current, say one _coulomb_, may be obtained in a second or
in an hour.
_The ampere is the unit rate of flow._
What is called the electric current is simply the relation of any quantity
of electricity passed to the time it is passing; that is
quantity in coulombs = current in amperes × time in seconds, or
simply
coulomb = ampere × second.
Again:
10 coulombs = 2 amperes × 5 seconds = 10 amperes × 1 second =
1 ampere × 10 seconds, etc.
One _ampere-hour_ is simply another way of saying 3,600 coulombs. Of
course 3,600 coulombs of electricity may be obtained in any desired time.
It all depends on the rate of flow or the current strength in amperes.
For instance, 2 amperes in 1/2 hour, or 4 amperes in 1/4 hour
will also give one ampere-hour of 3,600 coulombs.
It is well to keep the distinction between coulombs and amperes in mind,
as even in text books very lately published these units are confounded. To
illustrate further the difference between coulombs and amperes, the
following example is given.
It is sometimes estimated that the quantity of electricity in a
flash of lightning is 1/10 coulomb, and the duration of the
discharge 1/20000 part of a second. What is the current in
amperes?
Now since
coulombs = amperes × seconds (1)
solving (1) for the current,
amperes = coulombs/seconds (2)
substituting the given values in (2),
amperes= (1/10) / (1/20000) = 2000
=Power.=--The term power means _the rate at which work is done;_ it is
usually expressed as _the number of foot pounds done in one minute_, that
is
power = (foot pounds) / minutes
_Power exerted for a certain time produces work._
=Ques. What is the mechanical unit of power?=
Ans. The horse power.
=Ques. What is one horse power?=
Ans. 33,000 foot pounds per minute.
The unit is due to James Watt as being the power of a strong
London draught horse to do work during a short interval and used
by him to measure the power of his steam engines. One horse
power = 33,000 ft. lbs. per minute = 550 ft. lbs. per sec. =
1,980,000 ft. lbs. per hour.
=Ques. What is one horse power hour?=
Ans. Work done at the rate of one horse power for one hour.
=Ques. What is the electrical unit of power?=
Ans. The watt.
=Ques. What is a watt?=
Ans. It is the power due to a current of one ampere flowing at a pressure
of one volt. One watt = one ampere × one volt. It is equal to one joule
per second.
=Ques. What is a kilowatt?=
Ans. 1,000 watts.
=The Watt-Hour.=--The elements which may be measured are, however, not
only the volume of current, the unit of which is the ampere, and time, the
unit of which is the hour, but also the _pressure_, the unit of which is
the volt.
It is evident that a perfect system of electrical measurement should take
account of the total amount of energy consumed, and should depend not only
upon the volume of current, but _also upon the pressure_ at which the
current is applied.
The basis of such a system if provided in a unit which is the product of
the two units of current and pressure, and which is termed a _volt-ampere_
or _watt_.
_The watt-hour represents the amount of work done by an electric current
of one ampere strength flowing for one hour under a pressure of one
volt._
EXAMPLE--An incandescent lamp taking one-half an ampere of
current on a circuit having a pressure of 100 volts, or a lamp
taking one ampere on a circuit having a pressure of 50 volts,
would each be consuming 50 watts of energy, and this multiplied
by the number of hours would give the total number of watt-hours
for any definite time.
_The watt, then, is an accurate and complete unit of measurement and is
generally applicable to all forms of electrical consumption._
[Illustration: FIG. 85.--Tyndall’s experiment illustrating the production
of heat by friction. A brass tube about 7 inches in length and 3/4 of an
inch in diameter, is fixed on a small wheel. By means of a cord passing
round a much larger wheel, this tube can be rotated with any desired
velocity. The tube is three parts full of water, and is closed by a cork.
In making the experiment, the tube is pressed between a wooden clamp,
while the wheel is rotated with some rapidity. The water rapidly becomes
heated by the friction, and its temperature soon exceeds the boiling
point. The pressure caused by the formation of steam forces out the cork
and projects it to a height of several yards.]
A watt of electrical energy corresponds to 1/746 of a horse
power of mechanical energy; hence, if a lamp or motor require
energy equivalent to 1/746 of a horse power for one hour, it
might be said to take one watt-hour.
=Mechanical Equivalent of Heat.=--The eminent English physicist, James
Prescott Joule, worked for more than forty years in establishing the
relation between _heat_ and _mechanical work_; he stated the doctrine of
the conservation of energy and discovered the law, known as Joule’s law,
for determining the relation between the heat, current pressure, and time
in an electric circuit.
=Ques. What is heat?=
Ans. A form of energy.
Heat is produced in the agitation of the molecules of
matter--the energy expended in agitating these molecules is
transformed into heat.
[Illustration: FIG. 86.--Joules’ experiment on the mechanical equivalent
of heat, in which he caused paddle-wheels to rotate in a vessel of water
by means of falling weights _W_. The amount of work done by gravity upon
the weights in causing them to descend through any distance _d_ was equal
to their weight _W_ times the distance. If the weights descended slowly
and uniformly, this work was all expended in overcoming the resistance of
the water to the motion of the paddle-wheels through it; that is, it was
wasted in eddy currents in the water. Joule measured the rise in the
temperature of the water and found that the mean of his three best trials
gave 427 gram meters as the amount of work required to develop enough heat
to raise a gram of water one degree. He then repeated the experiment,
substituting mercury for water, and obtained 425 gram meters as the work
necessary to produce a calorie of heat. The difference between these
numbers is less than was to have been expected from the unavoidable errors
in the observations. He then devised an arrangement in which the heat was
developed by the friction of iron on iron, and again obtained 425. This
corresponds to 772 foot pounds, but later experiments show that the
correct value is 778 foot pounds.]
=Ques. How is heat measured?=
Ans. In British thermal units (B.t.u.).
=Ques. What is the British thermal unit?=
Ans. The quantity of heat required to raise the temperature of 1 lb. of
pure water 1° Fahr., at or near 39.1° F., the temperature of maximum
density.
=Ques. What is the mechanical equivalent of heat?=
Ans. The number of foot pounds of mechanical energy equivalent to one
British thermal unit.
Joule’s experiments 1843-50 gave the figure 772 ft. lbs. which
is known as Joule’s equivalent. Later experiments gave higher
figures, and _the present accepted value is 778 ft. lbs._, that
is: 1 B.t.u. = 778 ft. lbs.
=Electrical Horse Power.=--It is desirable to establish the relation
between _watts_ and _foot pounds_ in order to determine the _capacity_ of
an electric generator or motor in terms of _horse power._
One watt is equivalent to one joule per second or 60 joules per minute.
One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal:
60 × .7374 = 44.244 ft. lbs.
Since one horse power = 33000 ft. lbs. per minute, the electrical
equivalent of one horse power is
33000 ÷ 44.244 = 746 watts.
or,
746 / 1000 = .746 kilowatts (K.W.)
Again, one kilowatt or 1000 watts is equivalent to
1000 ÷ 746 = 1.34 horse power.
=The Farad.=--The measure constructed to hold a gallon of water may be
called the gallon measure. The capacity of a condenser which would contain
a charge of one coulomb under one volt pressure is the _farad._ It may
seem strange that there is a unit of quantity and another of capacity to
hold that quantity, when in the case of water the term “gallon” may
suffice for the measure and the liquid it can hold. Electricity in this
respect, however, corresponds to a _compressible fluid_ or a _gas_.
A gallon measure may hold a gallon of gas or ten; it depends entirely upon
the pressure. So a condenser of a certain size may hold any number of
coulombs, according to the electrical pressure.
The farad being inconveniently large for practical use, one-millionth of a
farad, called a _microfarad_, is generally adopted.
CHAPTER VIII
EFFECTS OF THE CURRENT
The term “electric current,” in the present state of our knowledge, should
be regarded as denoting the existence of a state of things in which
certain definite experimental effects are produced, for some of which
there certainly is no analogy exhibited in ordinary hydraulic currents.
The following are the most important of these effects:
1. Thermal effect;
2. Magnetic effect;
3. Chemical effect.
It is rather to these effects than to any imaginary current flow in the
conductor that the mind of the reader should be directed.
With this preliminary caution, which should never be lost sight of, the
use of familiar words and expressions connected with the flow of water in
pipes is justified in order to avoid roundabout and cumbrous phrases
which, though perhaps more nearly in accord with present knowledge of the
facts, would not tend to clearness or conciseness.
The three most important effects of the current just mentioned, may be
presented in more detail as follows:
1. The _Thermal effect:_--
The conductor along which the current flows becomes heated. The
rise of temperature may be small or great according to
circumstances, but some heat is always produced.
2. The _Magnetic effect;_
The space both outside and inside the substance of the
conductor, but more especially the former, becomes a “magnetic
field” in which delicately pivoted or suspended magnetic needles
will take up definite positions and magnetic materials will
become magnetized.
[Illustration: FIG. 87.--Lenz’s apparatus for measuring the heat given off
by an electric current. It consisted of a wide mouthed stoppered bottle
fixed upside down, with its stopper, _b_ in a wooden box; the stopper was
perforated so as to give passage to two thick platinum wires, connected at
one end with binding screws, _s_, while their free ends were provided with
platinum cones by which the wires under investigation could be readily
affixed; the vessel contained alcohol, the temperature of which was
indicated by a thermometer fitted in a cork inserted in a hole made in the
bottom of the vessel. The current is passed through the platinum wires,
and its strength measured by means of a galvanometer interposed in the
circuit. By observing the increase of temperature in the thermometer in a
given time and knowing the weight of the alcohol, the mass of the wire,
the specific heat, and the calorimetric values of the vessel, and of the
thermometer, compared with alcohol, the heating effect which is produced
by the current in a given time can be calculated.]
3. The _Chemical effect;_--
If the conductor be a liquid which is a chemical compound of a
certain class called _electrolytes_, the liquid will be
decomposed at the places where the current enters and leaves
it.
=Thermal Effect.=--If a quantity of electricity were set flowing in a
closed circuit and the latter offered no _resistance_, it would flow
forever, just as a wagon set rolling along a circular railway would never
stop if there were no _friction_.
[Illustration: FIG. 88.--The Seebeck effect: If in a complete metallic
circuit having junctions of dissimilar metals, the junctions are at
different temperatures, then generally a steady current will flow in the
circuit as long as the differences of the temperatures of the junction is
maintained. To demonstrate this, a piece of copper K bent in the shape
seen in the figure, was placed on a block of bismuth A B, carrying a
pivoted magnetic needle N S; as soon as the equality of temperatures was
altered by either heating or cooling one of the junctions of the two
metals, the needle indicated a current which continued to flow as long as
the difference of temperature was maintained at the junctions. The
movement of the needle indicated the direction in which the current
flowed. If, for instance, the north junction B were heated, the N pole
moved eastwards, showing that at the heated junction the current flows
from the bismuth to the copper, at the cold junction from the copper to
the bismuth.]
When matter in motion is stopped by friction, the energy of its motion is
converted into heat by the friction thus causing the matter to come to
rest. Similarly, when electricity in motion, that is, an electric current
is stopped by resistance, the energy of its flow is transformed into heat
by the resistance of the circuit.
If the terminals of a battery be joined by a short thick wire of low
resistance, most of the heat will be developed in the battery, whereas, if
a thin wire of high resistance be used it will become hot, while the
battery itself will remain comparatively cool.
To investigate the development of heat by a current, Joule and Lenz used
instruments on the principle of fig. 87, in which a thin wire joined to
two stout conductors is enclosed within a glass vessel containing alcohol,
into which is placed a thermometer. The resistance of the wire being
known, its relation to the other resistances can be calculated. Joule
found that the number of heat units developed in a conductor is
proportional to:
1. The resistance;
2. The square of the current strength;
3. The time that the current lasts.
Joules’ law may be stated as follows:
_The heat generated in a conductor by an electric current is proportional
to the resistance of the conductor, the time during which the current
flows, and the square of the strength of the current._
The quantity of heat in calories may be calculated by use of the
equation,
calories per second = volts × amperes × .24. (1)
The total number of calories H developed in t seconds will be
given by
H = P.D. × C × t × .24. (2)
EXAMPLE--If a current of 10 amperes flows in a wire whose
terminals are at a potential difference of 12 volts, how much
heat will be developed in 5 minutes?
Substituting in equation (2):
10 × 12 × (60 × 5) × .24 = 8640 calories.
Since by Ohm’s Law potential difference = I × R substituting IR
for P.D. in (2)
H = I^{2} R × t × .24
=Use of Heat from Electric Current.=--In the transmission of electricity
from place to place, it is very desirable that none of the energy be
expended in heating the conductor. Hence copper wires of the proper size
must be used.
In wiring a building for electric lights, the insurance rules require that
the wires be of a certain size and that they be put up in a certain
manner. Otherwise they will not insure a building against fire.
It is often desirable, however, to use the electric current for the
purpose of producing heat. The carbons of the arc and incandescent lamps
are intensely heated that they may produce light. Coils of German silver
wire or other high resistance wire are heated by the passage of a current
through them. In this manner the electric stove is made.
Soldering coppers, smoothing irons, and baking ovens are heated in a
similar manner.
=Magnetic Effect.=--An electric current flowing in a wire causes it to be
surrounded by a _magnetic field_, which consists of _lines of force_
encircling the wire. The field is strongest near the wire and diminishes
gradually in strength at increasing distances therefrom. The presence of
this magnetic field is shown by various experiments and the subject is
fully explained in chapter IX on _magnetism_.
=Chemical Effect.=--Pats van Trostwyk (1789) pointed out that an electric
discharge was capable of decomposing water; to show this he used gold
wires, which he allowed to dip in water, connecting one of them with the
inner, and another with the outer coating of a Leyden jar, and passing the
discharge through the water. The gas bubbles collected proved to consist
of oxygen and hydrogen gas.
Nicholson and Carlisle (1800) dipped a copper wire which was connected
with one of the poles of a voltaic pile into a drop of water, which
happened to be on the plate connected with the other pole; gas bubbles
appeared, and the drop of water became smaller and smaller.
This experiment was repeated in a somewhat different manner, the
brass wires from a pile being brought under a tube filled with
water and closed at the top. Gas bubbles were produced by the
wire in connection with the negative pole of the pile, and the
water was observed to diminish gradually. At the positive wire,
on the contrary, no gas came off, but the metal lost its
metallic lustre, became dark, and finally crumbled away. The gas
which had collected in the tube proved to be hydrogen; while on
examining the black mass it was found that the constituents of
brass, viz., copper and zinc, had become oxidized.
[Illustration: FIG. 89.--An electrolytic cell. The parts are: A, cell; B,
electrolyte; C, positive electrode or cathode; D, negative electrode or
cathode.[9]]
=Electrolysis.=--_Electric analysis_ or more briefly _electrolysis_ was
the term applied by Faraday to the process of decomposing a liquid by the
passage of a current of electricity through it.
The vessel containing the liquid is known as an _electrolytic cell._ In
fig. 89, A is the cell, which may be of glass or of any other suitable
material, and B is the liquid which is to be electrolyzed. Current enters
by the _positive electrode_ C, also known as the _anode_, traverses the
liquid, and leaves by the _negative electrode_, or _cathode_, D.
[Illustration: FIG. 90.--Modern apparatus for decomposing water by
electrolysis. Platinum electrodes P and P′ are placed at the bottom of two
upright tubes O and H, and are connected to the terminals T and T′ by
platinum wires, which are fused through the glass of the tubes. These
tubes have glass stop cocks S and S′ at their upper ends, and at their
lower ends are connected by a short glass tube, from the center of which
rises the large central tube which expands with a bulb at its upper end,
which is open at the top. The three tubes can be filled with acidulated
water from the central tube, the previously contained air being allowed to
escape through the stop cocks, which are afterwards closed. If it be so
filled, and the terminal T be attached to the positive and T′ to the
negative pole of a suitable battery, bubbles of gas will be observed to
rise from the plates P and P′, and finding their way to the top of the
respective tubes, will displace the liquid, which will be driven into the
open central tube. The gas rising from the anode P is oxygen (O), and that
rising from the cathode P′ is hydrogen (H). If the tubes are graduated,
the latter will be found to occupy about twice the volume of the former.
The proportion is theoretically 2 to 1; however, on account of the
different solubilities of the two gases in water, oxygen being the more
soluble of the two, is deficient in quantity.]
The passage of current through the water splits up its molecules into
their constituent atoms of oxygen and hydrogen, the former being given off
in bubbles at the anode, and the latter at the cathode.
When current is passed through a solution of copper sulphate between
platinum electrodes, the liquid is decomposed, atoms of copper being
deposited at the cathode, bubbles of oxygen being given off at the anode,
and sulphuric acid being formed in the liquid, which latter becomes more
and more acid as the copper is withdrawn.
[Illustration: FIG. 91.--Grotthuss’ theory of electrolysis. Grotthuss in
(1806), announced his theory that the molecules in an electrolyte have
their individual electro-positive and electro-negative atoms charged
positively and negatively respectively. In an ordinary liquid, for
instance in water, the molecules are arranged indifferently, like row 1,
with their positive and negative ends pointing in all directions. When the
charged plates A and B connected to the + and - poles of a battery are
inserted in the water, the molecules under the action of the laws of
electrostatic action turn as shown in row 2, so that all the hydrogen or
shaded ends (+) are turned towards the (-) plate B and all the oxygen or
unshaded ends (-) towards the (+) plate A. All along the row the
electrical forces are supposed to tear the molecules asunder, depositing H
on B and O on A. The atoms in the middle of the liquid, however,
recombine, for the hydrogen atoms in their journey towards B meet the
oxygen atoms travelling in the opposite direction, and we get the state of
affairs represented in row 3. The next step is to rotate once more the
atoms into the positions shown in row 2, and so on. In this way the theory
accounts for the products only appearing at the electrodes and not in the
body of the liquid.]
If, however, the anode be of copper instead of platinum, no sulphuric acid
will be formed, neither will oxygen be given off at the anode. As copper
is deposited at the cathode, an equal quantity will be dissolved at the
anode, so that the original constitution of the liquid is maintained.
The atoms separated from each other by the electric current were called
_ions_ by Faraday; those going to the anode being _anions_, and those
going to the cathode being _kathions_.
Anions are generally regarded as _electro-negative_, because they move as
if attracted to the positive electrode, while kathions are regarded as
_electro-positive_.
In order to explain the transfer of electricity and the transfer of matter
through the electrolyte, Grotthuss put forward the hypothesis that when
two metal plates at different potentials are placed in a cell, the effect
produced in the liquid is that the molecules of the liquid arrange
themselves in innumerable chains, as shown in fig. 91, in which every
molecule has its atoms pointing in a certain direction, the
electro-positive atom being attracted towards the cathode and the
electro-negative towards the anode. An interchange then takes place all
along the line, the free atoms appearing at the electrodes, and every atom
discharging a minute charge of electricity upon the electrode at which it
is liberated.
=Electro-chemical Series.=--This is an arrangement of the metals in a
series in such a manner that the most electro-positive is at one end and
the most electro-negative at the other.
The order of the metals varies with the electrolyte in which the metals
are tested.
