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Title: Induction Coils, How to Make, Use, and Repair Them. - Including Ruhmkorff, Tesla, and medical coils, Roentgen - Radiography, etc. etc.
Author: Norrie, H. S.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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                            INDUCTION COILS

                   How to Make, Use, and Repair Them


                            ON PRIMARY AND
                           SECONDARY BATTERY

                             H. S. NORRIE
                         (NORMAN H. SCHNEIDER)


                       [Illustration: Colophon]

                               NEW YORK:
                 SPON & CHAMBERLAIN, 12 CORTLANDT ST.

                   E. & F. N. SPON, Ltd., 125 STRAND


        Entered, according to Act of Congress, in the year 1896
                   Re-entered for Copyright in 1901
                         By SPON & CHAMBERLAIN
     in the office of the Librarian of Congress, Washington, D. C.

                         BURR PRINTING HOUSE,
                       NEW YORK, N. Y., U. S. A.


PAGE 55. Should read, "Cords being attached to binding posts Nos. 1 and
2 are in circuit with the _Secondary_ Coil only. When at Nos. 2 and 3
they receive the induced current or extra current in the _Primary_."

INDEX. "Tesla coil, descriptive," should read "Tesla coil,


The great favor with which the first edition of this little work
has been received and the steadily growing interest in its subject,
together with many valuable improvements and researches, may be given
as the reasons for this new edition.

The book has been thoroughly revised, partly rewritten, and
considerable new matter, with twenty-six new illustrations, added. It
has been brought up to date as far as electrical science has gone.

To detail all that has been done is too great a task for a preface; we
may briefly mention the following new matter:

Coils for gas and automobile engines; medical coils, concise directions
for operation and repairs; new forms of contact breakers, including
electrolytic and mechanical; gas-lighting apparatus; primary and
secondary batteries.

The chapter on X-Ray Apparatus has been entirely rewritten, and is
thoroughly practical; and an entire chapter on Wireless Telegraphy has
been added. In a book of this size it is not feasible to give specific
directions and full dimensions for the manufacture of all the apparatus
described. Indeed, much of the latter must be adapted to the particular
purpose for which it is to be utilized. Again, the same amount of
material will not always produce the same results. A little closer
winding, greater pressure applied to the cooling wax of a condenser,
and the output or capacity of either is changed.

Matters purely of design or taste are to be governed by the creative
faculty of the worker; but such general details and rules are given
as will be sufficient to enable one possessing ordinary constructive
ability to make his own apparatus.

The whole process of coil-making does not require high mechanical
skill, but chiefly patience and attention to details; and, perhaps best
of all, but few tools are needed, all of a simple kind.

We beg to acknowledge courtesies received from Messrs. Queen & Co., the
_Scientific American_ for frontispiece and Fig. 13, Mr. Goldingham's
book on Oil Engines for Fig. 12, and others who have been of assistance
to the author. The best American and English practice has been adopted;
the American standard gauges and sizes of wires are used, except where

A list of works, particularly of value to the coil worker, will be
found following the index.

  (Norman H. Schneider.)

APRIL, 1901.




  Construction of Ruhmkorff Coils. Sizes of
  Wires. Winding of Primary and Secondary.
  Assembling. Connecting Up.
  Insulation. Coils in Series. Oil Immersed
  Coils. "Tesla" Coil. Disruptive
  "Tesla" Coil. Coils for Gas
  Engines. Spark Coils. Resistance
  Coils. General Remarks on Coils. The
  Testing of a Coil for Polarity. Failure
  to Work. Medical Coils. Medical Coil
  with Tube Regulation. Medical Coil
  with Interchangeable Secondaries. Bath
  Coils                                                 1-64



  Construction of Contact Breakers. Various
  Forms of Simple Contact Breakers.
  The Mercury Vibrator. Polechanging
  Vibrator. Wehnelt Interrupter. Dessauer
  Contact Breaker. Steel Ribbon
  Interrupter. Contact Breakers in
  Vacuo. Queen Contact Breaker. Adjustable
  Contact Breaker for Medical
  Coils. The Queen Contact Breaker for
  Large Coils, Adjustable Cone Vibrator.
  Contacts                                             65-91



  Selection of Insulating Materials. Mineral
  Oil. Paraffin Wax. Resin Oils. Beeswax.
  Shellac Varnishes. Silk. Insulating
  Compounds                                            92-98



  Construction of Condensers. Leyden Jar.
  Glass Plate Condenser. Paper Condensers.
  Series Condenser. Rolled-Up
  Condensers. Adjustable Condensers.
  Application of Condensers                           99-119



  Luminous Effects Obtained by Means of a
  Ruhmkorff Coil. Materials Used.
  Spark Experiments. The Luminous
  Pane. Luminous Designs, etc.                       120-130



  Color Produced by Burning Different
  Metals. The Spectroscope Shown in
  Connection with the Coil. The Screen.
  The Color Spaces in the Solar Spectrum.
  Color Values                                       131-139



  Different Forms of Mercury Air Pumps.
  Geissler Tubes. Discharges in Vacuo.
  Characteristic Colors of Different Gases
  in Tubes, etc.                                     140-152



  Effects of Discharges in Rotating Tubes.
  Construction of Rotating Wheels. Arrangement
  of Tubes, etc.                                     153-163



  The Application of the Ruhmkorff Coil for
  Lighting Gas. Gas Lighting in Series.
  Gas Lighting in Multiple. Gas Lighting
  Diagram. Jump Spark Burner.
  Automatic Burners                                  164-177



  The Selection of Suitable Batteries. Open
  Circuit Cells. Closed Circuit Cells.
  Description of Cells. Formulæ for Solutions
  for Different Kinds of Batteries.
  The Grenet Battery. Fuller Battery.
  Gravity Battery. Dun Cell. Gethins
  Cell. Gordon Battery. New Standard.
  Edison-Lalande Cell. Dry Batteries.
  Dry Cell Construction, etc.                        178-199



  Construction of a Storage Cell. Connecting
  Up Cells. Charging Storage Batteries.
  Diagram for Charging from Dynamo
  Using a Rheostat. Diagram for Charging,
  using Lamp instead of Rheostat.
  Charging from. Primary Batteries.
  Testing Solutions. Setting Up the
  Storage Cell. The Harrison Cell. The
  "U. S." Storage Cell, etc.                         200-223



  Currents of High Frequency. Electric Resonator.
  The "Tesla" Effects. Coil
  Connected to Discharger. High Frequency
  Currents in Electro-Therapeutics,
  etc.                                               224-234



  General Arrangement of Connections for
  Coil and Crookes Tube for Making X Ray
  Negatives. The Fluoroscope. Phosphorus
  Tube. The Queen Self-Adjusting
  Crookes Tube. General Remarks,
  etc.                                               235-247



  Arrangements of Simple Circuits of Coil and
  Coherer for Receiving and Sending
  Messages. The Coherer. Carbon Coherer.
  Coherer without Filings. Aluminum
  Coherer. Steel Ball Coherer. The
  Oscillator. Clarke's Oscillator. Triple
  Oscillator. The Coil. Translating
  Devices. Air Conductor, etc.                       248-265