The following table shows such series for the most common metals, in three
different solutions:
_Sulphuric acid._ _Hydrochloric acid._ _Caustic potash._
Zinc Zinc Zinc
Cadmium Cadmium Tin
Tin Tin Cadmium
Lead Lead Antimony
Iron Iron Lead
Nickel Copper Bismuth
Bismuth Bismuth Iron
Antimony Nickel Copper
Copper Silver Nickel
Silver Antimony Silver
Gold
Platinum
Faraday stated several laws of electrolysis, as follows:
1. _The quantity of an ion liberated in a given time is proportional to
the quantity of electricity that has passed through the voltameter[10] in
that time._
2. _The quantity of an ion liberated in a voltameter is proportional to
the electro-chemical equivalent of the ion._
3. _The quantity of an ion liberated is equal to the electro-chemical
equivalent of the ion multiplied by the total quantity of electricity that
has passed._
=Electric Osmose.=--Porret observed that if a strong current be led into
certain liquids as if to electrolyze them, a porous partition being placed
between the electrodes, the current mechanically carries part of the
liquid through the porous diaphragm, so that the liquid is forced to a
higher level on one side than on the other. This phenomenon is known as
_electric osmose_.
=Electric Distillation.=--Closely connected with the preceding phenomenon
is that of the _electric distillation of liquids_. It was noticed by
Beccaria that _an electrified liquid evaporates more rapidly than one not
electrified_.
Gernez has recently shown that in a bent closed tube, containing
two portions of liquid, one of which is made highly + and the
other highly -, the liquid passes over from + to -. This
apparent distillation is not due to difference of temperature,
nor does it depend on the extent of surface exposed, but is
effected by a slow creeping of the liquid along the interior
surface of the glass tubes. Bad conductors, such as turpentine,
do not thus pass over.
[Illustration: FIG. 92.--Effect of the electric current on a frog’s legs;
discovered in 1678 by Galvani.]
=Muscular Contractions.=--It was discovered in 1678 that when a portion of
muscle of a frog’s leg, hanging by a thread of nerve bound with a silver
wire, was held over a copper support so that both nerve and wire touched
the copper, the muscle immediately contracted.
More than a century later Galvani’s attention was drawn to the
subject by his observation of spasmodic contractions in the legs
of freshly killed frogs under the influence of the “return
shock” experienced every time a neighboring electric machine was
discharged.
The limbs of the frog, prepared as directed by Galvani, are
shown in fig. 92. After the animal has been killed the hind
limbs are detached and skinned; the crural nerves and their
attachments to the lumbar vertebrae remaining. For some hours
after death the limbs retain their contractile power. The frog’s
limbs thus prepared form an excessively delicate galvanoscope.
=Electroplating.=--This is the process of depositing a layer or coating of
a rarer metal upon the surface of a baser, or of a metal upon any
conducting surface, by electrolysis.
The electric current used may be obtained from a battery or other source.
The battery has its positive plate connected to a rod extending across a
trough or tank containing the plating bath.
Suspended from the rod are anodes of gold, silver, or copper or whatever
metal from which a deposit is desired. The other plates of the battery or
the negative elements, are connected with another rod across the trough,
to which are suspended the articles to be plated.
=Electrotyping.=--This is the process by which, type, wood cuts, etc., are
reproduced in copper by the process of electroplating. A mould is first
made of the set type in wax; this mould is next coated with black lead to
give it a metallic surface, as the wax is an insulator; the mould is then
subjected to the process of electro deposition, resulting in the formation
of a film of copper on the prepared surface.
The copper shell is removed from the mould by applying hot water; the
shell is then backed up with electrotype metal to render it strong enough
for use.
Almost all the illustrations in this book, for example, are printed from
electrotype copies, and not from the original wood blocks, which would not
wear so well.
CHAPTER IX
MAGNETISM
=Magnetism.=--The ancients applied the word “magnet,” _magnes lapes_, to
certain hard black stones which possess the property of attracting small
pieces of iron, and as discovered later, to have the still more remarkable
property of pointing north and south when hung up by a string. At this
time the magnet received the name of _lodestone_ or “leading stone.” It is
commonly, though incorrectly, spelled loadstone.
[Illustration: FIG. 93.--Simple compass. It consists of a magnetic needle
resting on a steel pivot, protected by a brass case covered with glass,
and a graduated circle marked with the letters N, E, S, W, to indicate the
cardinal points; _a b_ is a lever which arrests the needle by pushing it
against the glass when the button _d_ is pressed.]
=Ques. Describe two kinds of magnetism.=
Ans. Magnets have two opposite kinds of magnetism or magnetic poles, which
attract or repel each other in much the same way as would two opposite
kinds of electrification.
=Ques. What is the nature of each kind of magnetism?=
Ans. One has a tendency to move toward the north and the other toward the
south.
[Illustration: FIGS. 94 to 96.--Simple _bar magnet_ and _horse shoe
magnet_ with _keeper_. These are known as _permanent magnets_ in
distinction from _electromagnets_. The horse shoe magnet will attract more
than the bar magnet because both poles act together. A piece of soft iron,
or keeper is placed across the ends of a horse shoe magnet to assist in
preventing the loss of magnetism.]
[Illustration: FIGS. 97 and 98.--Horizontal magnetic needle, and magnetic
“dip” needle. A magnetic needle consists of a small bar magnet, supported
upon a pivot or suspended so that it is free to turn in a horizontal or
vertical plane. The form of magnetic needle illustrated in fig. 97 is
arranged to show the magnetic meridian; the needle moves upon a
perpendicular axis or pivot _ab_. In fig. 98, the needle _sn_ turns upon a
horizontal axis _ab_. This needle indicates the dip or inclination, that
is, the angle which it makes with the horizontal plane, due to the fact
that in most places the lines of force are not horizontal. In the northern
hemisphere the N pole of the needle is depressed, in the southern
hemisphere the S pole is similarly affected. When used, the dip needle
must be set so that the plane in which the needle swings contains the
magnetic meridian, as indicated by the horizontal needle.]
=Ques. Where is the magnetism the strongest?=
Ans. In two regions called the _poles_.
=Ques. Describe the distribution of magnetism in a long shaped magnet.=
Ans. The strongest magnetism resides in the ends, while all around the
magnet half way between the poles there is no attraction at all.
[Illustration: FIG. 99.--Magnetic poles. If a bar magnet be plunged into
iron filings and then lifted, as illustrated in the figure, a mass of
filings will cling to the ends of the magnet but not to the middle. The
ends are called the _poles_ of the magnet.]
=Ques. How are the poles designated?=
Ans. They are called the _north pole_ and the _south pole_.
=Ques. What is the distinguishing feature of each?=
Ans. The north pole points approximately to the earth’s geographical
north, while the south pole of a magnet points approximately to the
earth’s geographical south.
The north pole is the positive (+) pole and the south pole is
the negative. The north and south poles were formerly called in
France, the austral and boreal poles respectively.
=Magnetic Field.=--When a straight bar magnet is held under a piece of
card board upon which iron filings are sprinkled, the filings will arrange
themselves in curved lines radiating from the poles. If a horse shoe
magnet be held at right angles to the plane of the card board, the filings
will arrange themselves in curved lines, as shown in fig. 108. These lines
are called _magnetic lines of force_ or simply _lines of force_; they show
that the medium surrounding a magnet is in a state of stress, the space so
affected being called the _magnetic field_.
[Illustration: FIG. 100.--Badly magnetized bar. Properly magnetized
magnets have only two poles; it is possible, however, by special or
careless magnetization, to produce magnets with more than two poles, but
no process will produce a magnet with a single pole. If an abnormal magnet
with more than two poles be dipped into iron filings, the latter will
adhere at places other than the two ends, as shown in the illustration.
The polarities are alternately N and S; that is, the regions N, B, N, have
north polarity, while A and C have south polarity. These are known as
_consequent poles_.]
[Illustration: FIGS. 101 to 107.--Effect of breaking a magnet into several
parts. If a magnetized needle be broken, each part will be found to be a
complete magnet having a N and S pole. The sub-division may be continued
indefinitely, but always with the same result as indicated in the figure.
This is evidence of the correctness of the molecular theory of magnetism,
which states that _the molecules of a magnet are themselves minute magnets
arranged in rows with their opposite poles in contact_.]
=Ques. What is the extent and character of the magnetic field?=
Ans. The influence of a magnet is supposed to extend in all directions
indefinitely, however, the effect is very slight beyond a comparatively
limited area.
[Illustration: FIG. 108.--The region about a magnet in which its magnetic
forces can be detected is called the _magnetic field_. This can be
represented graphically by placing a piece of cardboard over the magnet
and sprinkling iron filings on the paper, gently tapping at the same time.
Each filing becomes a temporary magnet by _induction_, and sets itself,
like the compass needle, in the direction of the _line of force_ of the
magnetic field.]
[Illustration: FIG. 109.--Tracing lines of force with a suspended magnet.
If a small magnetic needle, suspended by a thread, be held near a magnet,
it will point in some fixed direction depending on the proximity of the
poles of the magnet. The direction taken by the magnet is called the
direction of the force at the point, and if the suspended needle be moved
forward in the direction of the pole, it will trace out a curved line
which will be found to start from one of the poles, and end at the other.
Any number of such lines can be traced; the space filled by these lines of
force is called the _magnetic field_.]
[Illustration: FIG. 110.--Magnetic action: _Unlike poles of magnets
attract each other_.]
[Illustration: FIG. 111.--Magnetic action: _Like poles of magnets repel
each other_.]
=Magnetic Force.=--This is the force with which a magnet attracts or
repels another magnet or any piece of iron or steel. The force varies with
the distance, being greater when the magnet is nearer and less when the
magnet is farther off. The following are the laws relating to magnetic
force:
1. _Like magnetic poles repel one another; unlike magnetic poles attract
one another._
2. _The force exerted between two magnetic poles varies inversely as the
square of the distance between them._
=Magnetic Circuit.=--The path taken by magnetic lines of force is called a
magnetic circuit; the greater part of such a circuit is usually in
magnetic material, but there are often one or more air gaps included. The
total number of lines of force in the circuit is known as the _magnetic
flux_.
=Ques. How is magnetic flux measured?=
Ans. By a unit called the maxwell.
Named after James Clerk Maxwell the Scottish physicist.
=Ques. What is the maxwell?=
Ans. _The amount of magnetism passing through every square centimetre of a
field of unit density._
=Ques. What is the unit of field strength?=
Ans. The gauss.
=Ques. What is a gauss?=
Ans. _The intensity of field which acts on a unit pole with a force of one
dyne. It is equal to one line of force per square centimetre_. Named
after Karl Friedrich Gauss, the German mathematician.
=The Magnetic Effect of the Current.=--Hans Christian Oerstead, the Danish
scientist, discovered in 1819 that a magnet tends to set itself at right
angles to a wire carrying an electric current. He also found that the way
in which the needle turns, whether to the right or left of its usual
position, depends: 1, upon the position of the wire that carries the
current, whether it be above or below the needle, and 2, on the direction
in which the current flows through the wire.
[Illustration: FIG. 112.--Illustrating Maxwell’s “corkscrew rule” for
relative directions of current and lines of force. According to the rule:
_the direction of the current and that of the resulting magnetic force are
in the same relation to each other as is the forward travel and rotation
of an ordinary corkscrew_, Thus, in the figure, if a current flow through
the wire _ab_ in the direction from _a_ to _b_, the magnetic lines will
encircle the wire in the direction of the curved arrow _ro_ which shows
the direction in which the corkscrew must be turned to advance in the
direction of the arrow _n_.]
To keep these movements in mind numerous rules have been suggested, of
which the following will be found convenient:
=Corkscrew Rule=.--_If the direction of travel of a right handed corkscrew
represent the direction of the current in a straight conductor, the
direction of rotation of the corkscrew will represent the direction of the
magnetic lines of force._
[Illustration: FIG. 113.--Experiment showing direction of lines of force
in the magnetic field surrounding a conductor carrying an electric
current. A piece of copper wire is pierced through the center of a sheet
of cardboard, and carried vertically for two or three feet then bent
around to the terminals of a battery or other source of current. If iron
filings be sprinkled over the card while the current is passing, they will
arrange themselves in circles around the wire, thus indicating the form of
the magnetic field surrounding the conductor. Compass needles may also be
used to show the direction of the lines of force at any point.]
[Illustration: FIG. 114.--Right hand rule to determine the direction of
magnetic field around a conductor carrying a current. The thumb of the
right hand is placed along the conductor, pointing in the direction in
which the current is flowing, then, if the fingers be partly closed, as
shown in the illustration, the finger tips will point in the direction of
the magnetic whirls.]
=Ques. What is the effect of a current flowing in a loop of wire?=
Ans. If, in figs. 116 and 117, the current flow in the direction indicated
by the arrow, the lines for magnetic force are found to surround the loop
as shown; all the lines leave on one side of the loop and return on the
other; accordingly, a north pole is formed on one side, and a south pole
on the other.
[Illustration: FIG. 115.--Right hand palm rule to determine the direction
of the magnetic field around a conductor carrying a current: Place the
palm of the outstretched right hand above and to the right side of the
wire with the fingers pointing in the direction of the current, and the
thumb extended at right angles, that is, pointing downward. The direction
in which the thumb points will indicate the direction of the magnetic
whirls.]
[Illustration: FIG. 116.--Lines of force of a circular loop. If a current
flow through the loop in the direction indicated, the lines of force both
inside and outside the loop, will cross the plane of the loop at right
angles, and all those which cross the loop on the inside will pass through
the plane in one direction (downwards in the figure), while all on the
outside will return through the plane in the opposite direction.]
=Solenoids.=--A solenoid consists of a spiral of conducting wire wound
cylindrically so that, when an electric current passes through it, its
turns are nearly equivalent to a succession of parallel circular circuits,
and it acquires magnetic properties similar to those of a bar magnet.
[Illustration: FIG. 117.--Lines of force of a circular loop. If the loop
pass through a piece of cardboard at right angles to its plane, and the
current flow as indicated, the dotted lines on the cardboard will
represent the direction of the lines of force in the plane of the
cardboard. The student should verify the lines of force as here given by
applying the corkscrew rule.]
=Ques. What is the character of the lines of force of a solenoid in which
a current is flowing?=
Ans. The lines of force must be thought of as closed loops linked with the
current. The conductor conveying the current passes through all the loops
of force, and these are, so to speak, threaded or slung on the
current-line of flow, as in fig. 116.
=Ques. What is the distribution of the lines of force?=
Ans. The lines of force form continuous closed curves running through the
interior of the coil; they issue from one end and enter into the other end
of the coil, as shown in fig. 117.
=Ques. What are the properties of a solenoid?=
Ans. A solenoid has north and south poles, and in fact possesses all the
properties of an ordinary permanent magnet, with the important difference
that the magnetism is entirely under control.
Since a solenoid carrying a current attracts and repels by its
extremities the poles of a magnet, two such solenoids will
attract and repel each other.
[Illustration: FIG. 118.--Magnetic field of a solenoid. This is best
observed by cutting a piece of cardboard and fitting it around the
solenoid, as shown. If iron filings be sprinkled on the cardboard and a
current passed through the solenoid, the character of the field is
indicated. With the current in the direction shown, it will be found that
wherever small compass needles are placed, the direction in which their
north poles turn is along arrows marked on the card. The card only
exhibits the field in one of the sectional planes of the coil, but it is
obvious that the field is the same for all sectional planes.]
=Ques. How does the magnetic strength of a solenoid vary?=
Ans. It is proportional to the strength of the electric current passing
through it.
=Ques. On what, besides the current strength, does the magnetizing power
of a solenoid depend?=
Ans. _The magnetic effect or the magnetizing power is proportional to the
number of turns of wire composing the coil._
=Ques. How may the magnetizing power of a solenoid be increased?=
Ans. By inserting in the solenoid an _iron core_ or round bar of soft
iron.
[Illustration: FIG. 119.--Right hand rule for polarity of a solenoid: If
the solenoid be grasped in the right hand, so that the fingers point in
the direction in which the current is flowing in the wires, the thumb
extended will point in the direction of the north pole.]
=Ques. Describe the action of an iron core.=
Ans. At first, the presence of an iron core greatly increases the strength
of the field; after a time, however, as the strength of the current
flowing in the exciting coils is increased, the _conductibility_ of the
iron for the lines of force appears to decrease, until a point is
eventually reached when the presence of the iron core appears to have no
effect in increasing the strength of the field.
=Permeability.=--Permeability is a measure of the ease with which
magnetism passes through any substance. It is defined as: _the ratio
between the number of lines of force per unit area passing through a
magnetizable substance, and the magnetizing force which produces them_.
[Illustration: FIGS. 120 and 121.--Illustrating the effect of introducing
an iron core into a solenoid. In the upper figure, the air space or “air
core” surrounded by the solenoid offers considerable resistance to the
passage of magnetic lines, allowing only a small number to pass through.
If a piece of iron be introduced, as in the lower figure, the number of
lines will be greatly increased. The number of lines B passing through a
unit cross section of the iron core divided by the number of lines H,
passing through a unit cross section of the air core is called the
_permeability_ and designated by the Greek letter μ.]
In other words, it is the ratio of flux density to magnetizing force.
Permeability is a measure of the ease with which magnetism passes through
any substance. The permeability of good soft wrought iron is sometimes
3000 times that of air, varying with the quality of the iron.
=Ques. What is the effect of increasing the magnetization?=
Ans. The magnetic permeability decreases as the magnetization increases.
=Ques. What is magnetic saturation?=
Ans. The state of a magnet which has reached the highest degree of
magnetization.
[Illustration: FIG. 122.--Action of currents on solenoids. To demonstrate
this fact experimentally, a solenoid is constructed as shown, so that it
can be suspended by two pivots in the cups _a_ and _c_. The solenoid is
then movable about a vertical axis, and if a rectilinear current QP be
passed beneath it, which at the same time traverses the wires of the
solenoid, the latter is seen to turn and set at right angles to the lower
current; that is, in such a position that its circuits are parallel to the
fixed current; moreover, the current in the lower part of each of the
circuits is in the same direction as in the rectilinear wire. If, instead
of passing a rectilinear current below the solenoid, it be passed
vertically on the side, an attraction or repulsion will take place,
according as the two currents in the vertical wire, and in the nearest
part of the solenoid, are in the same or in contrary directions.]
A magnet, just after being magnetized, will appear to have a
higher degree of magnetism than it is able to retain
permanently; that is, it will appear to be super-saturated,
since it will support a greater weight immediately after being
magnetized than it will after its armature has been once
removed.
For all practical purposes, magnetic saturation may be defined as: That
point of magnetization where _a very large increase in the magnetizing
force does not produce any perceptible increase in the magnetization_.
From tests it has been shown that permeability increases with the flux
density up to a certain point and then decreases, indicating that the iron
is approaching a state of saturation.
=Magnetomotive Force.=--This is a force similar to electromotive force,
that is, magnetic pressure. When a coil passes around a core several
times, its magnetizing power, or magnetomotive force, (m.m.f.) is
proportional both to the strength of the current and to the number of
turns in the coil. The product of the current passing through the coil
multiplied by the number of turns composing the coil is called the _ampere
turns_.
It is known by experiment that one ampere turn produces 1.2566 units of
magnetic pressure, hence:
magnetic pressure = 1.2566 × turns × amperes
that is,
magnetomotive force (m.m.f.) = 1.2566 × n × I.
The unit of magnetic pressure is the _gilbert_ (named after William
Gilbert, the English physicist) and is equal to
1 ÷ 1.2566 ampere turn = .7958 ampere turn.
=Reluctance.=--The magnetic pressure (magnetomotive force) acting in a
magnetic circuit encounters a certain opposition to the production of a
magnetic field, just as electromotive force in an electric circuit
encounters opposition to the production of a current. In the magnetic
circuit this opposition is called the _reluctance_; it is simply _magnetic
resistance_ and may be defined as: _the resistance offered to the magnetic
flux by the substance magnetized, being the ratio of the magnetomotive
force to the magnetic flux_.