  INDEX                                                  266

  Bibliography                                           270



  GOOD PROPORTIONS OF CORE LENGTHS                                     7

  TABLE OF "SECONDARY" WINDINGS                                       24

  POLARITY TESTS                                                      45

  LENGTHS                                                             50

  SILVER WIRE                                                         64

  SPECIFIC INDUCTIVE CAPACITY                                        119

  WAVE LENGTHS AND TEMPERATURES                                  138-139

  TABLE OF RELATIVE COSTS OF MATERIALS                               191



  FIG.                                                              PAGE

  1.  Section of Coil                                                  4

  2.  Insulating Tube Ends                                            10

  3.  Sectional Winding                                               11

  4.  Section      "    First Method                                  12

  5.     "         "    Second Method                                 13

  6.  Proportional Diagram of Coil                                    15

  7.  Section Winder, End View                                        17

  8.     "       "    Face View                                       17

  9.  Assembly of Coils                                               18

  10.  Polechanging Switch                                            31

  11.  Disruptive Tesla Coil                                          35

  12.  Spark Coil for Gas Engine                                      38

  13.  Reproduction of a 32-inch Spark                                47

  14.  Simple Medical Coil                                            53

  15.  Connections for Simple Medical Coil                            55

  16.  Interchangeable Medical Coil                                   56

  17.  Vibrator for Medical Coil                                      60

  18.  Simple Contact Breaker                                         65

  19.  Imperfect Form of Contact Breaker                              67

  20.  Superior Form of Contact Breaker                               67

  21.  Spotteswoode Contact Breaker                                   69

  22.  Polechanging Contact Breaker                                   74

  23.  Wehnelt Interrupter                                            78

  24.  Ribbon Vibrator                                                81

  25.  Queen Contact Breaker                                          82

  26.  Adjustable Contact Breaker                                     86

  27.  Cone Contact Breaker                                           88

  28.  Coil Head Contact Breaker                                      89

  29.  Leyden Jar                                                    101

  30.  Plate Condenser                                               102

  31.  Arrangement of Condenser Plates                               104

  32.  Condenser Charging, First Method                              110

  33.      "         "     Second Method                             112

  34.  Adjustable Condenser                                          118

  35.  Spark between Balls                                           125

  36.  Short Spark between Balls                                     125

  37.  Sparkling Pane                                                125

  38.  Luminous Design                                               128

  39.  Electric Brush                                                128

  40.  Spectrum—Solar                                                132

  41.  Spectroscope and Coil                                         133

  42.  Simple Air Pump                                               141

  43.  Geissler Air Pump                                             144

  44.  Sprengel Air Pump                                             144

  45.  Solution Tube                                                 150

  46.  Fluorescent Bulbs                                             150

  47.  Ruby Tube—Crookes                                             150

  48.  Iridio-platinum Tube—Crookes                                  151

  49.  Revolving Wheel                                               154

  50.  Tube Holder                                                   157

  51.  Side View of Wheel                                            157

  52.  Geissler Tubes                                                160

  53.  Triangle on Disc                                              161

  54.  Maltese Cross on Disc                                         161

  55.  Gas Lighting Circuit                                          165

  56.  Connections for Gas Burners                                   169

  57.  Bartholdi Automatic Burner                                    172

  58.  Connections for Automatic Burner                              174

  59.  The Grenet Cell                                               180

  60.  The Fuller Cell                                               184

  61.  The Gethins Cell                                              193

  62.  Lead Plate for Storage Cell                                   201

  63.  Wooden Separator                                              201

  64.  Charging with Rheostat                                        207

  65.  Charging with Lamps                                           207

  66.  Harrison Electrodes                                           211

  67.  Hydrometer                                                    221

  68.  Hertz Resonator                                               227

  69.  Tesla Circuit                                                 229

  70.  Tesla Cut Out                                                 231

  71.  Tesla Cut Out, Top Plan                                       232

  72.  Circuit for X Ray Apparatus                                   237

  73.  Queen's Self-Regulating X Ray Tube                            240

  74.  Transmitter for Wireless Telegraphy                           250

  75.  Receiver for Wireless Telegraphy                              252

  76.  The Branley Coherer                                           254

  77.  Clarke's Oscillator                                           259

  78.  Triple Oscillator                                             259

  79.  Air Wire Insulators                                           263



In commencing a description of the Ruhmkorff coil and its uses, a brief
mention of the fundamental laws of induction directly bearing on its
action will assist in obtaining an intelligent conception of the proper
manner in which it should be constructed and handled.

Any variation or cessation of a current of electricity flowing in one
conductor will induce a momentary current in an adjacent conductor; and
if the second conductor be an insulated wire coiled around the first
conductor, also a coil of insulated wire, the effect is heightened. The
intensity of the secondary or induced current increases with the number
of turns of its conductor, the abruptness and completeness of the
variation of current in the first or primary coil, and the proximity of
the coils. And the insertion of a mass of soft iron within the primary
coil by its consequent magnetization and demagnetization augments still
further the inductive effect. There are other contributing causes which
cannot be treated of here, but are of not so much importance as the

In the Ruhmkorff coil, which is an application of the above laws, the
primary coil is of large wire and the secondary coil of extremely fine
wire, of a length many thousand times greater than the wire of the
primary coil. The current is abruptly broken in the primary circuit
by a suitable device—the contact breaker or rheotome. The current
induced in the secondary at the make of the circuit is in the opposite
direction to that of the primary coil and battery, but the current
at the break of the circuit is in the same direction as that of the
primary. The effect of the current at the break of the circuit is
more powerful than that at the make, which latter is also somewhat
neutralized by the opposing battery current. A condenser or Leyden jar
is connected across the contact breaker to absorb an _extra current_
induced in the primary coil by the break of the circuit, which would
tend to prolong the magnetization of the core beyond the desired limit.

The whole apparatus is mounted on a wood base, having the condenser in
a false bottom for the sake of compactness.

It is not herein intended to describe all the minor operations in the
construction of a Ruhmkorff coil. A sufficient description and review
of the main points to be considered, however, will be given to enable
a person fairly proficient in the use of simple tools to construct a
serviceable instrument.

The parts and their arrangement in relation to one another are shown in
Fig. 1, but are not drawn strictly to scale, although very nearly so.

 [Illustration: FIG. 1.]

_C_ is the core, consisting of a bundle of soft iron wires as fine as
can be obtained. The greater the subdivision of the core the quicker
will it respond to the magnetizing current in the primary coil, and
lose its magnetism when the current ceases. It has another advantage,
in that the disadvantageous eddy, or Foucault currents, are lessened,
which fact, however, is of not enough importance to need extended

Many coil-makers saturate the core with paraffin or shellac, which
is of slight benefit. This core is wrapped in an insulating layer of
paraffined paper or enclosed in a rubber shell, there not being any
great necessity to use more than ordinary insulation between the core
and the primary coil.

In the majority of induction coils or "transformers" used in the
alternating current system of electric lighting, the iron cores form
a closed magnetic circuit. A closed magnetic circuit in a Ruhmkorff
coil could be obtained by extending the iron core at each end and then
bending and securing the ends together, forming, as it were, a ring
partly inside and partly outside the coil. But although the inductive
effects would be heightened and less battery power required, the
slowness of the circuit to demagnetize would alone be detrimental to
rapid oscillations of current.

There would also be a loss from a greater hysteresis (energy lost in
the magnetization and demagnetization of iron). A core magnetizes
quicker than it demagnetizes, and the latter is rarely complete; a
certain amount of residual magnetism remains, hysteresis being strictly
due to this retention of energy (Sprague). Hysteresis shows itself in
heat, but must not be confounded with Foucault or eddy currents. The
latter are corrected by subdividing the metal, but the former depends
upon the quality of the metal, and increases with its length.

Moreover, a coil with a closed magnetic circuit requires an independent
contact breaker.

In most of the alternating currents used in lighting their rapidity of
alternation is but one hundred and twenty-five periods per second. As
in the simple electromagnet, the proportions of diameter and length
of the primary coil and core will determine its rapidity of action. A
short fat coil and core will act much quicker than a long thin one. But
on a short fat coil the outside turns would be too far removed from the
intensest part of the primary field. A good proportion of core length
is given in the following table:

  Spark Length   Iron Core.
    of Coil.
      ¼           4″ × ½″
      ½           5″ × 10∕16″
     1            7″ × ¾″
     2            9″ × 1″
     6           12″ × 1⅛″
    12           19″ × 1½″

The primary coil _P_ consists of two or not more than three layers of
insulated copper wire of large diameter, being required to carry a
heavy current in a 2-inch spark coil, probably from 8 to 10 amperes.
In designing the primary coil a great advantage arises from using
comparatively few turns but of large wire. Each turn of wire in the
primary has a choking effect upon its neighbor by what is termed

As the primary coil and core may be considered as an electro magnet,
it may not be out of place to notice the rule governing such.
Magnetization of an iron core is mainly dependent upon the ampere turns
of the coil surrounding it—that is, one ampere carried around the core
for one hundred turns (100 ampere-turns) would equal in effect ten
amperes flowing through ten turns. Practically speaking, there would
be certain variations to the rule, for one difficulty would arise in
that the smaller wire used in conveying the smaller current would fit
more compactly and allow more turns to be nearer the core, the active
effect of the turns always decreasing with their distance from the
core. And although a large current and few turns would not have so much
self-induction, there would be trouble at the contact breaker, owing
to the large current it would have to control.

The most suitable sizes of wire for the primary coil are: No. 16 B.
& S. for coils up to 1 inch spark; No. 14 B. & S. up to 4 inches of
spark, and No. 12 B. & S. for a 6-inch spark coil. The coil should be,
say, one-twelfth of the core length shorter than the core.

_I_ is the insulating tube between the primary coil and the secondary
coil _S_. Here great precaution is necessary to prevent any liability
of short circuiting or breaking through of sparks from the secondary
coil. This danger cannot be underestimated, and the tube should be
either of glass or hard rubber, free from flaws, varying in thickness
with the dimensions of the coil. It should extend at least one-tenth
of the total length of the primary coil beyond it at each end. The end
of this tube can be turned down so as to allow of the hard rubber reel
ends being slipped on and held in position by outside hard rubber
rings (Fig. 2).

 [Illustration: FIG. 2.]

The secondary coil consists of many turns of fine insulated copper wire
separated from the primary coil by the insulating tube and a liberal
amount of insulating compound at each end. In coils giving under 1 inch
of spark this coil may be wound in two or more sections.

 [Illustration: FIG. 3.]