The unit of reluctance or magnetic resistance is the _oersted_ (named
after Hans Christian Oersted, the Danish physicist) and is defined as:
_the reluctance offered by a cubic centimetre of vacuum_.
[Illustration: FIG. 123.--Mutual action of solenoids. When two solenoids
traversed by a current are allowed to act on each other, one of them being
held in the hand and the other being movable about a vertical axis, as
shown in the figure, attraction and repulsion will take place just as in
the case of two magnets (see figs. 110 and 111).]
=Analogy Between Electric and Magnetic Circuits.=--The total number of
magnetic lines of force, or magnetic flux, produced in any magnetic
circuit will depend on the magnetic pressure (m.m.f.) acting on the
circuit and the total reluctance of the circuit, just as the current in
the electrical circuit depends upon the electrical pressure and the
resistance of the circuit.
To make this plain, Ohm’s law states that
electric current = electromotive force / resistance or I = E/R
expressed in units
amperes = volts / ohms
The resistance, as already explained, depends on the materials of which
the circuit is composed, and their geometrical shape and size.
Similarly, in the magnetic circuit, the total number of magnetic lines
produced by a given magnetizing solenoid depends on the magnetic pressure,
the material composing the circuit, and its shape and size.
That is,
magnetic flux = magnetomotive force / reluctance
expressed in units, the equation becomes:
maxwells = gilberts / oersteds
_The gilbert is the unit of magnetomotive force, equivalent to the
magnetomotive force of .7958 ampere turn._
It should be noted that in the electric circuit resistance causes heat to
be generated and therefore energy to be wasted, but in the magnetic
circuit reluctance does not involve any similar waste of energy.
=Ques. Upon what does the reluctance of a magnetic circuit depend?=
Ans. _The reluctance is directly proportional to the length of the
circuit, and inversely proportional to its cross sectional area_.
The reluctance of a magnetic circuit is calculated according to
the following equation:
reluctance = length in centimetres / (permeability × cross section
in square centimetres)
=Hysteresis.=--The term hysteresis has been given by Ewing to the subject
of _lag of magnetic effects behind their causes_. Hysteresis means to “lag
behind,” hence its application to denote the _lagging of magnetism, in a
magnetic metal, behind the magnetizing flux which produces it_.
=Ques. What is the cause of hysteresis?=
Ans. It is due to the friction between the molecules of iron or other
magnetic substance which requires an expenditure of energy to change their
positions.
[Illustration: FIGS. 124 and 125.--Experiment illustrating the molecular
theory of magnetism. Coarse steel filings are placed inside a small glass
tube and the contents magnetized. It will be found that filings which at
first had no definite arrangement will rearrange themselves under the
influence of magnetic force, and assume symmetrical positions, each one
lying in line with, or parallel to its neighbor, as shown in the lower
figure.]
=Ques. When do the molecules change their positions?=
Ans. Both in the process of magnetization and demagnetization.
=Ques. What becomes of the loss of energy due to hysteresis?=
Ans. It is converted into heat in changing the positions of the molecules
during magnetization and demagnetization.
Ewing gives the value for the energy in ergs dissipated per
cubic centimetre, for a complete cycle of doubly reversed strong
magnetization for a number of substances as follows:
Substance Energy dissipated
(ergs)
Very soft annealed iron 9,300
Less “ “ “ 16,300
Hard drawn steel wire 60,000
Annealed “ “ 70,000
Same steel glass hard 76,000
Piano steel wire annealed 94,000
“ “ “ normal temper 116,000
“ “ “ glass hard 117,000
Approximately 28 foot pounds of energy are converted into heat
in making a double reversal of strong magnetization in a cubic
foot of iron.
=Residual Magnetism.=--When a mass of iron has once been magnetized, it
becomes a difficult matter to entirely remove all traces when the
magnetizing agent has been removed, and, as a general rule, a small amount
of magnetism is permanently retained by the iron. This is known as
_residual magnetism_, and it varies in amount with the quality of the
iron.
Well annealed, pure wrought iron, as a rule, possesses very little
residual magnetism, while, on the other hand, wrought iron, which contains
a large percentage of impurities, or which has been subjected to some
hardening process, such as hammering, rolling, stamping, etc., and cast
iron, possess a very large amount of residual magnetism.
Residual magnetism in iron is of great importance in the working of the
_self-exciting_ dynamo, and is, indeed, the essential principle of this
class of machine.
That is, without residual magnetism in the field magnet core,
the dynamo when started would not generate any current unless it
received an initial excitation from an external source.
CHAPTER X
ELECTROMAGNETIC INDUCTION
The word _induction_, introduced by Faraday, has various meanings so far
as it relates to electricity. It signifies, in general, phenomena produced
in bodies by the influence of other bodies, having no necessary material
connection with them.
_A body charged with electricity causes or “induces” charges on
neighboring bodies._ The process in this case is called _electrostatic
induction_.
A magnet induces magnetism in neighboring masses of iron or other magnetic
materials by the process of _magnetic induction_.
Again, a moving magnet induces electric currents in neighboring conductors
by the process of _electromagnetic induction_.
=Faraday’s Discovery.=--All dynamos of whatever form, are based upon the
discovery made by Faraday[11] in 1831, which may be stated as follows:
_Electric currents are generated in conductors by moving them in a
magnetic field, so as to cut magnetic lines of force._
=Ques. What does the expression “cut lines of force” mean?=
Ans. A conductor, forming part of an electric circuit, _cuts lines of
force_ when it moves across a magnetic field in such manner as to _alter_
the number of magnetic lines of force which are embraced by the circuit.
It is important to clearly understand the meaning of this
expression, which will be later explained in more detail.
[Illustration: FIG. 126.--Faraday’s dynamo which embodies his discovery in
1831 of _electromagnetic induction_, the principle upon which all dynamos
work, as well as induction coils, transformers, and other electrical
apparatus.]
=Faraday’s Machine.=--After various experiments, Faraday made his “new
electrical machine” as shown in fig. 126. This piece of apparatus is
preserved and was shown in perfect action by Prof. S. P. Thompson in a
lecture delivered April 11th, 1891, after an interval of sixty years.
It consists of a horse shoe magnet and a copper disc attached to
a shaft and supported so as to turn freely. The magnet is so
placed that its inter-polar lines of force traverse the disc
from side to side. There are two copper brushes, one bears
against the shaft, and the other against the circumference of
the disc. A handle serves to rotate the disc in the magnetic
field.
Now, if the north pole of the magnet be nearest the observer and
the disc be rotated clockwise, the current _induced_ in the
circuit will flow out at the brush which touches the
circumference, and return through the brush at the shaft.
=Faraday’s Principle.=--The principle deduced from Faraday’s experiment
may be stated as follows:
_When a conducting circuit is moved in a magnetic field so as to alter the
number of lines of force passing through it, a current is induced therein,
in a direction at right angles to the direction of the motion, and at
right angles also to the direction of the lines of force, and to the right
of the lines of force, as viewed from the point from which the motion
originated._
Faraday’s principle may be extended as follows to cover all cases of
electromagnetic induction:
_When a conducting circuit is moved in a magnetic field, so as to alter
the number of lines of force passing through it, or when the strength of
the field is varied so as to either increase or decrease the number of
lines of force passing through the circuit, a current is induced therein
which lasts only during the interval of change in the number of lines of
force embraced by the circuit._
=Ques. Explain just what happens when a current is induced by
electromagnetic induction.=
Ans. In order to induce an electromotive force by moving a conductor
across a uniform magnetic field, it is necessary that the conductor, in
its motion, should so cut the magnetic lines as to alter the number of
lines of force that pass through the circuit of which the moving conductor
forms a part.
=Ques. What is the proper name for a “conductor” which moves across the
magnetic field?=
Ans. An _inductor_, because it is that part of the electric circuit in
which induction takes place.
In the case of a dynamo, an inductor may be either a copper wire
or copper bar.
=Ques. How may a conducting circuit be moved across a magnetic field
without having a current induced therein?=
Ans. If a conducting circuit--a wire ring or single coil, for example--be
moved in a uniform magnetic field, as shown in fig. 127, so that only the
same number of lines of force pass through it, no current will be
generated, for since the coil is moved by a motion of translation to
another part of the field, as many lines of force will be left behind as
are gained in advancing from its first to its second position.
[Illustration: FIG. 127.--Electromagnetic induction: In order to induce a
current by electromagnetic induction, a conductor must be so moved through
a magnetic field _that the number of lines of force passing through it
(that is, embraced) are altered_. If a coil be given a simple motion of
translation in a uniform magnetic field as indicated in the figure, no
current will be induced _because the number of lines of force passing
through it are not changed_, that is, during the movement as many lines
are lost as are gained.]
=Ques. Describe another movement by which no current will be induced.=
Ans. If the coil be merely rotated on itself around a central axis, that
is, like a fly wheel rotating around a shaft, the number of lines of force
passing through the coil will not be altered, hence no current will be
generated.
=Ques. State the essential condition for current induction in a uniform
field.=
Ans. The coil in which a current is to be induced, must be tilted in its
motion across the uniform field, or rotated around any axis in its plane
as in fig. 128, _so as to alter the number of lines of force which pass
through it_.
[Illustration: FIG. 128.--Electromagnetic induction: If a coil be given a
motion of rotation from any point within its own plane so that it passes
through a uniform magnetic field, a current will be induced in the coil
_because the number of lines of force passing through it is altered_.]
=Ques. In what direction will the current flow in the coil, fig. 128?=
Ans. The current induced in the coil will flow around it in a clockwise
direction (as observed by looking along the magnetic field in the
direction in which the magnetic lines run) if the effect of the movement
be to diminish the number of lines of force that pass through the coil.
The current will flow in the opposite direction, (counter-clockwise) if
the movement be such as to increase the number of intercepted lines of
force.
=Ques. If the magnetic field be not uniform, as in fig. 129, what will be
the result?=
Ans. The effect of moving the coil by a simple motion of translation from
a dense region of the field to one less dense, or vice versa, will be to
induce a current because in either case, the number of lines of force
passing through the coil is altered.[12]
=Laws of Electromagnetic Induction.=--There are certain laws of
electromagnetic induction which, on account of the importance of the
subject, it is well to carefully consider. The facts presented in the
preceding paragraphs are embodied in the following fundamental laws:
[Illustration: FIG. 129.--Electromagnetic induction: If a coil be given a
simple motion of translation in a non-uniform or variable magnetic field,
a current will be induced in the coil, whether the motion be from the
dense to the less dense region of the field or the reverse, _because the
number of lines of force passing through the coil is altered_.]
1. _To induce a current in a circuit, there must be a relative motion
between the circuit and a magnetic field, of such a kind as to alter the
number of magnetic lines embraced in the circuit._
2. _The electromotive force induced in a circuit is proportional to the
rate of increase or decrease in the number of magnetic lines embraced by
the circuit._
For instance, if _n_ equal the number of magnetic lines embraced
by the circuit at the beginning of the movement, and _n′_ the
number embraced after a very short interval of time t, then
the average induced electromotive force = (n - n′)/t
It would require the cutting of 100,000,000 lines per second to
produce an electromotive force equal to that of one Daniell
cell.
The unit of electromotive force, called the _volt_, is the
electric pressure produced by cutting 100,000,000 lines per
second, usually expressed 10^{8}.
[Illustration: FIG. 130.--Experiment illustrating Lenz’s law which states
that in all cases of electromagnetic induction, _the direction of the
induced current is such as to tend to stop the motion producing it_. In
the experiment, in order to produce the induced current, energy must be
expended in bringing the magnet to the coil and in taking it away, which
is in accordance with the law of conservation of energy.]
3. _By joining in series a number of conductors or coils moving in a
magnetic field, the electromotive forces in the separate parts are added
together._
The reason for this is apparent by considering a coil of wire
having several turns and moving in a magnetic field so as to cut
magnetic lines. During the movement, the lines cut by the first
turn are successively cut by all the other turns of the coil,
hence, the total number of lines cut is equal to the number cut
by a single turn multiplied by the number of turns. The
electromotive forces therefore of the separate turns are added.
EXAMPLE--If a coil of wire of 50 turns cut 100,000 lines in
1/100 of a second, what will be the induced voltage?
The number of lines cut per second per turn of the coil is
100,000 × 100 = 10,000,000.
The total number of lines cut by the coil of 50 turns is
10,000,000 × 50 = 500,000,000.
which will induce a pressure of
500,000,000 ÷ 10^{8} = 5 volts.
[Illustration: FIG. 131.--Experiment illustrating Lenz’s law. If a copper
ring be held in front of an ordinary electromagnet, and the current
circulating through the coil of the magnet be in such a direction as to
magnetize the core as indicated by the letters S N, then as the current
increases in the coil more and more of the lines of force proceeding from
N pass through the ring O O from left to right. While the field is thus
increasing currents will be induced in the copper ring in the direction
indicated by the arrows, such currents tending to set up a field that
would pass through the ring from right to left, and would therefore
_retard_ the growth of the field due to the electromagnet M.]
4. _A decrease in the number of magnetic lines which pass through a
circuit induces a current around the circuit in the positive direction._
The term positive direction is understood to be the direction
along which a free N pole would tend to move.
5. _An increase in the number of magnetic lines which pass through a
circuit induces a current in the negative direction around the circuit._
The reason for the change of direction of the current for
decrease or increase in the number of lines cut, as stated in
the fourth and fifth laws, will be seen by aid of the formula
given under the second law, viz:
electromotive force = (n - n′)/t (1)
but by Ohm’s law
current = electromotive force / resistance or, I = E/R (2)
Substituting (1) in (2)
current = ((n - n′)/t)/R or (n - n′)/(Rt) (3)
[Illustration: FIG. 132.--Fleming’s rule for direction of induced current.
Extend the thumb, forefinger and middle finger of the right hand so that
each will be at right angles to the other two. Place the hand in such
position that the thumb will point in the direction in which the conductor
moves, the forefinger in the direction of the lines of force (N to S),
then will the middle finger point in the direction in which the induced
current flows.]
Now in equation (3) if there be a _decrease_ in the number of
lines cut _n′_ will be less than _n_ hence the current will be
positive (+); again, if the lines _increase_ _n′_ will be greater
than _n_, which will give a minus value, that is, the current
will be negative or in a reverse direction.
6. _The approach and recession of a conductor from a magnet pole will
yield currents alternating in direction._
Since the strength of the field depends on the proximity to the
pole, the approach and recession of a conductor involve an
_increase_ and _decrease_ in the rate of cutting of magnetic
lines, hence a reversal of current.
7. _The more rapid the motion, the higher will be the induced
electromotive force._
In other words, the greater the number of lines cut per unit of
time, the higher will be the voltage.
[Illustration: FIG. 133.--A rule for direction of induced current which,
in some cases, is more conveniently applied than Fleming’s rule: Hold the
thumb, forefinger and remaining fingers of the right hand at right angles
to each other; place the hand in such position that the forefinger points
in the direction of motion of the conductor, the three fingers in the
direction of the lines of force, then will the thumb point in the
direction of the induced current.]
8. _Lenz’s law. The direction of the induced current is always such that
its magnetic field opposes the motion which produces it._
This is illustrated in figs. 130 and 131.
=Rules for Direction of Induced Current.=--There are a number of rules to
quickly determine the direction of an induced current, when the direction
of the lines of force, and motion of the conductor are known. The first
rule here given was devised by Fleming and is very useful. It is sometimes
called the “dynamo rule.”
=Fleming’s Rule.=--_If the forefinger of the right hand be pointed in the
direction of the magnetic lines, and the thumb (at right angles to the
forefinger) be turned in the direction of the motion of the conductor,
then will the middle finger, bent at right angles to both thumb and
forefinger, show the direction of the induced current._
The application of this rule is shown in fig. 132. Here the
right hand is so placed at the north pole of a magnet, that the
forefinger points in the direction of the magnetic lines; the
thumb in the direction of motion of the conductor; the middle
finger pointed at right angles to the thumb and forefinger
indicates the direction of the current induced in the conductor.
[Illustration: FIG. 134.--The palm rule for direction of induced current:
If the palm of the right hand be held against the direction of the lines
of force, the thumb in the direction of the motion, then the fingers will
point in the direction of the induced current.]
=Ampere’s Rule.=--_If a man could swim in a conductor with the current,
then the north seeking_ (+) _pole of a magnetic needle placed directly
ahead of him, will be deflected to the left, while the south seeking_ (-)
_pole will be urged to the right._
For certain particular cases in which a fixed magnet pole acts
on a movable circuit, the following converse to Ampere’s rule
will be found useful: If a man swim in the wire with the
current, and turn so as to look along the direction of the lines
of force of the pole (that is, as the lines of force run, _from_
the pole if it be north seeking, _toward_ the pole if it be
south seeking), then he and the conducting wire with him will be
urged _toward his left_.
=The palm rule.=--_If the palm of the right hand be held facing or against
the lines of force, and the thumb in the direction of the motion, then
will the fingers point in the direction of the induced current._
=Self-induction.=--This term signifies _the property of an electric
current by virtue of which it tends to resist any change of value._
Self-induction is sometimes spoken of as _electromagnetic inertia_, and is
analogous to the mechanical inertia of matter.
It is on account of self-induction of the induced currents in the armature
winding of a dynamo, that sparks appear at the brushes when the latter are
not properly adjusted, hence the importance of clearly understanding the
nature of this peculiar property of the current.
Self-induction is fully explained in the chapter following.
CHAPTER XI
INDUCTION COILS
The induction coil has always been a popular piece of apparatus with those
interested in electrical science; the experiments which can be performed
with its aid are very numerous. It is of considerable importance,
especially in its application to such useful purposes as X ray work,
wireless telegraphy and _ignition_ for gas engines. The latter has caused
manufacturers to give much attention to the development of the induction
coil, resulting in many refinements of design and construction.
Induction coils may be divided into two general classes:
1. Primary coils;
2. Secondary coils.
The subject of electromagnetic induction has been fully explained in
chapter X, but it may be said, with special reference to induction coils,
that the operation of the two classes just mentioned is respectively due
to:
1. Self-induction;
2. Mutual induction.
=Self-induction.=--This is the property of an electric current by virtue
of which _it tends to resist any change in its rate of flow_. It is
sometimes spoken of as _electromagnetic inertia_ and is analogous to the
mechanical inertia of matter.
Self-induction is due to the action of the current upon itself during
variations in strength. It becomes especially marked in a coil of wire, in
which the adjacent turns act inductively upon each other upon the
principle of _mutual induction_ arising between two separate adjacent
circuits. Self-induction manifests itself by giving “_momentum_” to the
current so that _it cannot be instantly stopped when the circuit is
broken_, the result being a bright spark at the moment of breaking the
circuit. On account of this spark a primary induction coil is used in low
tension or “make and break” ignition systems.
[Illustration: FIG. 135.--Diagram illustrating the action of mutual
induction between two circuits; the one including a source of electrical
energy and a switch; the other including a galvanometer, but having no
cell or other electrical source. During the increase or decrease in the
strength of the current as on closing or opening the key a current is
_induced_ in the secondary circuit in a direction opposite to that of the
primary current as indicated by the arrows.]
In a single circuit, consisting of a straight wire and a parallel return
wire there is little or no self-induction. When a circuit containing a
primary induction coil and a battery is closed there is no spark because
at the instant of closing the circuit the current is at rest and on
account of self-induction _the current cannot at once rise to its full
value_.
=Mutual Induction.=--This is a particular case of electromagnetic
induction in which the magnetic field producing an electromotive force in
a circuit is due to the current in a neighboring circuit.
The effect of mutual induction may be explained with the aid of fig. 135.