The usual manner of constructing these sections is to divide up the
space on the insulating tube by means of hard rubber rings placed at
equal distances apart, in number according to the number of sections
desired (Fig. 3). The space between each set of rings, or between
the coil end and a ring, is wound with the wire selected, the filled
sections constituting a number of complete coils, which are finally
connected in series. The sectional method of winding prevents the
liability of the spark jumping through a short circuit, but heightens
its tendency to pass into the primary coil at the ends, where it must
be therefore specially insulated from it.

 [Illustration: FIG. 4.]

In winding these sections there is a method now generally adopted which
has many good points, although at first it may seem complicated. The
old way of filling two sections was to wind both in the same direction
as full as desired, then join the outside end of the left-hand coil
to the inside end of the right-hand coil. This necessitated bringing
the outside end down between two disks, or in a vertical hole in the
sectional divider, and thereby rendered it liable to spark through into
its own coil. This is shown in Fig. 4, _A_ and _C_ inside ends, _B_ and
_D_ outside ends, the disk being between _B_ and _C_.

 [Illustration: FIG. 5.]

Reference to Fig. 3 shows the new method, and Fig. 5 shows an enlarged
diagram of sections 2 and 3 of Fig. 3.

Sections 1 and 3, Fig. 3, are filled with as many turns as desired; the
spool is then turned end for end, and sections 2 and 4 are wound, being
thus in the opposite direction of winding to sections 1 and 3.

The inside ends of 1 and 2 and 3 and 4 are soldered together, and the
outside ends of 2 and 3 are also soldered together.

The outside ends of 1 and 4 serve as terminals for the coil.

This method of connection leaves all the turns so joined that the
current circulates in the same direction through them all, as will be
seen by an examination of the enlarged diagram, Fig. 5.

Sprague, in his "Electricity: Its Theory, Sources, and Application,"
recommends that the turns of wire in the secondary coil shall gradually
increase in number until the middle of the spool is reached, and then
decrease to the spool end, in order that the greatest number of turns
be in the strongest part of the magnetic field (see Fig. 6). _D D D_
are section dividers, _S_ secondary windings, _P_ primary coil. The
selection of the size of wire to be used depends on the requirements as
to the spark. If a short thick spark be desired, use a thick wire, say
No. 34 B. & S.; if a long thin one, use No. 36 to No. 40 B. & S.

 [Illustration: FIG. 6.]

Although it is impossible to lay down rules for determining the exact
amount of wire to be used to obtain a certain sized spark, yet a fair
average is to allow 1¼ pounds No. 36 B. & S. per inch spark for small
coils and slightly less for large ones.

The most satisfactory and perhaps the easiest way for large coils is to
wind the secondary in separate coils, made in a manner similar to that
employed in winding coils for the Thompson reflecting galvanometer.
This method, first described by Mr. F. C. Alsop in his treatise on
"Induction Coils," is somewhat as follows:

A special piece of apparatus (Figs. 7 and 8) is necessary, but presents
no great difficulty in manufacture. A metal disk, _D_, one-sixth of
an inch thick and 7 inches in diameter, is mounted on the shaft _S_.
A second disk is provided with a collar and set screw, _A_, in order
that it may be adjusted on the shaft at any desired distance from the
stationary one. When the diameter of the coil to be wound has been
decided upon, a wooden collar, _W_, with a bevelled surface is slipped
on the shaft, it corresponding in diameter with the desired diameter of
the hole through the centre of the secondary coil. As these coils are
going to be made as flat rings and slipped on over the insulating tube,
a remark here becomes necessary on this diameter. Reference to Fig. 9
will show that it is intended that the coils near the reel ends shall
fit very loosely on the tube _T_ (Fig. 1)—in fact, that there shall be
a clearance of possibly one-half inch in the extreme end, diminishing
gradually to a fifteenth of an inch in the centre coils. Therefore it
becomes necessary to provide a number of wooden rings equal to the
desired diameter of the central hole in the coil. The thickness of
the wood determining the width of the individual coil depends on the
selection of the operator; but the rule may be laid down that the
narrower the coils the better the insulation of the complete coil will
be on completion.

 [Illustration: FIG. 7.]

 [Illustration: FIG. 8.]

 [Illustration: FIG. 9.]

One-sixteenth of an inch is a very fair average, and has been generally
adopted by the writer.

A quantity of paper rings are now cut out of stout writing paper which
has been soaked in melted paraffin. If a block or pad of letter paper
be soaked in paraffin and allowed to become cold under pressure, the
ring may be scratched on the surface of it and the block cut through on
a jig saw. The central apertures of course will vary in size with their
position on the tube _T_ (Fig. 9).