If, as illustrated, a circuit including a battery and a switch, be placed
near another circuit, formed by connecting the two terminals of a
galvanometer by a wire, it will be found that whenever the first circuit,
1, is closed by the switch, allowing a current to pass in a given
direction, a momentary current will be induced in the second circuit, 2,
as shown by the galvanometer. A similar result will follow on the opening
of the battery circuit, the difference being that the momentary induced
current occurring at closure moves in a direction opposite to that in the
battery circuit, while the momentary current at opening moves in the same
direction.
Currents, besides being induced in circuit 2 at _make_ or _break_ of
circuit 1, are also induced when the current in 1 is fluctuating in
intensity.
The most marked results are observed when the make or break is sudden,
_the action being strongest at the break of the current in 1._
The inductive effect of the current in the arrangement shown in fig. 135
is very weak.
=Ques. What name is given to circuit 1?=
Ans. The _primary circuit_.
=Ques. What name is given to circuit 2?=
Ans. The _secondary circuit_.
=Ques. What names are given respectively to the currents in circuits 1 and
2?=
Ans. The _primary_ and _secondary_ or _induced currents_.
=Primary Induction Coils.=--These represent the simplest form of coil, and
are used chiefly in low tension ignition to intensify the spark when a
battery forms the current source.
A primary coil consists of a long iron core wound with a considerable
length of low resistance insulated copper wire, the length of the core and
the number of turns of the insulated wire winding determining the
efficiency. The effect of the iron core is to increase the self-induction.
[Illustration: FIG. 136.--Primary induction coil as used for low tension
ignition. Coils of this type are made in a great variety of form and size.
Ordinarily the winding consists of about six convolutions of No. 14 copper
wire. The winding is usually covered and the ends capped with ebonite
heads so that the core wires are not exposed.]
The spark produced, as previously explained, is due to self-induction, and
it should be remembered that in the operation of the coil, the _spark
occurs at the instant of breaking the circuit_, _not at the instant of
making_.
=Secondary Induction Coils.=--The arrangement shown in fig. 137, may be
considered as a very simple or rudimentary form of secondary induction
coil. In the actual coil, the primary and secondary circuits
(corresponding to 1 and 2 in fig. 135) are made up of coils of insulated
wire, as shown in fig. 143, the primary coil P, being wound over a core C
and the secondary coil S being wound over the primary.
The one property of such an arrangement that makes it of great value for
most purposes is that _the voltage of the induced currents may be
increased or diminished to any extent depending on the relation between
the number of turns in the primary and secondary winding._
This relation may be expressed in the following rule:
_The voltage of the secondary current is (approximately) to the voltage of
the primary current as the number of turns of the secondary winding is to
the number of turns of the primary winding._
[Illustration: FIG. 137.--Production of spark with plain coil. Connect the
ends or leads of the secondary winding to fixed insulators and bend the
ends so they are from one-sixteenth to one-eighth inch apart. Connect one
end of the primary winding to an electric battery, and with the other lead
of the primary winding brush against the other terminal of the battery, as
indicated. When the contact is broken there will be a spark both at the
point of rupture in the primary circuit and at the gap. An electric
impulse is also induced in the secondary circuit when the primary circuit
is closed and the current flowing in it gradually rises to its maximum
value, but this impulse is too feeble to cause a spark to jump across the
gap. Only the impulse induced in the secondary during the dying out of the
current in the primary is utilized.]
For instance, if the voltage of the primary current be 5 volts,
the primary winding have 10 turns and the secondary 100 turns,
then
Secondary voltage: 5 :: 100 : 10 from which
Secondary voltage = 50 volts (approximately)
The watts in each circuit are approximately the same; hence: if,
for instance, the current strength in the primary circuit be 5
amperes, the watts in primary circuit are 5 × 5 = 25.
Accordingly, for the secondary circuit the current strength is:
25 watts / 50 volts = 1/2 ampere (approximately)
From this, it is seen that where the voltage is raised in the
secondary circuit, the current flow is small as compared to that
in the primary circuit; therefore, heavy wire is used in the
primary winding and fine wire in the secondary, as indicated in
figs. 137 and 143.
For most purposes a very much higher secondary voltage is
required than in the example just given.
[Illustration: FIG. 138.--Diagram of battery and coil connections for jump
spark ignition as applied to a motor cycle. Coils are usually plainly
labeled with the abbreviations: “Bat.,” “Pri.,” “Sec.,” indicating that
the wires are to be connected to the battery, the primary circuit or
contact maker, and the spark plug. The battery and primary wires being for
the low tension circuit are easily distinguished from the secondary wire
by the small amount of insulation surrounding them.]
Secondary induction coils may be divided into three general classes:
1. Plain coils;
2. Vibrator coils;
3. Condenser coils.
The plain coil gives but one spark when the primary circuit is made and
broken, while the vibrator coil gives a series of sparks following each
other in rapid succession.
=Plain Secondary Induction Coils.=--Coils of this class are very simple
and consist of:
1. Core;
2. Primary winding;
3. Secondary winding.
The construction of a plain coil, such as would be suitable for ignition
service, is about as follows:
The core is made of soft annealed iron wires (No. 20 B and S
gauge) from one-half to three-quarters of an inch in diameter
and about six inches long. Over this core is slipped a spool of
insulating material (hard rubber or composition), on which is
wound first the primary winding of the coil, which consists of
several layers of about No. 18 B and S gauge silk insulated
magnet wire.
After the _primary winding_ has been wound over the insulated
core, and the ends have been properly brought out through the
heads of the spool to be connected to binding posts thereon, a
layer of insulating material is applied over the primary wire,
and the secondary winding is then wound on.
The wire for the secondary winding consists of about No. 36 B
and S gauge silk covered magnet wire, the amount used varying
considerably, depending on the desired voltage of the secondary
current.
When all the wire has been wound on, the ends are brought out to
the binding posts, the coil is soaked in shellac dissolved in
alcohol and baked, or in melted paraffin or a paraffin compound,
and allowed to cool. It is then placed in either a cylindrical
hard rubber shell or in a hard wood box.
The proportions of such coils vary greatly; for motor cycle use
they are made long and of small diameter (10×2-1/2 inches for
instance), while for some other purposes short and thick coils
are found more convenient.
=Ques. How may the coil just described be connected for demonstrating
purposes?=
Ans. Connect the ends of the secondary winding to fixed insulators and
bend the ends so they are about 1/8 inch or less apart. Connect one end
of the primary winding to a battery and brush the other end of the primary
winding against the other terminal of the battery as indicated in fig.
137.
=Ques. What happens when the primary circuit is made?=
Ans. An electric pressure is induced in the secondary circuit, but of not
enough intensity to cause a spark to jump across the air gap.
[Illustration: FIG. 139.--A Medical coil with armature and attachments
consisting of electrodes, foot place, sponge, induction coil etc. A
current of any degree of intensity may be obtained. The currents furnished
are: 1, primary, 2, secondary; and 3, primary and secondary combined.]
=Ques. What happens when the primary circuit is suddenly broken?=
Ans. A spark is produced both at the point of break in the primary circuit
and at the air gap in the secondary circuit.
=Ques. Why is a spark produced at the air gap at break and not at make of
the primary current?=
Ans. Because when the current is flowing it cannot be stopped instantly on
account of self-induction, that is, it acts as though it possessed weight.
If the reader has charge of a gas engine with a make and break
ignition system, he will often avoid vexatious delays in
locating ignition troubles, if he remember that one of the most
important conditions for obtaining a good spark is that the
_break take place with great rapidity_. This, of course,
involves that the ignition spring be adjusted to the proper
tension.
[Illustration: FIG. 140.--Rhumkorff induction coil. A secondary coil with
vibrator and condenser; a type generally used in the laboratory. The name
Rhumkorff was formerly very widely applied to induction coils for the
reason that some of the earliest coils were constructed by Rhumkorff.]
=Secondary Induction Coils with Vibrator and Condenser.=--A plain
secondary coil, such as just described, will only give feeble sparks for
its size for the following reasons: The inductive effect of the primary
winding in the secondary depends as previously explained on the rate at
which the current in the primary winding decreases or dies out.
If a strong inductive effect is to be produced in the secondary, the
current in the primary must stop suddenly. This is prevented by
self-induction in the primary winding, which opposes any change in the
current strength. The direct result is that, as the primary circuit is
broken, a spark appears at the break, which means that the current
continues to flow after the break has occurred, dying down comparatively
slowly, hence, the inductive effect on the secondary winding is small.
[Illustration: FIG. 141.--Conventional diagram of a condenser. A condenser
is a device designed to absorb or hold an electric charge in about the
same manner as a vessel will hold a liquid. Every conductor of electricity
forms a condenser and its capacity for holding a charge depends upon the
extent of its surface. A condenser is therefore made of conductive
material formed into such shape as to present the maximum surface for a
given amount of material.]
The spark at the break in the primary circuit is even larger than that in
the secondary circuit, and as this primary spark serves no useful purpose,
but, on the contrary, quickly burns away the contact points, such an
arrangement is obviously defective.
The vibrator-condenser coil is designed to overcome this trouble and also
to give a series of sparks following in rapid succession instead of one.
It should be noted that a series of sparks following each other
with considerable rapidity may be obtained with a plain coil by
placing a _mechanical vibrator_ in the primary circuit, as used
on some motor cycle ignition circuits.
The object of the vibrator, of a vibrator-condenser coil, is to rapidly
make and break the primary circuit during the time the primary circuit is
closed externally. It consists of a flat steel spring secured at one end,
with the other free to vibrate. At a point about midway between its ends,
contact is made with the point of an adjusting screw, from which it
springs away and returns in vibrating. The points of contact of blade and
screw are tipped with platinum. One wire of the primary circuit is
connected to the blade and the other to the screw, hence, the circuit is
made when the blade is in contact with the screw and broken when it
springs away.
[Illustration: FIG. 142.--Construction of condenser for an induction coil.
The conducting material used is tinfoil, of which a large number of sheets
are prepared, all cut to the same size. These are placed, one on top of
the other, like the pages of a book, with a thin layer of insulating
material between, usually two sheets of paraffined paper. Numbering the
successive sheets of tinfoil serially, all sheets of even number are
connected together and all sheets of odd number are connected together,
these connections forming the terminals of the condenser. The condenser is
then connected across the break in the primary circuit.]
A condenser is used to absorb the self-induced current of the primary
winding and thus prevent it opposing the rapid fall of the primary
current.
Every conductor of electricity forms a condenser and its
capacity for absorbing a charge depends upon the extent of its
surface. Hence, a condenser is constructed of conductive
material so arranged as to present the greatest surface for a
given amount of material.
The usual form of condenser for induction coils as shown in figs. 141 and
142 is composed of a number of layers of tin foil separated by paraffin
paper, each alternate layer being connected at the ends.
Fig. 143 is a diagram of a vibrator coil. CC represents the core composed
of soft iron wires. PP is the primary winding and SS the secondary. There
is no connection between these windings and they are carefully insulated.
Y is the vibrator or _trembler_ and D the center about which it vibrates.
W is a switch used for opening and closing the primary circuit; B, a
battery of five cells. The point of adjusting screw A rests against a
platinum point R soldered upon the vibrator.
[Illustration: FIG. 143.--Diagram of a vibrator coil. The parts are as
follows: A, contact screw; B battery; C, core; D, vibrator terminal; G,
condenser; P, primary winding; S, secondary winding; W, switch; Y,
vibrator. When the switch is closed, the following cycle of actions take
place: 1, the primary current flows and magnetizes core; 2, magnetized
core attracts the vibrator and breaks primary circuit; 3, the magnetism
vanishes, including a momentary high tension current in the secondary
winding; 4, magnetic attraction of the core having ceased, vibrator spring
renews contact; 5, primary circuit is again completed and the cycle begins
anew.]
If the switch W be closed, the electric current generated by the battery B
will flow through the primary winding. This will cause the core CC to
become magnetized, and the vibrator Y will at once be drawn toward it.
This will break the connection at R. The core, being made of soft iron,
immediately upon the interruption of the current, will again lose its
magnetism, and the vibrator will return to its original position. This
again closes the circuit, after which the operation of opening and closing
it is repeated with great rapidity so long as the switch W remains closed.
[Illustration: FIG. 144.--Circuit diagram of a master vibrator coil. B, is
the battery; C, the unit coils; C1, C2, etc., the condensers; P, the
primary windings and S, the secondary windings; H1, H2, etc., the spark
plugs; T, the timer; MP, the master primary; V, the vibrator; W, the
common primary connection; 1, 2, etc., the stationary contacts of the
timer.]
The cycle of actions may be briefly stated as follows:
1. A primary current flows and magnetizes the core;
2. The magnetized core attracts the vibrator which breaks the primary
circuit;
3. The core loses its magnetism and the vibrator springs back to its
original position;
4. The vibrator, by returning to its original position, closes the primary
circuit and the cycle begins again.
[Illustration: FIG. 145.--The Splitdorf master vibrator coil. As shown in
the illustration the several unit coils are indicated by the figures 1, 2,
3, and 4. A fifth unit V at the left contains the master vibrator. The
primary wires P connect with the timer and the secondary wires S with the
plugs. B B shows the battery connections.]
=Magnetic Vibrators.=--Many types of vibrator are used on induction coils,
the most important requirement being that _the break occur with great
rapidity_. In order to render the break as sudden as possible, different
expedients have been resorted to, all tending to make the mechanism more
complicated, yet having sufficient merit in some cases to warrant their
adoption.
In the plain vibrator, the circuit is broken at the instant the spring
begins to move, hence, the operation must be comparatively slow.
In order to render the break more abrupt some vibrators have two moving
parts, one of which is attracted by the magnetic core of the coil and
moved a certain distance before the break is effected. A vibrator of this
type is shown in fig. 146 and described under the illustration.
[Illustration: FIG. 146.--A hammer vibrator. When at rest, the upward
tension of the spring, which carries the armature A, holds the platinum
points in contact and causes the upper spring C, to leave shoulder of
adjusting screw D, and rest against the heavy brass plate above it. When
the iron core B, attracts the armature A, the downward tension on the
upper spring, C, causes the latter to follow the armature down, holding
the platinum point in contact, until the end of the upper spring C,
strikes the lower shoulder of the adjusting screw, D, which gives it a
“hammer break.” The adjusting screw is held firmly in position by a bronze
spiral spring under shoulder D.]
=Vibrator Adjustment.=--When a vibrator coil is used, the quality of the
spark depends largely upon the proper adjustment of the vibrator. The
following general instructions for adjusting a plain vibrator should be
carefully noted:
1. Remove entirely the contact adjusting screw.
2. See that the surfaces of the contact points are flat, clean
and bright.
3. Adjust the vibrator spring so that the hammer or piece of
iron on the end of the vibrator spring stands normally about
one-sixteenth of an inch from the end of the coil.
4. Adjust the contact screw until it just touches the platinum
contact on the vibrator spring--be sure that it touches, but
very lightly. Now start the engine; if it miss at all, tighten
up, or screw in the contact screw a trifle further--just a
trifle at a time, until the engine will run without missing
explosions.
TABLE OF INDUCTION COIL DIMENSIONS.
Length of spark | 3/8 inch | 1/2 inch | 1 inch | 2 inches
------------------------------------------------------------------------
Size of bobbin | 2-1/8 × | 2-1/2 × | 3 × 3/8 | 4 × 2-3/4 ×
ends | 1-1/4 | 5/16 | | 3/8
------------------------------------------------------------------------
Length of bobbin | 4 | 5-1/2 | 6-1/2 | 6-1/2
------------------------------------------------------------------------
Length and | 4-1/4 × | 6 × 5/8 | 6-1/2 × 3/4 | ----
diameter of core | 7/16 | | |
------------------------------------------------------------------------
Size of base | 7-1/4 × | 9 × 5 × 2 | 14-1/2 × | 12 × 7-1/2 ×
| 3-1/4 × | | 6 × | 3-1/4
| 1-1/2 | | 1-3/4 |
------------------------------------------------------------------------
Size of tinfoil | 4 × 2 | 5-1/2 × | 6 × 4 | 6 × 6
sheets | | 3-1/4 | |
------------------------------------------------------------------------
Number of tinfoil| 36 | 40 | 40 | 60
sheets | | | |
------------------------------------------------------------------------
Size of paper | 5 × 3 | 6-1/2 × | 9 × 5 | ----
sheets | | 4-1/4 | |
------------------------------------------------------------------------
Primary coil | No. 18 | No. 18 |2 layers No. | 2 layers 14
| | |16, silk | B. W. G. silk
| | |covered. | covered.
------------------------------------------------------------------------
Secondary coil | 3/4 lb. | 1 lb. | 1-1/4 lbs. | 2-1/2 lbs.
| No. 40 | No. 40 | No. 38 | No. 36
------------------------------------------------------------------------
TABLE OF SPARKING DISTANCES IN AIR.[13]
| Distance.
Volts. | (Inches.)
-------------------------
5000 | .225
10000 | .47
20000 | 1.00
30000 | 1.625
35000 | 2.00
45000 | 2.95
60000 | 4.65
70000 | 4.85
80000 | 7.1
100000 | 9.6
130000 | 12.95
150000 | 15.00
=Points Relating to Ignition Coils.=--1. Most ignition induction coils or
“spark coils” as they are called, have terminals marked “battery,”
“ground,” etc., and to short circuit the timer for the purpose of testing
the vibrator, it is only necessary to bridge with a screw driver from the
“battery” binding post to the “ground” binding post.
2. In adjusting the vibrator of an ignition coil, the latter should not
require over one-half ampere of current.
[Illustration: FIG. 147 to 161.--Wiring diagrams showing connections of
some standard spark coils. Key: B, to battery; C, to commutator or timer;
G, to ground (engine frame); P, to plugs; S, to switch. 1, 6 terminal
standard non-vibrator coil; 2, 3 terminal standard vibrator coil; 3 and 4,
terminal standard vibrator coil; 5, standard double vibrator coil; 6,
standard triple vibrator coil; 7, standard quadruple vibrator coil; 8,
single dash coil; 9, single dash coil with switch; 10, double dash coil;
11, double dash coil with switch; 12, triple dash coil; 13, triple dash
coil with switch; 14, quadruple dash coil; 15, sextuple dash coil.]
3. A half turn of the adjusting screw on a coil will often increase the
strength of the current four or five times the original amount, hence, the
necessity of carefully adjusting the vibrator. When the adjustment is not
properly made it causes, 1, short life of the battery, 2, burned contact
points, and 3, poor running of the engine.
4. In adjusting a multi-unit coil, if any misfiring be noticed, hold down
one vibrator after another until the faulty one is located, then screw in
its contact screw very slightly.
5. The number of cells in the circuit should be proportioned to the design
of the coil.
If the coil be described by the maker as a 4 volt coil, it
should be worked by two cells of a storage battery or four dry
cells. The voltage of the latter will be somewhat higher, but
since their internal resistance is also greater, the current
delivery will be about the same. Most coils are made to operate
on from 4 to 6 volts.
6. It is a mistake to use a higher voltage than that for which the coil is
designed, because it does not improve the spark and the contact points of
the vibrator will be burned more rapidly, moreover, the life of the
battery will be shortened.
CHAPTER XII
THE DYNAMO
The dynamo is a machine which converts mechanical energy into electrical
energy by electromagnetic induction.
The word dynamo is used to designate a machine which produces _direct
current_ as distinguished from the _alternator_ or machine generating an
_alternating current_. In a broader sense, the word _generator_ is used to
denote any machine generating electric current by electromagnetic
induction; the term therefore includes both dynamos and alternators.
=Operation of a Dynamo.=--A dynamo does not create electricity, but
generates or produces an _induced electromotive force_ which causes a
current of electricity to flow through a circuit of conductors in much the
same way as a force pump causes a current of water to flow in pipes. The
electromotive force generated in the dynamo causes the current of
electricity to pass from a lower to a higher potential in the machine, and
from the higher back to the lower potential in the external circuit; that
is, the dynamo generates electrical pressure which overcomes the
_resistance_ or opposition to the current flow in the circuit. The pump
produces a mechanical pressure which, for instance, may be used to force
water into an elevated reservoir against the back pressure due to its
weight.
[Illustration: FIG. 162.--Holzer Cabot type “M” dynamo. The design of the
base is such that it allows the field ring or frame to drop down, lowering
the center of gravity, which gives increased stability. The pedestals are
bolted directly to the base. Both front and rear pedestals are removable,
so that the armature may be taken out from either end without disturbing
the brushes or connections. The journals are provided with oil rings which
keep the oil in continual circulation around the shaft by means of oil
grooves in the journal. The pole pieces are cylindrical in shape and are
fitted with shoes which retain the field coils in place and assist
commutation. The field coils are former wound, the insulation being
reinforced with mica. They are soaked in varnish and baked for 24 hours at
225° Fahr. The armature is wound as desired, series, shunt or compound.
The armature core is of the drum type and is laminated, the discs being
held by end plates locked without through bolts. The armature coils are
formed of round, ribbon or bar copper, and are without joint except at the
commutator; they lie in troughs of insulating material, the upper layers
being insulated from the lower layers; they are retained in place by maple
wedges secured by binding wires, soldered throughout their length. The
commutator segments are drop forged in the smaller, and hard drawn in the
larger sizes. Radial brushes are used. The efficiency of this type machine
is stated by the maker at from 80% to 90%, according to size.]
The point to be emphasized is that _the dynamo does not create
electricity_ (nor the pump water) _but sets into motion something already
existing by generating sufficient pressure to overcome the opposition to
its movement_.
[Illustration: FIG. 163.--General Electric 16 KW multi-polar dynamo
designed to operate at moderate and slow speeds. The outer structure of
the machine consists of a magnet frame having feet in one casting.
Adjustment is provided for moving the machine on its bed plate to tighten
the belt. The field coils are former wound and the series windings permit
of any degree of compounding up to 10% by the use of suitable German
silver shunts connected across the series field.]
=Essential Parts of a Dynamo.=--The dynamo in its simplest form consists
of two principal parts:
1. The field magnet;
2. The armature.
[Illustration: FIG. 164.--General Electric dynamo with end shield and
armature removed showing construction. The core of the armature consists
of laminations keyed to spider with space blocks inserted at intervals to
provide ventilating ducts for cooling the core and windings. The armature
is _former wound_--that is, the inductors are bent to the proper shape on
a form; they are, therefore, interchangeable.]
=Ques. What is the object of the field magnet?=
Ans. To provide a field of magnetic lines or lines of force to be _cut_ by
the armature inductors as they revolve in the field.
=Ques. What is an armature?=
Ans. A collection of _inductors_ mounted on a shaft and arranged to rotate
in a magnetic field with provision for collecting the currents induced in
the inductors.
A simple loop or turn or wire may be considered as the simplest
form of armature.
=Ques. How do armatures and field magnets differ in dynamos and
alternators?=
Ans. A characteristic feature is that in the dynamo the field magnet is
the stationary part and the armature the rotating part, while in the
alternator the reverse conditions usually obtain.
=Ques. With respect to this feature, what names are sometimes given to the
armature and field magnet?=
Ans. The _stator_ and the _rotor_ depending on which moves.
=Ques. What is the real distinction between an armature and a field
magnet?=
Ans. The name field magnet is properly given to that part which, whether
stationary or revolving, _maintains its magnetism steady during
operation_; the name armature is properly given to that part which,
whether revolving or fixed, _has its magnetism changed in a regularly
repeated fashion when the machine is in motion_.
=Construction of Dynamos.=--In the make up of a dynamo, as actually
constructed, there are five principal parts, as follows:
1. Bed plate;
2. Field magnets;
3. Armature;
4. Commutator;
5. Brushes.
CHAPTER XIII
THE DYNAMO: BASIC PRINCIPLES
A dynamo is a machine for converting mechanical energy into electrical
energy, by means of electromagnetic induction, the amount of electric
energy thus obtained depending upon the mechanical energy originally
supplied.
The word dynamo is properly applied to a machine which generates[14]
direct current, as distinguished from the alternator, which generates
alternating current.
=Ques. Define a dynamo with respect to its principle of operation.=
Ans. A dynamo is _a machine for filling and emptying conducting loops with
magnetic flux, and utilizing the electromotive force thus induced in them
for the production of current in the external circuit_.
The fitness of this definition is apparent, having in mind the
principles of electromagnetic induction.
=Ques. What are the three essential parts of a dynamo?=
Ans. The field magnet, armature, and commutator.
=Ques. What is the object of the field magnet?=
Ans. To provide a magnetic field, through which the conducting loops
arranged on a central hub and forming the _armature_ are carried, or the
flux carried through them, so that they are successively filled and
emptied of magnetic lines.
=Ques. What is a commutator?=
Ans. A device for causing the alternating currents generated in the
armature to flow in the same direction in the external circuit.
=Ques. Upon what does the voltage depend?=
Ans. Upon the _rate_ at which each conducting loop is filled and emptied
of lines of force and the number of such loops with their grouping or
connection.
=Ques. How is the operation of a dynamo best explained?=
Ans. By considering first the action of the simplest form of current
generator, or elementary alternator.
=Ques. Describe an elementary alternator.=
Ans. It consists, as shown in fig. 165, of a single rectangular loop of
wire A B C D, one end being attached to a ring F and the other to the
shaft G, and arranged so as to revolve around the axis X X′, which is
located midway between the two poles of the magnet. Two metallic strips or
_brushes_ M and S connected with the external circuit, bear on the ring F
and shaft G, respectively, in order to “collect” the current generated in
the armature when the machine is in operation. The long, straight,
horizontal arrows joining the two poles of the magnet, represent the
_lines of force_ which make up the magnetic field between the poles. The
field is here assumed to be uniform, as indicated by the equal spacing of
the arrows.
=Ques. What happens when the loop is rotated?=
Ans. According to the law of electromagnetic induction, when the loop is
rotated around its horizontal axis in the direction indicated by the
curved arrow, an electromotive force will be induced in the loop, the
magnitude of which depends on the _rate_ of change of the number of lines
of force threading through, or embraced by the loop.
[Illustration: FIG. 165.--Simple elementary alternator. Its parts are a
single conducting loop, A B C D, placed between the poles of a permanent
magnet, and having its ends connected with a ring, F, and shaft, G, upon
which bear brushes M and S, connected with the external circuit. When the
loop is rotated clockwise the induced current will flow in the direction
indicated by the arrows during the first half of the revolution.]
That is, if the number of lines embraced by the loop be
increased from, say, 0 to 1000, or decreased from 1000 to 0, in
one second, the electromotive force generated will be two times
as great as if the increase or decrease were only 500 lines per
second.
=Ques. Upon what does the direction of the induced current depend?=
Ans. Upon the direction of the lines of force and direction of rotation of
the loop.
=Ques. How is Fleming’s rule applied to determine the direction of
current?=
Ans. In applying this rule, the horizontal portion of the loop, such as A
B or C D (fig. 165), is to be considered as moving up or down; that is,
the component of its motion at right angles to the lines of force is taken
as the direction of motion. When the loop is in the position A B C D, such
that its plane is vertical or perpendicular to the lines of force, the
maximum number of magnetic lines thread through it, but when it is in a
horizontal position, A′ B′ C′ D′, so that its plane is parallel to the
lines of force, no lines pass through the loop. During the rotation from
position A B C D to A′ B′ C′ D′, the number of lines passing through the
loop is _reduced_ from the maximum to zero, the reduction taking place
with _increasing rapidity_ as the loop approaches the horizontal position,
the electromotive force thus induced _increasing in like proportion_.
Continuing the rotation from the horizontal position A′ B′ C′ D′ to the
inverted vertical position A B C D (fig. 166), the number of lines passing
through the loop is increased from zero to the maximum, the increase
taking place _with decreasing rapidity_ as the loop approaches the
inverted vertical position, the electromotive force thus induced
_decreasing in like proportion_.
=Ques. How does the current flow during the first half of the revolution
of the loop?=
Ans. It flows in the direction A B C D (fig. 165), as is easily
ascertained by aid of Fleming’s rule.
[Illustration: FIG. 166.--Simple elementary alternator, showing reversal
of current when the loop has made one half revolution from the position of
fig. 165. It should be noted that A B, for instance, which has been moving
_downward_ during the first half of the revolution (fig. 165), moves
_upward_ during the second half (fig. 166); hence, the current during the
latter interval flows in the opposite direction.]
=Ques. What is the path of the current to the external circuit?=
Ans. It flows out through brush M (fig. 165) and returns through brush S,
thus making M positive and S negative.
=Ques. What occurs during the second half of the revolution?=
Ans. The wire A B (fig. 166), which before was moving in a downward
direction, moves in an upward direction; hence, the current is reversed
and flows around the loop in the direction A D C B (fig. 166), going out
through brush S and returning through brush M. This makes M negative and S
positive.
[Illustration: FIG. 167.--Illustrating the increase and decrease in the
rate magnetic lines are cut by a revolving loop. The initial position of
the loop is taken at right angles to the direction of the lines of force.
Since the loop rotates at a constant speed, it is evident that it does not
cut the magnetic lines at uniform rate, because the intercepted arcs 0-1,
1-2, etc., are unequal. These arcs, rectified at the right by the
horizontal lines 0-1, 1-2, etc., show more clearly the increase and
decrease in the rate at which the magnetic lines are cut.]
=Ques. What may be said of the electromotive force during the second half
of the revolution?=
Ans. It varies in a similar manner as in the first half of the revolution;
that is, the magnetic lines are cut _with increasing rapidity_ during the
third quarter, _and with decreasing rapidity_ during the fourth quarter
of the revolution, which causes the electromotive force to increase and
decrease during these intervals.
The cycle of events just described may be summed up as follows: During the
revolution of the loop:
1. From 0° to 90°, the electromotive force increases from 0 to maximum;
2. From 90° to 180°, the electromotive force decreases from maximum to
zero;
3. From 180° to 270°, current reverses and the electromotive force
increases from zero to maximum;
4. From 270° to 360°, the electromotive force decreases from maximum to
zero.
It was stated that, during the revolution of the loop, the
magnetic lines were cut “with increasing or decreasing
rapidity,” causing the electromotive force to rise or fall. The
reason for this is illustrated in fig. 167. The loop is here
shown in a horizontal position at right angles to the direction
of the magnetic field; the latter, as indicated by the even
spacing of the vertical arrows representing the magnetic lines,
is assumed to be uniform.
The wire C D of the loop, as it rotates at _constant speed_,
cuts the magnetic lines at the points 0, 1, 2, 3, etc., but the
distances 0-1, 1-2, 2-3, etc., between these points, are
unequal; that is, the wire C D travels farther in cutting the
lines 0 and 1, than it does in cutting 1 and 2, and still less
in cutting the lines 2 and 3. After cutting the line 4, which
passes through the axis of revolution, the opposite conditions
obtain.
If the arcs 0-1, 1-2, etc., of the dotted circle, which are
intercepted by the magnetic lines and passed through by the
wire, be rectified and laid down under each other, as lines 0-1,
1-2, etc., the time of passage of the wire between successive
magnetic lines will vary as the length, since the speed is
uniform. Thus the wire in passing from line 0 to line 1, takes
much more time than in passing from 1 to 2, as indicated at the
left of the figure by 0-1 and 1-2, and still less in passing
from 2 to 3; that is, the rate of cutting the lines increases as
C D rotates from 0 to 4 and decreases from 4 to 8.
Since similar conditions prevail with respect to A B, for its
corresponding movement, it is evident that the number of lines
which thread through the loop are _decreased with increasing
rapidity_ as the loop rotates through the first quarter of a
revolution, and _increased with decreasing rapidity_ during the
second quarter of the revolution. Moreover, it must be evident
that the reverse conditions obtain for the third and fourth
quarters of the revolution.
=The Sine Curve.=--In the preceding paragraph it was shown that an
alternating current is induced in the armature of either an alternator or
dynamo; that is, the current: 1, begins with zero electromotive force, 2,
rises to a maximum, 3, decreases again to zero, 4, increases to a maximum
in the opposite direction, and 5, decreases to zero.
[Illustration: FIG. 168.--Application and construction of the sine curve.
The sine curve is a wave-like curve used to represent the changes in
strength and direction of an alternating current. An elementary alternator
is shown at the left to illustrate the application of the sine curve to
the alternating current cycle. It consists of a loop of wire A B C D,
whose ends are attached to the ring F and shaft G, being arranged to
revolve in a uniform magnetic field indicated by the vertical arrows which
represent magnetic lines at equidistances. The alternating current induced
in the loop is carried to the external circuit through the brushes M and
S. Now, as the loop rotates, the induced electromotive force will vary in
such a manner that its _intensity at any point of the rotation is
proportional to the sine of the angle corresponding to that point_, this
is represented by the wave-like curve. The mean value of the sine curve,
or _average electromotive force_ developed during the revolution, or
_period_, is equal to 2 ÷ π, or .637 of that of the maximum ordinate,
that is, average electromotive force = .637 × _amplitude_. The sine curve
lies above the horizontal axis during the first half of the revolution and
below it during the second half, which indicates that the current flows in
one direction for a half revolution and in the opposite direction during
the remainder of the revolution.]
A wave-like curve, as shown in fig. 168, is used to represent these
several changes, in which the horizontal distances represent time, and the
vertical distances, the varying values of the electromotive force. It is
called the sine curve because a perpendicular at any point to its axis is
proportional to the sine of the angle corresponding to that point.
=Ques. Describe the construction and application of the sine curve.=
Ans. In fig. 168, at the left, is shown an elementary armature in the
horizontal position, but at right angles to the magnetic field. The dotted
circle indicates the circular path described by A B or C D during the
revolution of the loop. Now, as the loop rotates, the induced
electromotive force will vary in such a manner that _its intensity at any
point of the rotation is proportional to the sine of the angle
corresponding to that point_. Hence, on the horizontal line which passes
through the center of the dotted circle, take any length, as 08, and
divide it into any number of parts representing fractions of a revolution,
as 0°, 90°, 180°, etc. Erect perpendiculars at these points, and from the
corresponding points on the dotted circle project lines parallel to 08;
the intersections with the perpendiculars give points on the sine curve.
Thus the loop passes through 2 at the 90° point of its revolution, hence,
projecting over to the corresponding perpendicular gives 2 2′, a point
whose elevation from the axis is proportional to the electromotive force
at that point. In like manner other points are obtained, and the curved
line through them will represent the variation in the electromotive force
for all points of the revolution.
At 90°, the electromotive force is at a maximum; hence, by using
a pressure scale such that the length of the perpendicular 2 2′
for 90° will measure the maximum voltage the length of the
perpendicular at any other point will represent the actual
pressure at that point.
The curve lies above the horizontal axis during the first half
of the revolution, and below it during the second half, which
indicates that the current flows in one direction for a half
revolution and in the opposite direction during the remainder of
the revolution.
The application of the sine curve to represent the alternating cycle, is
further illustrated in figs. 169 to 173, which show the position of the
armature at each quarter of the revolution.
[Illustration: FIGS. 169 to 173.--The sine curve with view of armature for
each 90° of the revolution, showing progressively the application of the
sine curve to the alternating current cycle.]
In fig. 179, the loop A B C D is in the vertical position at the
beginning of the revolution. At this instant the electromotive
force is zero, hence the sine curve as shown begins at E, the
zero point--that is, on the axis or line of no pressure.
As soon as the loop rotates out of the vertical plane, the
electromotive force rises and the current begins to flow in the
direction indicated by the arrows, going out to the external
circuit through brush M, and returning through brush S.
Continuing the rotation, the electromotive force increases in
proportion to the sine of the angle made by the plane of the
loop with the horizontal, until the loop comes into the
horizontal position illustrated in fig. 170. This increase is
indicated by the gradual rise of the sine curve from E to F. The
loop has now made one quarter of a revolution and the
electromotive force reached its maximum value.
As the loop rotates past the horizontal position of fig. 170,
the electromotive force gradually decreases in intensity,
reaching the zero point at the end of the second quarter--that
is, when the loop has turned one half revolution. This is
indicated by the gradual fall of the curve from F to G.
When the loop turns out of the vertical position shown in fig.
171 the current reverses, because the movement of A B and C D is
reversed; at this instant the brush M becomes negative, and S
positive. This reversal of current is indicated by the curve
falling _below_ the axis from G to I.
During the second half of the revolution, figs. 171 to 173, the
changes that occur are the same as in the first half, with the
exception that the current is in the reverse direction; these
changes are as shown by the curve from G to I.
CHAPTER XIV
THE DYNAMO: CURRENT COMMUTATION
=How the Dynamo Produces Direct Current: The Commutator.=--The essential
difference between an alternator and a dynamo is that the alternator
delivers alternating current to the external circuit while the dynamo
delivers direct current. In both machines, as before stated, alternating
currents are induced in the armature, but the kind of current delivered to
the external circuit depends on the manner in which the armature currents
are collected.
In the case of an alternator, the method is quite simple. As
previously explained, each end of the loop is connected with an
insulated collector ring carried by the shaft, the current being
collected by means of brushes which bear against the rings. This
principle, rather than the actual construction, is shown in the
preceding illustrations. Its important point, as distinguished
from other methods of collecting the current, is that _each end
of the loop is always in connection with the same brush_.
=Ques. How is direct current obtained in a dynamo?=
Ans. A form of switch called the _commutator_ is placed between the
armature and the external circuit and so arranged that it will reverse the
connections with the external circuit at the instant of each reversal of
current in the armature.
=Ques. How is a commutator constructed?=
Ans. It consists of a series of copper bars or segments arranged side by
side forming a cylinder, and insulated from each other by sheets of mica
or other insulating material.
[Illustration: FIGS. 174 to 178.--Commutation of the current. These
figures show how a dynamo transforms alternating into the so-called direct
current. During the first half of the revolution the current flows in the
direction A B, out through segment F of the commutator and brush M,
returning through brush S and segment G, figs. 174 and 175. At the
beginning of the second half of the revolution, fig. 176, the current in
the armature reverses and flows around the loop in the direction B A. At
this instant the brushes M and S pass the gaps between the commutator
segments, thus reversing contact with the segments, and causing the
current in the external circuit to remain in the same direction.]
=Ques. Where is the commutator placed?=
Ans. It is attached to the shaft at the front end of the armature.
=Ques. What are inductors?=
Ans. The insulated wires wound on the armature core, and in which the
electric current is induced.
=Ques. How are the inductors connected to the commutator?=
Ans. The ends of each conducting loop or coil must be connected with the
commutator segments in a certain order to correspond with the type of
winding.
=Ques. Explain in detail how direct current is obtained in a dynamo.=
[Illustration: FIGS. 179 to 181.--Elementary dynamo armatures. Fig. 1,
single turn loop; fig. 2, coil of two turns _in series_; fig. 3, coil of
two turns _in parallel_. In operation the amplitude or maximum pressure
induced with the two turn coil, fig. 180, is double that of a single turn
loop, fig. 179. In fig. 180, the pressure is double that induced in fig.
181, while the amount of current generated with series turns, fig. 180, is
only half that generated with turns in parallel fig. 181.]