The coil winder is now either mounted in a lathe or fixed in a hand
magnet winder in such manner that it can be steadily and rapidly
rotated. The wire to be wound comes on spools, which can be so
threaded on a piece of metal rod that they turn readily. A metal dish
containing melted paraffin is provided with a round rod, preferably of
glass, fixed under the paraffin surface, so that it can rotate freely
when the wire passes under it through the paraffin. Two paper rings are
slipped on the winder that they may form, as it were, reel ends for the
coil, and if the metal disks have been warmed it is an easy matter to
lay them flat.

The end of the wire is then passed through the paraffin under the
glass rod and through the hole _H_ in the metal disk for a distance
of, say, 6 inches, and held to the disk outside with a dab of paraffin
or beeswax. Then the winder is rotated and the space between the paper
disks is filled with wire. The paraffin, being hot, will adhere to the
wire, and cooling as the wire lays down on the winder, hold the turns
together and at the same time insulate them from each other. It will
not be possible to lay the wire in even layers, as would be necessary
in winding a wider coil, but the spaces can be filled up, using
ordinary care that no radical irregularity occurs—that is, that only
adjacent layers are likely to commingle.

When the space is filled up to the level of the paper disks and the
paraffin is hard, loosen the set screw, and removing the outside disk,
the coil can be slipped off, or a slight warming will loosen it. Any
number of these coils can be made, and there are the advantages in
this mode of construction that a bad coil will not spoil the whole
secondary, and that the wire can be obtained in comparatively small

As each coil will not be of very high resistance, the continuity of
the wire can be readily tested by means of a few cells of battery,
connecting one end of the coil to one pole of the battery, and the
other pole of the battery and coil end touched to the tongue. If a
burning sensation is experienced, the connection is not broken. Where
possible the coils should be measured as to their resistance on a
Wheatstone bridge.

When the requisite number of coils has been prepared, they are
assembled in the following manner (Fig. 9): The coils, having their
aperture diameter graded, are placed in order, and starting with the
one having the largest hole, it is slipped over the primary protection
tube _T_, one end being brought out through a hole in the reel end
drilled vertically or between the reel end and the coil. A couple of
paper rings are then slipped on the tube, and another coil placed over
them, having its ends connected as in Fig. 3. This process is continued
until all the coils are in place. The annular space between the coils
and the tube _T_ (Fig. 9) is filled in with melted paraffin and the
coils gently pressed together, so as to form a compact mass, paraffin
being poured over the outside of the whole combination. Before winding
any wire used in this work it must be perfectly dry, which end can be
accomplished by subjecting the whole spool to a short period of baking
in a moderately warm oven.

The accompanying table gives the length of No. 36 silk-covered wire
that will fill a linear space equal to one thickness of the wire in
different-sized rings. This size wire wound tight will give 125 turns
per linear inch. Therefore on a ring having a middle aperture of 1½
inches and an outside diameter of 4 inches, there will be 156 turns, or
a total length of 1347 inches. This is obtained as follows: 1½ inches
× 3.1416 = 4.7124 (or 4.712); 4 inches × 3.1416 = 12.5664 (or 12.56);
(4.712 + 12.56)∕2 = mean circumference—viz., 8.635 inches.

This mean × number of turns in thickness of ring between the two
circumferences—viz., 156 = 1347 inches.


                           |       1½″        |   2″             |
  NO. 36 SILK-COVERED WIRE.|Aperture Diameter,|Aperture Diameter,|
  125 TURNS PER LINEAR     |    4.712″        |   6.283″         |
  INCH. 13,306 FEET PER    |Aperture          |Aperture          |
  POUND.                   |  Circumference.  |  Circumference.  |
  Outside diameter         |  4″  |  5″ |  6″ |  4″  |  5″ |  6″ |
  Outside circumference    |12.56 |15.70|18.84|12.56 |15.70|18.84|
  Mean circumference       | 8.635|10.20|11.78| 9.421|10.99|12.56|
  Turns between            |      |     |     |      |     |     |
    circumferences         | 156  | 219 |282  |  125 | 188 | 250 |
  Distance between aperture|      |     |     |      |     |     |
    and outside, in inches | 1.25 | 1.75| 2.25|   1  | 1.50| 2   |
  Length of wire, in inches| 1347 | 2234| 2650| 1178 | 2066| 3140|
                           |    2½″           |
  NO. 36 SILK-COVERED WIRE.|Aperture Diameter,|
  125 TURNS PER LINEAR     |    7.854″        |
  INCH. 13,306 FEET PER    |Aperture          |
  POUND.                   |  Circumference.  |
  Outside diameter         |  5″  |  6″ | 7″  |
  Outside circumference    |15.70 |18.84|21.99|
  Mean circumference       |11.78 |13.35|14.92|
  Turns between            |      |     |     |
    circumferences         | 156  | 219 | 282 |
  Distance between aperture|      |     |     |
    and outside, in inches | 1.25 | 1.75| 2.25|
  Length of wire, in inches| 1838 | 2924|4207 |

To obtain the length of wire necessary for a ring occupying more than
the space of one turn on the primary insulating tube, multiply the
length before obtained by the number of turns in the space it occupies.
Thus a flat ring one-tenth of an inch thick would equal 1347 inches ×

This rule is necessarily only approximate, owing to the way the wires
bed on each other from their cylindrical section. In actual practice,
when the wire is run through the paraffin bath not more than 50 per
cent of the calculated wire will occupy the space. And the thickness of
the paper rings must also be added when figuring the total length of
the coil. In the iron-clad transformers or induction coils of highest
efficiency used in the alternating current electric light system, the
rule for determining the windings of the coils is based on the ratio
of the turns of wire in the primary to the turns in the secondary, the
electromotive force in the primary, and the lines of force cut by the

The secondary ends can be attached to binding posts mounted on the reel
ends. Unless these reel ends be very high and clear the outside of the
coil considerably, it is better to mount the binding posts on the top
of the hard rubber pillars. A neat plan is to mount on the top of the
coil a hard rubber plate reaching from reel end to reel end, and place
the binding posts on that.

A discharger consists of two sliding metal rods with insulated handles
passing through pillars attached to the secondary coil. The inside ends
of these rods is provided with screw threads for the ready attachment
of the balls, points, etc., which are to be used. The substance to be
acted upon is laid on a rubber or glass table midway between the rod
pillars and slightly below the level of the rods.

By hinging the rod pillars, or using a ball and socket joint, the
discharger can be inclined so as to be better brought near the
substance on the table.

The next important part of the coil is the contact breaker.

The armature _R_ is a piece of soft iron carried at the end of a stiff
spring, in about the middle of which, at _B_, is riveted a small
platinum disk or stud. The adjusting screw _A_ has its point also
furnished with a piece of platinum, which is intended to touch the
platinum on the spring when the latter is in its normal position. The
core _C_ of the coil serves as an electro-magnet. When the current
flows from the battery (represented by the figure at _L_) through the
primary coil and armature spring to the adjusting screw, it causes the
armature to be drawn to the magnetized core, but thereby draws the
platinum disk away from the adjusting screw. In so doing it breaks the
circuit, the magnet loses its power, and the elasticity of the spring
reasserting itself, carries the armature back, thereby reclosing the
circuit. This is repeated many times in a second, the result being a
continual vibration of the spring, and a consequent interruption to the

The condenser or Leyden jar _J_, connected as in the diagram to the
base of the vibrating spring at _K_ and to the adjusting screw wire
_M_, is constructed as follows: On a sheet of insulated paper is laid
a smaller sheet of tinfoil, one edge of which projects an inch or so
over one end of the paper. Another sheet of paper covering this carries
a second sheet of tinfoil, one end of which projects as in the first
sheet, but at the opposite end of the paper. Tinfoil and paper sheets
are laid in this manner alternately until a sufficient number is
attained. The projecting ends are then clamped together and the whole
pile immersed in melted paraffin, as will be described in a subsequent
chapter. Wires are affixed to these clamped ends which serve to connect
the condenser with the contact breaker. The conventional sign for a
condenser is that used at _J_, showing the two series of plates, the
insulation or dielectric, as it is called, being understood.

The size of condenser to use with different-sized coils varies
according to the winding of the primary and the battery used. A primary
coil of few turns would not necessitate as large a condenser as one of
a large number of turns. At the same time, a condenser may be made of
too great a capacity, and thereby weaken the action of the coil.