Ans. It will be easily seen by the aid of a series of illustrations just
how the alternating armature currents are transformed into direct current.
Figs. 174 to 178 show, in several positions, a single loop of wire with
its ends joined to a commutator; the latter has only two segments, one for
each end of the loop. In fig. 174 the loop is shown in the vertical
position, and it should be noted that the division between the two
segments forming the commutator is in the same plane as the loop. When the
loop is in the vertical position, as shown in fig. 174, brush M is in
contact with segment F, and S with G. As the armature rotates, the current
flows for one half revolution in the direction A B, through segment F and
out to the external circuit through brush M as shown in figs. 174 and 175,
returning through brush S and segment G. At the beginning of the second
half of the revolution, fig. 176, the current in the loop reverses and
flows in the opposite direction B A as indicated by the arrows. At this
instant, however, the brushes M and S pass out of contact with segments F
and G, and come into contact with G and F respectively; that is, M leaves
F and contacts with G, while S leaves G and contacts with F. The effect of
this is _to reverse the connections with the external circuit at the
instant the alternation or reversal of current in the armature takes
place_, thus keeping the current in the external circuit in the same
direction.
[Illustration: FIG. 182.--Gramme ring armature with one coil, and
characteristic sine curve below. With one coil as shown, there are two
pulsations of the current per revolution of the armature.]
=Ques. How is this indicated by the sine curve?=
Ans. The sine curve, instead of falling below the axis, as in figs. 169 to
173, again rises as in the first half of the period, that is G′H′I′ is
identical with E′F′G′.
=Ques. Is the direct current indicated by the sine curve in figs. 174 to
178 continuous?=
Ans. _No_; it is properly described as a _pulsating current_, or one,
constant in direction, but periodically varying in intensity so as to
progress in a series of throbbings or pulsations instead of with uniform
strength.
=Ques. What is generally understood by the word “continuous” as applied to
the current obtained from a dynamo?=
Ans. It is usually accepted as meaning a steady or non-pulsating direct
current; one that has a uniform pressure and constant direction of flow as
opposed to an alternating current.
=Ques. Is a continuous current ever obtained with a dynamo?=
Ans. _No._
[Illustration: FIG. 183.--Gramme ring armature with two coils placed 180°
apart. This arrangement gives double the pressure of the one coil
armature, fig. 182.]
It should be clearly understood at the outset that it is
impossible to obtain a continuous current with a dynamo. The
so-called continuous current which it is said to produce is in
reality a pulsating current, but with pulsations so minute and
following each other with such rapidity that the current is
practically continuous, and as such is generally called
continuous.
=Ques. How is the so-called continuous current produced by a dynamo?=
Ans. In order to obtain a large number of small pulsations per revolution
of the armature instead of two large pulsations, as with the single loop
armature, the latter must be replaced by one having a great number of
loops properly connected to commutator segments and so arranged that the
successive loops begin the cycle progressively.
* * * * *
The difficulties encountered in connecting up numerous loops were overcome
by Gramme, who, in 1871 invented a “ring” armature. His method consists in
winding a ring with a continuous coil of wire, connections being made at
suitable intervals with the commutator.
[Illustration: FIG. 184.--Four separate coils wound on ring to illustrate
the action of a Gramme ring armature. If the ring be rotated the
electromotive forces induced in adjacent coils will be equal and tend to
produce currents in opposite directions; hence, if the inner ends be
joined, the junctions would be at a higher potential (+ or -) than the
loose ends. With proper connections current may be collected at the
junctions.]
In order to understand the action of such an arrangement, it will be well
to first consider four separate coils wound on a ring as shown in fig.
184. These coils are all similar, but at the moment occupy different
magnetic positions on the ring. The rotation being clockwise, 1 is about
to enter the field adjacent to the north pole, while 2 is emerging from
the field in the region of the south pole. Again, 3 is approaching the
south pole and 4 receding from the north pole.
=Ques. Describe in detail the action of the four coils wound around the
ring as in fig. 184.=
Ans. According to the laws of electromagnetic induction, pressures are set
up at the ends of the coils such as tend to produce currents in the
directions indicated by the arrows. Now, assuming the electromotive forces
in coils 1 and 2 to be equal, if the adjacent ends be joined, no flow of
current will take place, but the junction will be at a higher pressure
than the loose ends of the coils and if a wire be attached to this
junction, and the necessary circuits completed, a current will flow along
the wire outward from the junction. Similarly, if the adjacent ends of
coils 3 and 4 be joined, there will be no flow of current, but the
junction will be at a lower pressure than the loose ends, and if a wire be
attached to the junction and the necessary circuits completed, current
will flow from the junction around the coils.
[Illustration: FIG. 185.--Gramme ring armature with four coils. The
electromotive force induced in coils A, A′ reaches the zero point at the
instant that of coils B, B′ is at a maximum; hence, sine curve No. 1,
beginning at zero, and No. 2, at the maximum, show the pressure changes
for A, A′ and B, B′, respectively. The summation of these curves gives
_the resultant curve_ No. 3, showing changes in pressure of current
delivered to the external circuit.]
=Ques. What may be said with respect to the four coil Gramme ring armature
shown in fig. 185?=
Ans. According to the laws of electromagnetic induction, with the north
pole of the field at the left and clockwise rotation, the induced currents
flow _upward_ on both sides of the ring, hence, _the electromotive forces
oppose each other at only two of the junctions, namely: at the one
connected to brush M where the pressures on either side are both directed
toward the junction and the other at the junction connected to brush S, at
which the pressures are both directed from the junction._
[Illustration: FIG. 186.--Gramme ring armature with six coils. The sine
curves 1, 2 and 3, represent the conditions due to coils AA′, BB′ and CC′,
respectively, and 4, the resultant pulsations.]
It is evident, then, that the pressure at M is higher than at S;
that is, M is positive and S negative; consequently, the current
flows from M to the external circuit and returns through S.
=Ques. In what other way may the four coils of the armature in fig. 185 be
regarded?=
Ans. They may be considered as two pairs A A′ and B B′, the action of
either pair being identical with the two coil armature shown in fig. 183;
this, in turn, produces the same effect as the one coil armature of fig.
182, with the exception that the amplitude of the current generated with
two coils is twice as great as that with one coil of the same number of
turns.
Again considering the action of the four ring coil shown in fig. 185, and
starting at the beginning of the revolution, the variation of
electromotive force induced in coils AA′ is indicated by the dotted sine
curve 1, and of BB′ by dotted curve 2. It will be seen that 1 begins at
the axis or line of no pressure, and 2 at maximum pressure.
[Illustration: FIG. 187.--The resultant curves of figs. 183, 185 and 186
are here shown for comparison to illustrate the approach to uniform
pressure as the number of coils are increased. It should be noted that the
number of pulsations per cycle depends on the number of coils, and that as
the pulsations increase in number, the variation in pressure decreases.]
The two curves overlap each other, and in order to determine the
effect of this it is necessary to trace the resultant curve, 3.
This is easily done, as the resultant electromotive force
induced at any point in the revolution of the armature is equal
to the sum of the pressures induced in AA′ and BB′. Thus, at the
beginning of the revolution the pressure induced in AA′ is at
zero point, and in BB′ at its maximum J, hence, the resultant
curve begins at the point J. Again, for any point in the
revolution, as N, the height of the resultant curve is equal to
NP + NT = NV. For 45° or 1/8 revolution, the resultant curve
reaches its amplitude, which is equal to 2 × RZ = RW, and at 90°
it again reaches its minimum, XY.
=Ques. State the conditions upon which the steadiness of the current
depends.=
Ans. _It depends on the number of coils and the manner in which they are
connected._
Comparing curves 1 and 3, in fig. 185, it will be noted that
with four coils the variation of pressure or amplitude of the
pulsations is less than half that obtained with two; moreover,
with four coils the number of pulsations per cycle is doubled.
In order to further observe the approach to continuous current
obtained by increasing the number of coils, the effect of a six
coil armature is shown in fig. 186, the resultant curve being
obtained in the same manner as just explained. For comparison,
the curves for the three cases of two, four, and six coils are
reproduced under each other in fig. 187.
As the number of coils is further increased, the amplitude of
the pulsations decreases so that the resultant curve approaches
nearer the form of a straight line.
In the actual dynamo there are a great many coils, hence the
amplitude of the pulsations is exceedingly small; accordingly,
it is customary to speak of the current as “continuous,”
although as previously mentioned such is not the case.
CHAPTER XV
CLASSES OF DYNAMO
In order to adapt the dynamo to the varied conditions of service, its
design is modified in numerous ways, giving rise to the different “types.”
These may be classified with respect to:
1. Field magnets;
2. Field excitation;
3. Field winding.
The first division relates to the number of magnetic poles, as unipolar,
bipolar, and multi-polar dynamos; also inter-polar dynamos. Under the
second division are included the following:
1. _Self-exciting machines_ of which the magneto is the simplest. Its
magnetic field is obtained from permanent magnets, hence the electromotive
force generated is comparatively small. The more important type of
self-exciting machine is provided with electromagnets in which the field
of force is “built up” from the residual magnetism of the soft iron or
steel cores of the field magnets of the dynamo itself. Nearly all
commercial types of dynamo are of this class.
2. _Separately excited machines_ in which the field magnets are magnetized
when the machine is in operation by current supplied from a separate
source such as a battery or magneto generator.
With respect to the third division, based on the field winding, dynamos
are classed as:
1. Series wound;
2. Shunt wound;
3. Compound wound.
In addition to the foregoing there are further distinctions with
respect to the mechanical features. Most dynamos have a
revolving armature and stationary field magnets; however, in
some cases, both the armature and field magnets are stationary,
a revolving iron inductor being provided to intercept the
magnetic lines intermittently which produces the same effect as
is obtained in cutting the magnetic lines by a revolving
armature.
=Ques. What may be said of bipolar and multi-polar dynamos?=
Ans. Dynamos with bipolar field magnets were universally used prior to
1890, but since that time machines of this type are only made in very
small sizes; the multi-polar dynamo is the type now in general use.
=Ques. State some of the features of the multi-polar dynamo.=
Ans. In this class of machine, the armature and field magnets are
surrounded by a circular frame, or _ring yoke_ to which the field magnets
are attached. This ring arrangement has the advantages of strength,
simplicity, symmetrical appearance, and minimum magnetic leakage, since
the pole pieces have the least possible surface and the path of the
magnetic flux is shorter.
=Ques. What important advantage is gained by the use of multi-pole field
magnets?=
Ans. Commercial voltages are obtained at moderate armature speed.
The difficulty experienced with bipolar machines is that, with a
dynamo of large output, the speed at which its armature would
have to rotate to generate commercial voltages would be
excessive.
[Illustration: FIGS. 188 and 189.--Circuit diagrams to illustrate the
difference between a dynamo and a magneto. The former has its field
magnets F F magnetized by means of a small current flowing around a shunt
circuit. In a magneto the field magnets are permanently magnetized. The
strength of the magnet field of a magneto is constant while that of a
dynamo varies with the output.]
It is evident that with two or more magnetic fields, secured by
increasing the number of poles, the armature inductors revolving
between them cut more magnetic lines in one revolution than with
a single field, hence, a given voltage is obtained with less
speed of the armature than in the bipolar machine.
For instance, if a bipolar dynamo be required to run at say 900
revolutions per minute to generate 125 volts, a four pole
machine of equal output will require only 450 revolutions, and
one of eight poles only 225 revolutions per minute.
=Ques. What is a self-exciting dynamo?=
Ans. A machine in which the initial excitation of the field is due to the
residual magnetism retained by the cores.
=Ques. What may be said of the field due to this residual magnetism?=
Ans. It presents a very weak field, and the voltage that could be
generated by the armature revolving in such a field would be only about
two to ten volts.
[Illustration: FIG. 190.--Series wound dynamo, used for series arc
lighting, and as a booster for increasing the pressure on a feeder
carrying current furnished by some other generator. The coils of the field
magnet are in series with those of the armature and external circuit, and
consists of a few turns of heavy wire. The characteristic of the series
dynamo is to furnish current with increasing voltage as the load
increases. If overloaded, the voltage will drop.]
=Ques. How then can commercial voltages such as 100 or more volts be
obtained with a self-exciting dynamo?=
Ans. Part or all of the current induced in the armature is passed through
the windings of the field magnets, thus strengthening the field. The
voltage, therefore, will “build up,” increasing until the maximum has been
reached.
The maximum voltage will depend upon the capacity of the field
magnets as determined by the construction, and upon the strength
of current used to excite them.
=Ques. How long does the process of “building up” require?=
Ans. The time required to fully excite the field magnets is from ten to
twenty seconds, the rise in field strength being indicated on the
voltmeter or by the gradual increase in the brilliancy of the _pilot
lamp_.
=Ques. Name three important classes of dynamo.=
Ans. Series wound, shunt wound, and compound wound.
=Ques. Describe the winding of a series dynamo.=
Ans. In this machine, the field magnets are wound with a few turns of
thick wire joined in series with the armature brushes as shown in fig.
190.
=Ques. What is the effect of this arrangement?=
Ans. All of the current generated by the machine passes through the coils
of the field magnets to the external circuit. The current in passing
through the field magnets, energizes them and strengthens the weak field
due to the residual magnetism of the magnet cores, resulting in the
gradual building up of the field.
=Ques. For what service is the series dynamo adapted?=
Ans. It may be used for series arc lighting, series incandescent lighting,
and as a _booster_ for increasing the pressure on a feeder carrying
current furnished by some other generator.
=Ques. What is the effect of the series winding in the operation of the
machine?=
Ans. Its characteristic is to furnish current at an increased voltage as
the load increases. If sufficient current be drawn to overload the
machine, the voltage will drop.
Since the armature coils, field magnets and external circuits
are in series, any increase in the resistance of the external
circuit lessens the power of the machine to supply current,
because it diminishes the current in the coils of the field
magnets and therefore diminishes the effective magnetism. Again,
a decrease in the resistance of the external circuit will
increase the voltage because more current will flow through the
field magnets. Accordingly, when the external circuit has lamps
in series (as is common in an arc light circuit) the switching
on of an additional lamp both adds to the resistance of the
circuit and diminishes the power of the machine to supply
current. When the lamps are in parallel, the switching on of
additional lamps not only diminishes the resistance of the
circuit, but causes the field magnets to be further excited by
the increased current, so that the greater the number of lamps
put on, the greater becomes the risk of inducing too much
current.
The series dynamo has also the disadvantage of not starting
action until a certain speed has been attained, or unless the
resistance of the external circuit be below a certain limit.
=Regulation of Series Dynamos.=--The series dynamo is ordinarily used for
operating arc lamps connected in series. The current generally consumed is
about 10 amperes, and it is necessary that it should remain at this
strength to keep the lights burning steadily. If it increase, the lights
will be too bright, and if it decrease, they will be too dim or flicker.
With all the lamps connected in series it is evident that the resistance
of the circuit will vary widely as they are turned on or off, the
resistance increasing as the lamps are turned on, and decreasing as they
are turned off. It is necessary, therefore, that some means of regulation
be provided to enable the dynamo to increase or decrease the voltage in
proportion to the load. There are several methods of regulation, as by:
1. Variation of armature speed;
2. Variation of position of brushes;
3. Variation of field strength.
Whatever method be used the necessary regulation should be accomplished by
automatic devices, as it would not be practical to station a man in
constant attendance to regulate the voltage every time one or more lamps
were thrown on or off.
=Ques. When is the first method of regulation used?=
Ans. It is only used in special cases, as for constant load; if the
voltage be not just right to give the required current, it may be adjusted
by changing the speed of the engine.
=Ques. What may be said of the second method?=
Ans. In both the “ring” and “drum” types of armature, rotating in a
bipolar field, there are two points situated at opposite extremities of a
diameter of the commutator, at one of which the potential is a maximum and
at the other a minimum, and it is at these points that the brushes must be
placed in order to obtain the greatest difference of pressure, the
difference being less at other points. Hence, by rocking the brushes
around the commutator the pressure at the terminals of the machine may be
varied and regulated as required.
=Ques. What difficulty is experienced in rocking the brushes to regulate
the voltage?=
Ans. Sparking takes place at the brushes when they are moved any
considerable distance from the neutral position.
Special dynamos have been designed to overcome this
objectionable feature, still this method of regulation is not
extensively used.
=Ques. What may be said of the third method of regulation?=
Ans. The third method, that of variation of field strength, is the one in
general use.
=Ques. How is the field strength varied?=
Ans. This may be done by the _two path method_, or by the _variable field
coil method_.
=Ques. Describe the two path method of field regulation.=
Ans. An adjustable resistance or _rheostat_ is connected in parallel with
the field winding as shown in fig. 191. This shunts more or less of the
current from the field winding according to the amount of resistance made
active by the lever, _L_.
Thus, if the current in the armature and main circuit be 10
amperes and the resistance of the field winding 10 ohms, a
resistance of 40 ohms in parallel with the winding would cause
the current to split in the ratio of 40 to 10, or 4 to 1; 2
amperes would pass through the resistance and 8 amperes through
the field.
[Illustration: FIG. 191.--The two path method of regulating a series
dynamo. The ends of the series winding are connected by a shunt containing
a rheostat. The current induced in the armature, divides and flows through
the two paths thus offered, the amount flowing through the shunt being
regulated by the rheostat. In this way the field strength is easily
regulated.]
[Illustration: FIG. 192.--Regulation of series dynamo by variable field. A
multipoint switch is provided with connections to the field winding at
various sections, thus permitting more or less of the field winding to be
cut out to regulate its strength.]
=Ques. Describe the variable field coil method of field regulation.=
Ans. This consists in dividing the field winding into a number of sections
and throwing the sections in and out of circuit as shown in fig. 192.
Since the strength of any magnet depends on the number of ampere
turns in its field winding, reducing or increasing the number of
turns will respectively reduce or increase the field strength,
the current being kept constant.
=Ques. What is the objection to this method?=
Ans. This arrangement is undesirable for magnets of large size, because of
the tendency to flashing at the contacts of the regulating switch.
[Illustration: FIG. 193.--Shunt wound dynamo for parallel circuit
incandescent lighting, and for mill and factory power. The coils of the
field magnet form a shunt to the main circuit; they consist of many turns
of fine wire and consequently absorb only a small fraction of the current
induced in the armature. The characteristic of the shunt dynamo is that it
gives practically constant voltage for all loads within its range. If
overloaded the pressure will drop and the machine cease to generate
current.]
=The Shunt Dynamo.=--The shunt wound dynamo differs from the series wound
machine, in that an independent circuit is used for exciting its field
magnet. This circuit is composed of a large number of turns of fine
insulated copper wire, which is wound round the field magnets and
connected to the brushes, so as to form a shunt or “by pass” to the
brushes and external circuit, as shown in fig. 193. Two paths are thus
presented to the current as it leaves the armature, between which it
divides in the inverse ratio of the resistance. One part of the current
flows through the magnetizing coils, and the other portion through the
external circuit.
In all well designed shunt dynamos, the resistance of the shunt circuit is
always very great, as compared with the resistance of the armature and
external circuit, the strength of the current flowing in the shunt coils
being very small even in the largest machines.
=Ques. For what service is the shunt dynamo adapted?=
Ans. It is used for constant voltage circuits, as in incandescent
lighting.
=Ques. In the operation of a shunt dynamo what is its characteristic
feature?=
Ans. The voltage at the dynamo remains practically unchanged, and the
current varies according to the load.
=Ques. Does the voltage remain constant for all loads?=
Ans. There is a certain maximum load current that the shunt dynamo is
capable of supplying at constant voltage; beyond this, the voltage will
decrease, the machine finally demagnetizing itself, and ceasing to
generate current.
=Ques. Why does the voltage not remain constant for all loads?=
Ans. Because there is a drop in the voltage in forcing the current through
the armature windings which increases with the load.
=Ques. What is the usual method of regulation for shunt dynamos?=
Ans. The method of varying the current through the field coils by means of
a rheostat inserted in series with the field winding as shown in fig.
194.
Moving the lever L of the rheostat to the right increases the
resistance in series with the field winding, and this reduces
the amount of current in that winding, thus reducing the
strength of the magnet and consequently the voltage at the
brushes. The contrary movement of the lever, by cutting out the
resistance, produces the opposite effect.
=The Compound Dynamo.=--This class of generator is designed to
automatically give a better regulation of voltage on constant pressure
circuits than is possible with a shunt machine. It possesses the
characteristics of both the series and shunt machines, of which it is in
fact a combination.
[Illustration: FIG. 194.--Regulation of shunt dynamo by method of varying
the field strength. A rheostat is placed in series with the field coils,
and by varying the resistance, more or less current will flow through the
coils, thus regulating the field strength.]
The field magnets of the compound dynamo, as shown in fig. 195, are wound
with two sets of coils, one set being connected in series, and the other
set in parallel, with the armature and external circuit. The purpose of
the series winding is to strengthen the magnets by the current supplied
from the armature to the circuit, and thus automatically sustain the
pressure. If the series winding were not present, the pressure at the
terminals would fall as the load increased. This fall of pressure is
counteracted by the excitation of the series winding, which increases with
the load and causes the pressure to rise. The number of turns and relative
current strengths of the series and shunt windings are so adjusted that
the pressure at the terminals is maintained practically constant under
varying loads.
With respect to the ratio between the number of turns of the two field
windings, the dynamo is spoken of as:
1. Compound;
2. Over compounded.
=Ques. What is the difference between a compound and an over compounded
dynamo?=
Ans. In the first instance, there are just enough turns in the series
winding to maintain the voltage constant at the brushes for variable load.
If a greater number of turns be used in the series winding than is
required for constant voltage at the brushes for all loads, the voltage
will rise as the load is increased, and thus make up for the loss or drop
in the transmission lines, so that a constant voltage will be maintained
at some distant point from the generator. The machine is then said to be
_over compounded_.
=Ques. For what service is over compounding desirable?=
Ans. For incandescent lighting where there is considerable length of
transmission lines.
=Ques. What is the usual degree of over compounding?=
Ans. Generally for a rise of voltage of from five to ten per cent.
In construction, the field coils are wound with a greater number
of turns than actually required, the machine being accurately
adjusted by a running load test after completion.
=Ques. How is the degree of over compounding varied?=
Ans. A rheostat is placed in shunt with the series winding so that the
current passing through the winding may be regulated to control the
voltage of the machine.
[Illustration: FIG. 195.--Compound wound dynamo, used when better
automatic regulation of voltage on constant pressure circuits is desired
than is possible with the shunt machine. The compound dynamo is a
combination of the series and shunt types, that is, the field magnet is
excited by both series and shunt windings. With a proper selection of the
number of turns in the series coils, the voltage may be kept automatically
constant for wide fluctuations in the load. When the machine is _over
compounded_ its characteristic is to slightly increase the voltage with
increase of load, a desirable feature for long transmission lines in order
to compensate for the line drop.]
=Ques. How are the ends of the shunt winding of a compound dynamo
connected?=
Ans. There are two methods of connection, being known as the short shunt
and the long shunt.
=Ques. Describe the short shunt.=
Ans. In the short shunt, the ends of the shunt winding are connected
directly to the brushes as in fig. 196.
=Ques. Describe the long shunt.=
Ans. In the long shunt, one end of the shunt winding is connected to one
of the brushes and the other end to the terminal connecting the series
winding with the external circuit as in fig. 197.
=Ques. Which is the more desirable?=
Ans. Theoretically, the long shunt is preferable as being the more
efficient; however, in practice, the gain is not very appreciable and the
short shunt is generally used.
[Illustration: FIGS. 196 and 197.--Short and long shunt types of compound
wound dynamos. The distinction between the two is that the ends of the
short shunt connect direct with the brush terminals, while in the long
shunt type, fig. 197, one end of the shunt connects with one brush
terminal and the other with the terminal connecting the series winding
with the external circuit. R is the shunt field rheostat for regulating
the current through the shunt.]
=Ques. What may be said regarding the voltage in short, and long shunt
machines?=
Ans. In a short shunt machine, the shunt winding is subjected to a higher
voltage than with a long shunt. The pressure applied through a shunt
winding with a long shunt, for any particular load, is equal to the
voltage at the brushes plus the drop in the series winding.
=Ques. For what other service besides incandescent lighting are compound
dynamos adapted?=
Ans. They are employed in electric railway power stations where the load
is very fluctuating.
=Ques. What is the effect of a short circuit on a compound dynamo?=
Ans. It overloads the machine, since the excessive current flowing through
the series field tends to keep the voltage at its normal value.
Unless the line be automatically opened under such a condition
either by a fuse or circuit breaker, the machine and its driving
engine may be damaged. To avoid this danger fuses or automatic
circuit breakers are employed.
=Ques. Mention another service for which the compound dynamo is used.=
Ans. In some isolated plants, as small country residences where it is
frequently necessary to have a dynamo capable of charging a storage
battery during the day, and of furnishing current for lighting during a
certain portion of the evening.
Under such conditions the compound machine with slight
modification is used, the ordinary shunt dynamo not being
capable of maintaining the necessary consistency of voltage,
without attention to the shunt regulator in driving the lamps
direct, the ordinary compound dynamo on the other hand, being
unsatisfactory for charging storage batteries.
=Ques. How is the compound dynamo modified to adapt it to the dual service
of lighting and battery charging?=
Ans. It is furnished with _alternative compound winding_, in which the
series winding is provided with a switch, which may be fixed either upon
the machine itself or upon the switchboard. This switch permits the series
coils to be either short circuited in part or cut out of the circuit
entirely while the machine is charging the storage battery, being again
cut into circuit when the machine is required to furnish current for the
lamps.
[Illustration: FIG. 198.--Separately excited dynamo. Current for field
excitation is supplied by a second and smaller generator.]
=Separately Excited Dynamos=.--In this class of machine the current
required to excite the field magnets is obtained from some independent
external source. Though used by Faraday, the separately excited dynamo did
not come into favor until, in 1866, Wilde employed a small auxiliary
magneto machine to furnish currents to excite the field magnets of a
larger dynamo.
A separately excited dynamo is shown in fig. 198. This method of field
excitation is seldom used except for alternators; it is, however, to be
found occasionally in street railway power houses, the shunt fields of all
the dynamos being separately excited by one dynamo.
In common with the magneto, the separately excited machine possesses the
property that, with the exception of armature reactions, the magnetism in
its field and therefore the total voltage of the machine is independent of
variations in the load.
[Illustration: FIG. 199.--Diagram showing principle of Dobrowolski three
wire dynamo. This type of machine is shown in more detail in fig. 795 on
page 708.]
=Dobrowolski Three Wire Dynamo.=--This type of dynamo was designed to
operate a three wire system of distribution without a balancer. The
armature is provided with insulated slip rings connected to suitable
points in the armature winding and (by means of brushes) with choking
coils meeting at a common point, to which the neutral wire of the system
is connected, the main terminals being connected with the outside wires.
The machine is capable of feeding unbalanced loads without serious
disturbance of the pressure on either side of the system.
The principle of the Dobrowolski three wire dynamo is
illustrated in fig. 199. The armature A is tapped at two points,
B and B′, and connected to slip rings C C′. A compensator or
reactance coil D, between the two halves of which there is
minimum magnetic leakage, is connected to C and C′ by brushes,
and has its middle point tapped and connected to the neutral
wire E.
[Illustration: FIG. 200.--Armature of Westinghouse three wire dynamo.
Collector rings are mounted at one end of the armature as shown, and the
leads to them with the armature winding are similar to those employed on
the alternating current side of a rotary converter armature. The
connections from the armature to collector rings may be either single
phase, two phase, or three phase. The two phase connection with four
collector rings and two balance coils is used in the Westinghouse three
wire dynamo.]
It is clear, from the symmetry of the arrangement, that the
center point of the coil must always be approximately midway in
pressure between that of the brushes, and hence any unbalanced
current will return into the armature, dividing equally between
the two halves of the coil.
The arrangement forms a cheap and effective substitute for a
balancer set, but lacks the adjustable properties of the latter.
There are various modifications of the arrangement. Thus more
than two slip rings may be used. The compensator windings,
however, should always be arranged so that the magnetizing
effect of the neutral current is self-neutralized in the
windings, as otherwise saturation occurs causing a very heavy
alternating magnetizing component.
CHAPTER XVI
FIELD MAGNETS
The object of the field magnet is to produce an intense magnetic field
within which the armature revolves. It is constructed in various forms,
due in a large measure to considerations of economy, and also to the
special conditions under which the machine is required to work.
Electromagnets are generally used in place of permanent magnets on account
of: 1, the greater magnetic effect obtained, and 2, the ability to
regulate the strength of the magnetic field by suitably adjusting the
strength of the magnetizing current flowing through the magnet coils.
The field magnet, in addition to furnishing the magnetic field, has to do
duty as a framework which often involves considerations other than those
respecting maximum economy.
=The Make Up of a Field Magnet.=--In construction, the electromagnet, used
for creating a field in which the armature of a dynamo revolves, consists
of four parts:
1. Yoke;
2. Cores;
3. Pole pieces;
4. Coils.
These are shown assembled in figs. 201 to 204.
=Ques. What is the object of the yoke?=
Ans. The yoke serves to connect the two “limbs,” that is, the cores and
pole pieces, and thus provide a continuous metallic circuit up to the
faces of the pole pieces.
=Ques. How is the yoke constructed?=
Ans. It usually forms the frame of the dynamo as shown in figs. 205 and
206.
[Illustration: FIG. 201.--Salient pole, bipolar field magnet with single
coil wound around the yoke.]
=Ques. What may be said of the cores?=
Ans. The cores, which are usually of circular form, carry the coils of
insulated wire used to excite the magnets.
=Classes of Field Magnet.=--Although numerous forms of field magnet have
been devised, they can be classed into two groups according to the type of
pole, as:
1. Salient pole;
2. Consequent pole.
The distinction between these two types of pole is shown in
figs. 201 to 203. By inspection of the figures, it will be seen
that the term _salient_ applies to poles produced when the pole
pieces form the _ends_ of the magnet, as distinguished from
_consequent_ poles, or those formed by coils wound on a
continuous metal ring or equivalent.
In the salient pole bipolar magnet, the winding may be either
upon the limbs, M M fig. 202, or upon the yoke, Y as shown in
fig. 201. The magnetic circuit of salient and consequent poles
is indicated in the figures by the dotted lines.
[Illustration: FIG. 202.--Salient pole, bipolar field magnet with two
coils wound around the cores.]
[Illustration: FIG. 203.--Consequent pole, bipolar field magnet with two
coils on the cores. This is known as the “Manchester” type in which the
cores are connected at the ends by two yokes--so named from its original
place of manufacture at Manchester, England.]
=Multi-Polar Field Magnets.=--In the multi-polar machine, the subdivision
of the magnetic flux reduces the amount of material of both magnet and
armature. Moreover, there is less heating on account of the greater
capability of dissipating the heat, offered by the increased area of
surface per unit of volume in each magnet pole and winding.
[Illustration: FIG. 204.--Modern dynamo with four consequent pole field
magnets. In this construction the ring shaped yoke also serves as a frame;
the circular form of yoke gives the least chance for magnetic leakage.]
There may be four, six, eight, or more poles, arranged in
alternate order around the armature. Fig. 204 shows a four pole
field magnet having a common yoke or iron ring, with four pole
pieces projecting inwardly, and over which the exciting coils
are slipped.
In the larger machines the yoke is made in two parts bolted
together as shown in fig. 206, so that the upper portion may be
lifted off for examination of the armature.
=Ques. Can the number of poles in a multi-polar machine be advantageously
increased to 16, 32, or more?=
Ans. A large number of poles is not advisable except in very large
machines, since it involves an increase in the expense of machine work,
fittings, etc., somewhat out of proportion to the reduction in cost of
material and increase in efficiency.
=Ques. What materials are generally used for field magnets?=
Ans. Wrought iron, steel and copper.
There are a number of considerations which govern the selection
of the materials to be used in a particular machine, such as
initial cost, weight, efficiency, etc.
[Illustration: FIGS. 205 and 206.--Solid and split construction of yoke
for multi-polar dynamos. In the latter type the yoke is in two halves
joined along a horizontal diameter; while the upper half may be
conveniently removed to give access to the armature, it has the
disadvantage of the joint, which, no matter how well made, will add to the
reluctance of the magnetic circuit. The figures also illustrate the
circular and segmental forms of yoke construction.]
=Ques. In the construction of field magnets, what governs the choice of
materials?=
Ans. For cores, wrought iron is most desirable, as requiring the smallest
amount of material for a given flux. There is a saving in copper due to
using wrought iron for the core since, on account of its small size, the
length of each turn of the magnetizing coil is reduced. For heavy yokes,
where lightness is not essential, but very often the reverse, cast iron is
used, as its cross section can be made larger than that of the cores, this
increase in area serving to give strength and rigidity to the machine.
Cast steel occupies a place intermediate between cast iron and wrought
iron both in cost and magnetic properties.
[Illustration: FIGS. 207 to 209.--Various sections of cast iron yoke. In
form, these yokes may be either circular or segmental as shown in figs.
205 and 206.]
[Illustration: FIGS. 210 to 212.--Various sections of cast steel yoke. The
ribs shown in figs. 210 and 211 are provided to secure stiffness.]
=Ques. Name two forms of yoke in general use.=
Ans. The solid, and divided types as shown in figs. 205 and 206.
=Ques. What is the object of dividing a yoke?=
Ans. To permit access to the armature, where the construction does not
admit of removal of the latter from the side.
=Ques. How is the yoke usually divided?=
Ans. Across its horizontal diameter into an upper and lower half, as shown
in fig. 206, the lower half being seated on, or more frequently cast in
one piece with the bed plate.
=Ques. What is the objection to dividing a yoke?=
Ans. The joints introduced, even if carefully faced and well bolted
together, add a little reluctance to the magnetic circuit.
[Illustration: FIGS. 213 to 215.--Some methods of attaching detachable
cores. The core seat is machined to receive the core, it being necessary
to secure good contact in order to avoid a large increase in the
reluctance of the magnetic circuit.]
=Ques. How does this affect the poles adjacent to the points, and what
provision is made?=
Ans. It weakens them, and in order to overcome this, the coils of these
poles are given a few extra turns.
=Ques. How is the reluctance of a yoke joint reduced?=
Ans. By enlarging the area of contact; the flange for the bolts furnishes
the necessary increase.
=Ques. What determines chiefly the cost of field magnets?=
Ans. The material used in making the cores and their shape.
=Ques. How does this affect the cost?=
Ans. Since considerable cross sectional area of core is required, the
problem confronting the designer is to design the core by judicious
selection of material and shape, that the required number of turns in the
magnetizing coil is obtained with the shortest length of wire.
[Illustration: FIGS. 216 to 221.--Comparison of field magnet core
sections. The shorter the perimeter or outside boundary of the core for a
given cross sectional area, the less will be the amount of copper required
for the magnetizing coils. All the above sections are of equal area, and
the figures marked on each represent relative values for the perimeters,
the circle for convenience being taken at 100.]
=Ques. What is the principal objection to the use of cast iron for core
construction?=
Ans. Since its sectional area must be considerably more than wrought iron,
a much greater quantity of copper is required for the magnetizing coils.
Copper is expensive, while cast iron cores are less expensive
than equivalent ones of wrought iron; in this connection, it is
interesting to observe how different designers aim at true
economy in construction.
Steel is sometimes used in place of wrought iron, and though
less efficient magnetically, it can be cast into the desired
shape, thus avoiding the somewhat expensive processes of forging
and machining, which are necessary in the case of wrought iron.
=Ques. What form of core requires the least amount of copper for the
magnetizing coils, and why?=
Ans. The cylindrical core, because it has the shortest periphery or
boundary for a given area enclosed.
[Illustration: FIGS. 222 to 225.--Several forms of pole piece. Where the
extremities project as in figs. 222 and 223, they are called _horns_. The
object of these is to reduce the reluctance of the air gap. The width of
“fringe” of the magnetic field is influenced by the shape of the pole
piece; the margin of fringe should be such that the flux density will vary
from zero to a high value where the inductors enter.]
Figs. 216 to 221, show a series of cross sections, all of the
same area. The number marked on each section indicates the
length of the boundary line, that of the circle being taken for
convenience as 100.
=Ques. What are the pole pieces?=
Ans. These are the end portions of the field magnets, joined to, or cast
together with the core and placed adjacent to the armature.
The faces of the pole pieces are of circular shape, thus forming
the sides of the so-called armature chamber within which the
armature rotates.
[Illustration: FIG. 226.--Unsymmetrical pole piece introduced by Gravier
to concentrate the magnetic field. When the dynamo is working at small
loads, the flux in the gap is nearly uniform, but at heavy loads, the
distortion due to the armature current forces the flux forward and
saturates the forward horn, thus preventing much change in its flux
density, on account of the saturation, and the diminishing area. Lundell
combined the unsymmetrical and slotted forms of pole piece as shown in
fig. 237.]
=Ques. Why are the pole faces made larger than the coils?=
Ans. In order to reduce the reluctance of the air gap between the face and
the armature, thus enabling fewer magnetizing coils to be used.
[Illustration: FIG. 227.--Pole piece with oblique slots; a modification of
Lundell’s form of pole piece as suggested by Thompson. In operation, the
neck of the casting becomes saturated and offers considerable reluctance,
which tends to prevent distortion of the magnetic field.]
It is important that the field should be magnetically rigid,
that is, not easily distorted. This stiffness of field can be
partially secured by judicious shaping of the pole pieces. A few
forms of pole piece are shown in figs. 222 to 231.
If the projecting tips of the pole pieces, or _horns_ as they
are called, be widely separated, as in fig. 222, they are not
always good, even though thin. It is better that they should be
extended as in fig. 223 so that they may be saturated by the
leakage field or else cut off as in fig. 224.
An extreme design, suggested by Dobrowolski, as shown in fig.
225, surrounds the armature with iron.
[Illustration: FIG. 228.--Non-concentric pole faces; one method of
securing suitable magnetic “fringe” with fair magnetic rigidity of field.]
[Illustration: FIGS. 229 to 231.--Various shapes of pole piece for
securing a gradual entrance of the armature inductors into the magnetic
field.]
Another scheme, proposed by Gravier, employed the unsymmetrical
form shown in fig. 226. In this pole piece the forward horn is
elongated. The action due to this arrangement is such that when
the machine is working at small loads, the field in the gap is
nearly uniform, but at heavy loads with distorting reactions
which have a tendency to drive the flux into the forward horn,
the small section of the latter causes it to become saturated,
thus reducing the distortion to a minimum.
=Eddy Currents; Laminated Fields.=--The field magnet cores and pole
pieces, as well as the armature of a dynamo are subject to _eddy
currents_, that is, induced electric currents occurring when a solid
metallic mass is rotated in a magnetic field. These currents consume a
large amount of energy and often occasion harmful rise in temperature.
This loss may be almost entirely avoided by laminating the pole piece, or
both pole piece and core; in the latter case, both form one part without
any joint.
[Illustration: FIG. 232.--Illustrating the alteration of magnetic field
due to movement of mass of iron in the armature. If the masses of iron in
the armature are so disposed that as it rotates, the distribution of the
lines of force in the narrow field between the armature and the pole piece
is being continually altered, then, even though the total amount of
magnetism of the field magnet remain unchanged, eddy currents will be set
up in the pole piece and will heat it. This is shown in the above figures,
which represent the effect of a projecting tooth, such as that of a
Pacinotti ring, in changing the distribution of magnetism in the pole
piece.]
[Illustration: FIG. 233.--Eddy currents induced in pole pieces by movement
of masses of iron. These diagrams, which correspond to those of fig. 232,
show the eddy currents grouped in pairs of vortices. The strongest current
flows between the vortices and is situated just below the projecting
tooth, where the magnetism is most intense; it moves onward following the
tooth. At C is shown what occurs during the final retreat of the tooth
from the pole piece. These eddy currents penetrate into the interior of
the iron, although to no great depth. Clearly the greatest amount of such
eddy currents will be generated at that part of the pole piece where the
magnetic perturbations are greatest and most sudden. A glance at the
figures shows that this should be at the forward horn of the pole piece.
However, when a dynamo, with horned pole pieces, has been running for some
time as a motor the forward horns are cool and the hindward horns hot.]
[Illustration: FIG. 234.--Fort Wayne laminated pole piece before being
cast welded into frame. In the faces of solid pole pieces there exist
minute electric currents called _eddy currents_ which cause heating of the
iron and increase the energy required to maintain a magnetic circuit in
much the same manner as does reluctance. This loss is reduced by dividing
the magnetic circuit in the line of flux into numerous parallel paths
separated by some material of relatively high resistance. In construction,
the above core and pole piece is made up of sheets of annealed steel of
two different widths assembled together to form proper size and shape. The
minute spacing between these laminations and the slight oxidizing on each
surface is sufficient to reduce considerably the eddy currents. By cast
welding the pole piece into the frame, a low reluctance is secured.]
=Ques. What is a laminated pole?=
Ans. One built up of layers of iron sheets, stamped from sheet metal and
insulated, as shown in fig. 234.
=Ques. What may be said of this construction?=
Ans. It is a most approved method, and one frequently employed in the
construction of cores and pole pieces.
Fig. 234 shows a combined core and pole piece made entirely of
sheet iron punchings assembled and riveted together, and fig.
235, a core to be used with separate pole piece. It should be
noted that in both cases there is a longitudinal slot extending
from the end into the core. This was first suggested by Lundell,
the object being to prevent, as far as possible, the distortion
of the magnetic field due to armature reaction especially on
heavy overloads.
[Illustration: FIG. 235.--Fort Wayne laminated core without pole piece, as
used on large dynamos. It is constructed of punchings from sheet iron, and
riveted under pressure. The alternate end projections and grooved base
insure good mechanical union of metal in cast welding to magnet frame.
Reluctance between core and yoke is reduced to a minimum by cast welding.
The core is slotted parallel with the shaft to prevent, as far as
possible, the distortion of the magnetic field, especially on heavy
overloads.]
=Ques. What mode of construction is adopted to reduce the reluctance of
the magnetic circuit when laminated poles are used?=
Ans. They are cast welded into the frame.
[Illustration: FIG. 236.--Fort Wayne one piece frame with cast welded
combined cores and pole pieces. In any electrical apparatus a magnetic
circuit of low reluctance requires less energy to maintain a given flux
than one having a comparatively high reluctance. To reduce this to a
minimum the pole pieces and cores are combined into one part and then cast
welded into the yoke or frame. Thus the continuity of the magnetic circuit
is practically unbroken save for the air gap.]
The frame end of the core as shown in the illustrations has
irregularities in the heights of the different sheets, as well
as grooved undercut surfaces, in order to enable the molten
metal of the frame to key well into the laminations of the core,
making a good joint, both mechanically and electrically. By this
construction, the continuity of the magnetic circuit is
practically unbroken save for the air gap between the pole piece
and armature.
Fig. 236 shows a one piece frame of a six pole dynamo having
cast welded into it, combined cores and pole pieces.
=Ques. What is the disadvantage of laminating a core?=
Ans. It necessitates a nearly square or rectangular section, which
requires more copper for the winding than the cylindrical form.
[Illustration: FIG. 237.--Lundell type of combined core and pole piece; a
combination of Gravier’s unsymmetrical horns and longitudinal slot
designed to prevent distortion of field.]
=The Magnetizing Coils.=--The object of the magnetizing coils, is to
provide, under the various conditions of operation, the number of _ampere
turns_ of excitation required to give the proper flux through the armature
to produce the desired electromotive force.
With respect to the manner in which magnetizing coils are wound they are
said to be:
1. Spool wound;
2. Former wound.
=Ques. Describe the methods of constructing spool wound coils.=
Ans. The spool is made in various ways, sometimes entirely of brass, or of
sheet iron with brass flanges, or of very thin cast iron. Some builders
use sheet metal with a flange of hardwood, such as teak. If a spool be
simply put upon a lathe to be wound, the inner end of the wire, which must
be properly secured, should be brought out in such a way that it cannot
possibly make a short circuit with any of the wires in the upper layers.
To avoid this difficulty, the wire is sometimes wound on the spool in two
separate halves, the two inner ends of which are united, so that both the
working ends of the coil come to the outside as shown in fig. 238.
[Illustration: FIG. 238.--Method of winding magnet spool so that the two
ends of the coil will come to the outside. This method has also been used
for induction coils, where it is desirable to keep the ends of the wire
away from the core and primary coil.]
=Ques. Describe the construction of former wound coils.=
Ans. Former wound coils are wound upon a block of wood having temporary
flanges to hold the wire together during the winding. Such coils have
pieces of strong tape inserted between the layers and lapped at intervals
over the windings to bind them together. Coils are usually soaked with
insulating varnish and stove dried.
=Ques. What may be said with respect to the coil ends?=
Ans. Several methods of bringing out the ends of coils are shown in figs.
238 to 241. In fig. 239 copper strip, laid in behind an end sheet of
insulating material, makes connection to the inner end, as shown in the
right side of the figure, while another strip, shown on the left side
similarly inlaid, serves as a mechanical and electrical attachment for the
outer end of the winding.
[Illustration: FIG. 239.--Core and edge strip winding for shunt field
coils of large multi-polar dynamo. The winding consists of a copper strap
S carefully insulated and placed edgewise on the core C in a single layer
of winding. With this arrangement, the space occupied by insulation is
reduced to a minimum, and, although the cooling surface is small, each
turn of the winding has one edge on the outer surface, being ample for
adequate cooling.]
Two other methods are shown in figs. 240 and 241. A simple
device for securing the outer end is to fashion a terminal piece
so that it can be laid upon the winding, the last three or four
turns of which are wound over its base, and after winding, are
bared at the place and securely soldered.
=Ques. How are the coils insulated?=
Ans. The spools upon which the coils are wound are usually insulated with
several layers of paper preparations; a thickness of one-tenth of an inch
made up of several superposed layers is generally sufficient. Varnished
canvas is useful as an underlay, and vulcanized fibre for lining the
flanges. It is important to protect the joint between the cylindrical part
and the flanges. A core paper may be laid upon every four layers of
winding. Between series and shunt coils, in compound wound machines there
should be an insulation as efficient as that on the cores. When the
winding is completed, two layers of pressed board or equivalent are laid
over and bound with an external winding of hard rope or tape. This
protective external lagging covering the outer surface of the completed
coils is not altogether a benefit for it tends to prevent dissipation of
heat.
[Illustration: FIG. 240.--One mode of bringing out the coil ends, in which
copper strip is laid in behind an end sheet of insulating material.]
[Illustration: FIG. 241.--Another mode of bringing out the coil ends. A
narrow insulated strip of thin copper G, leading to terminal H, is
connected with the end _e_ of the coil before winding.]
[Illustration: FIGS. 242 and 243.--Square and hexagonal order of
“bedding.” The term bedding is an expression used to indicate the relation
between the cross sectional area of the winding when wound square, as in
fig. 242, and where wound in some other way, as in fig. 243. In the square
order of bedding, the degree of bedding equals zero.]
[Illustration: FIG. 244.--Method of securing coils in position when the
pole pieces are simply extensions of the core without enlargement.]
=Ques. How are the coils attached?=
Ans. Where the pole pieces are simply extensions of the cores without
enlargement, the coils can be slipped over the ends, but some kind of
clamping device is necessary to hold them in place, as for instance, the
method shown in fig. 244.
In case the pole piece be made larger than the core and separate
therefrom, it is put into position after the coils are in place,
thus serving the double purpose of pole piece and clamp.
=Ques. Describe the coil connections.=
Ans. Coils are generally united in series so that the same magnetizing
current may flow through all of them. The coils should be so connected
that they produce alternate north and south poles.
If all the coils be similarly wound with respect to the
terminals, and similarly placed; that is, so placed that the
winding, considered from the coil terminal nearest the pole
face, starts in all the coils in the same direction, then the
connections will come at the north end and at the south end of
the spools.
[Illustration: FIG. 245.--Western Electric set of former wound field coils
for four pole dynamo. These coils are wound around a former or template,
and are then slipped over the cores before the latter are bolted to the
yokes or frame.]
=Heating.=--The heat generated in the magnetizing coils is dissipated in
three ways; by:
1. Induction;
2. Radiation;
3. Convection.
In the first instance, it passes through the copper and the insulation,
either to the external surface, whence it passes off by radiation and
convection into the air, or to the magnet core and yoke, which in turn
conduct it away. In large multi-polar machines the masses of metal in the
pole cores and frame are more efficient in dissipating heat than the
external surface of the coil.
[Illustration: FIG. 246.--Fort Wayne compound wound rectangular ventilated
spool field coil. The series and shunt coils are wound side by side,
ventilating passages being provided lengthwise through each coil and
between the shunt and series coils as shown.]
=Ventilation.=--Sometimes provision is made for ventilation of the field
magnet coils as shown in fig. 246. Here the series and shunt coils are
wound side by side, ample ventilation being provided lengthwise through
and between the coils.
HAWKINS PRACTICAL LIBRARY OF ELECTRICITY
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=ELECTRICAL GUIDE, NO. 1=
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
=ELECTRICAL GUIDE, NO. 2=
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.
=ELECTRICAL GUIDE, NO. 3=
Electrical Instruments, Testing, Practical Management of Dynamos
and Motors.
=ELECTRICAL GUIDE, NO. 4=
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
=ELECTRICAL GUIDE, NO. 5=
Principles of Alternating Currents and Alternators.
=ELECTRICAL GUIDE, NO. 6=
Alternating Current Motors, Transformers, Converters,
Rectifiers.
=ELECTRICAL GUIDE, NO. 7=
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
=ELECTRICAL GUIDE, NO. 8=
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
=ELECTRICAL GUIDE, NO. 9=
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
=ELECTRICAL GUIDE, NO. 10=
Modern Practical Applications of Electricity and Ready Reference
Index of the 10 Numbers.
Theo. Audel & Co., Publishers 72 FIFTH AVENUE, NEW YORK
FOOTNOTES:
[1] NOTE.--In 1749, Benjamin Franklin, observing lightning to possess
almost all the properties observable in electric sparks, suggested that
the electric action of points, which was discovered by him, might be tried
on thunderclouds, and so draw from them a charge of electricity. He
proposed, therefore, to fix a pointed iron rod to a high tower, but
shortly after succeeded in another way. He sent up a kite during the
passing of a storm, and found the wetted string to conduct the electricity
to the earth, and to yield abundance of sparks. These he drew from a key
tied to the string, a silk ribbon being interposed between his hand and
the key for safety. Leyden jars could be charged, and all other electrical
effects produced, by the sparks furnished from the clouds. The proof of
the identity was complete. The kite experiment was repeated by Romas, who
drew from a metallic string sparks 9 feet long. In 1753, Richmann, of St.
Petersburg, who was experimenting with a similar apparatus, was struck by
a sudden discharge and killed.
[2] NOTE.--Suppose that the conditions are as in the fig. 34, that is, the
segment A_1 is positive and the segment B_1 negative. Now, as A_1 moves to
the left and B_1 to the right, their potentials will rise on account of
the work done in separating them against attraction. When A_1 comes
opposite the segment B_2 of the B plate, which is now in contact with the
brush Y, it will be at a high positive potential, and will therefore cause
a displacement of electricity along the conductor between Y and Y_1,
bringing a large negative charge on B_2 and sending a positive charge to
the segment touching Y_1.
As A_1 moves on, it passes near the brush Z and is partially discharged
into the external circuit. It then passes on until, on touching the brush
X, it is put in connection with X, and has a new charge, this time
negative, driven into it by induction from B_2. Positive electricity,
then, being carried by the conducting patches from right to left on the
upper half of the A plate, and negative from left to right on its lower
half.
A similar process is taking place on the B plate, but in this case the
negative electricity is going from left to right above, and the positive
from right to left below. On the whole, therefore, positive electricity is
being supplied to the left hand main conductor Z by both upper and lower
plates, and negative to Z_1.
[3] NOTE.--The discovery of this property of matter is due to Stephen
Gray, who, in 1729, found that a cork, inserted into the end of a rubbed
glass tube, and even a rod of wood stuck into the cork, possessed the
power of attracting light bodies. He found, similarly, that metallic wire
and pack thread conducted electricity, while silk did not.
Gray even succeeded in transmitting a charge of electricity through a
hempen thread over 700 feet long, suspended on silken loops. A little
later, Du Fay succeeded in sending electricity to no less a distance than
1,256 feet through a moistened thread, thus proving the conducting power
of moisture. From that time the classification of bodies into conductors
and insulators has been observed.
[4] NOTE.--Copper is pre-eminently the metal used for electric conduction,
being among the best conductors, it is excelled by one or more of the
other metals, but no other approaches it in the average of all qualities.
[5] NOTE.--A current of electricity always flows in a conducting circuit
when its ends are kept at different potentials, in the same way that a
current of water flows in a pipe when a certain pressure is supplied. The
same electrical pressure does not, however, always produce a current of
electricity of the same strength, nor does a certain pressure of water
always produce a current of water of the same volume or quantity. In both
cases the strength or volume of the currents is dependent not only upon
the pressure applied, but also upon the _resistance_ which the conducting
circuit offers to the flow in the case of electricity, and on the friction
(which may be expressed as resistance) which the pipe offers to the flow
in the case of water.
[6] NOTE.--The prefixes “meg” and “micro” denote million and millionth.
For example, a megohm equals 1,000,000 ohms, a microhm equals 1/1,000,000
of an ohm.
[7] NOTE.--The reciprocal of a number is equal to 1 ÷ the number; for
instance, the reciprocal of 3/20 = 1 ÷ 3/20 = 20/3 = 6-2/3
[8] NOTE.--A writer in the _New Science Review_ undertakes to answer the
question: “What is electricity?” In order to lead the reader up to the
main question, he first considers the natural forces, gravitation and
heat. Examples are given of how these forces are manifested and how energy
is changed from one form to another. Every form of force, the author says,
should be regarded as a different method in which energy makes itself
known to the senses. He calls particular attention to the important fact
that the “resistance of one kind or another is always the agent that acts
to alter energy from one form to another,” and suggests that electricity
is simply a form or manifestation that energy may assume under given
conditions, and generally is a mere transitory stage between the
mechanical form and the heat form. “In most operations,” he continues,
“mechanical force passes to the heat form without passing through the
electric form; but whenever magnetism is brought into play as a resistance
that must be overcome, then mechanical power applied to overcome this
resistance always becomes electricity, if only momentarily in its passage
from the mechanical to the heat form.” In conclusion, he asks if the
question: “What is electricity?” cannot be answered in a fairly
satisfactory way by saying that it is simply a form that energy may assume
while undergoing transformation from the mechanical or the chemical form
to the heat form or the reverse.
[9] NOTE.--The cathode is the conductor by which current flows away as
distinguished from the _anode_ or conductor through which the current
enters. The terms usually apply to conductors leading the current through
a liquid or gas, as an electrolytic cell, or vacuum tube.
[10] NOTE.--The name _voltameter_ was given by Faraday to an electrolytic
cell employed as a means of measuring an electric current by the amount of
chemical decomposition the current effects in passing through the cell.
[11] NOTE.--Faraday’s own description of his discovery is as follows: “Two
hundred and three feet of copper wire in one length were coiled round a
large block of wood; another two hundred and three feet of similar wire
were introposed as a spiral between the turns of the first coil, and
metallic contact everywhere prevented by twine. One of these helices was
connected with a galvanometer, and the other with a battery of one hundred
pairs of plates, four inches square, with double coppers, and well
charged. When the contact was made there was a sudden and very slight
effect at the galvanometer, and there was also a similar slight effect
when the contact with the battery was broken.”
[12] NOTE.--In reality it would be impossible to have a magnetic field
exactly like fig. 129, for in the less dense part, the magnetic lines
would be of curved complex form.
[13] NOTE.--These values are correct for effective sinusoidal voltages.
[14] NOTE.--It should be understood that a dynamo does not generate
electricity, for if it were only the quantity of electricity that is
desired, it would be of no use, as the earth may be regarded as a vast
reservoir of electricity. However, electricity without pressure is
incapable of doing work, hence a dynamo, or so-called “generator,” is
necessary to create an electromotive force by electromagnetic induction in
order to cause the current to flow against the resistance of the circuit
and do useful work.
* * * * *
TRANSCRIBER’S AMENDMENTS
Transcriber’s Note: Blank pages have been deleted. Some illustrations may
have been moved. Notes at the bottom of pages in the text were converted
to footnotes and footnote tags were added to the text itself. The
footnotes are now located prior to this section. When the author’s
preference can be determined, we have rendered consistent on a
per-word-pair basis the hyphenation or spacing of such pairs when repeated
in the same grammatical context. The publisher’s inadvertent omissions of
important punctuation have been corrected. Duplicative front matter has
been removed.
The following list indicates any additional changes. The page number
represents that of the original publication and applies in this etext
except for footnotes and illustrations since they may have been moved.
TOC = Table of Contents
TOC [Added INTRODUCTORY CHAPTER and SIGNS AND SYMBOLS]
TOC between electric and netic[magnetic] circuits
TOC action of Toepler-Holz[Holtz] machine
4 species inhabiting the Mediteranean[Mediterranean]
14 would fly from it without any elecrical[electrical]
16 pith balls [on] strips of paper C, D, E, as shown.
28 between the source and rerminal[terminal].
39 a certain anount[amount] of work
43 When metals differeing[differing] from each other
61 Various zincs; fig. 58 Fuller; fig. 59 Daniel[Daniell]
62 Various carbons; fig. 61 Cylindrical from[form],
63 fig. 68 Lockwood; fig. 69 fire alram[alarm].
66 A paralled[paralleled] or multiple connection
70 but no other aproaches[approaches]it
93 With this prelimary[preliminary] caution,
96 If a current of 10 amperes flow[flows] in a wire
98 as an electrotylic[electrolytic] cell,
99 rising from the kathode[cathode] P′ is hydrogen
100 hydrogen atoms in their journey towards b[B] meet
100 at the electrodues[electrodes] and not
133 VI.[6.] _The approach and recession of a conductor
142 3[2]. Vibrator coils;
142 2[3]. Condenser coils.
143 soaked in shellac dissolved in alchool[alcohol]
151 the hammer or piece of of[2nd ‘of’ del] iron
170 FIG.[FIGS.] 169 to 173.-- The sine curve
209 An extreme design, suggested by Dobrowolsky[Dobrowolski],
* * * * *
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