The base upon which the coil and its parts are mounted may be of dried
polished wood. But where the coil is designed to give large sparks—over
2 inches—it is an advantage to use hard rubber one quarter of an inch
and upward in thickness. Glass, were it not for the difficulty of
drilling it and its brittleness, would be a desirable material for a
coil base in a dry atmosphere. Hard red or black fibre coated with
shellac varnish is also serviceable, and, moreover, is extremely easy
to work. Slate must never be used; there is too much liability of
iron veins being found in it, which in such high tension experiments
as will be described would seriously impair the usefulness of the
apparatus. The material selected for the base must be one that will
not absorb moisture. A paraffined surface collects moisture up to a
certain point in isolated drops, whereas a glass and even a hard rubber
surface condenses the moisture as a film, which latter is extremely
undesirable. But unfortunately the fact that a paraffined surface
does not present a pleasing appearance would probably result in its
rejection. And lastly, by mounting the coil on hard rubber blocks, or
extending the reel ends to raise the coil body, a high insulation can
be obtained at the sacrifice perhaps of appearance or height. From
the care taken to insulate the secondary coil, it may be considered a
superfluous precaution to so carefully select a base, but practical
work with the instrument at some important crisis will demonstrate
the necessity of extreme care in the smallest details relating to
insulation. It may be well to note here that hard rubber is acted upon
by ozone, and is thereby impaired as an insulator.

 [Illustration: FIG. 10.]

The base forms the top of a flat box in which the condenser lies; but
there are a few points worth considering right here. As the connections
of the coil will probably be under the base, a sufficient space must
intervene between the base and the top of the condenser. It is a
good plan to lay the condenser at least one half inch below the top
of this box, and fill up to, say, one eighth of an inch with melted
paraffin, leaving the condenser wires projecting for attachment. The
connections of the primary coil and contact breaker should by all means
be soldered, not simply wires held under screw nuts. And, moreover,
all wires under the base should be so run that they do not cross one
another, which precaution only requires a little planning. Then, when
the connections are all made and the base laid on top of the box, it
can be pressed down if the paraffin be warm, so that the screw heads
and wires mark out their own channels and cavities in which to lie.

A commutator or pole-changing switch is often added to change the
polarity of the battery current. The diagram of connection is shown in
Fig. 10. When the levers are as in the figure, the circuit is broken
and no current flows through the coil.


Ruhmkorff coils can be connected in series, but it is not to be
recommended. When it becomes necessary, however, the cores should be
removed, and one long core inserted, extending through each primary.
This will bring the time constants of each primary coil together and
prevent the interference otherwise present. The primary coils and
secondary coils are connected in series by assuming that they are
but adjacent sections of one complete instrument. Of course, as the
resistance of the primary is raised, the electromotive force of the
battery must be raised also.


A highly satisfactory induction coil can be made without much labor and
few tools, and will prove useful in many experiments which would not
warrant a more expensive instrument.

Make a bundle of soft iron wires, No. 22 B W G, for the core, ten
inches in length and one inch or more in diameter. Wrap this with
insulating tape or even ordinary tape to prevent the primary coil from
coming in contact with the iron. Now, wind on a primary of two layers
No. 14 B & S gauge cotton-covered copper wire, and insert the coil into
a hard rubber (or glass preferred) tube large enough to hold the coil
tight and to project an inch or so beyond the core ends.

A secondary coil of about one pound No. 36 cotton-covered magnet wire
should now be made on a hard rubber spool, the hole through centre
of this spool must be at least one inch larger in diameter than the
diameter of the primary cover. This spool should not exceed four
inches in length, and is to be slipped over the primary coil and held
suspended by blocks of wood in such a manner that it does not touch
the primary coil or cover. The whole outfit is now immersed in an
earthenware or glass vessel filled with linseed or heavy paraffin oil.
The contact breaker and condenser will be mounted independently; the
condenser for the two-inch spark coil will be suitable (see Table on


The coil just described, without contact breaker or iron core, can be
connected up and used in place of a "Tesla coil," which it resembles.
The coils used by Nikola Tesla are so many and varied that it becomes a
difficult task to describe a mode of construction which will meet the
wants of those who ask for "Tesla" coils. The _American Electrician_
gives a description of one wherein a glass battery jar, 6 inches × 8
inches, is wound with 60 to 80 turns of No. 18 B & S magnet wire. Into
this is slipped a primary, consisting of 8 to 10 turns of No. 6 B & S
wire, and the whole combination immersed in a vessel containing linseed
or mineral oil.

 [Illustration: FIG. 11.]


For Fig. 11 the specification is as follows: Secondary, 300 turns of
No. 30 B & S silk-covered magnet wire, wound on rubber tube or rod,
and the ends encased in glass or rubber tubes. This is inserted _into_
the primary, which consists of two coils, each of 20 turns No. 16 B &
S rubber-covered wire, wound separately on a long rubber tube not less
than ⅛ inch thick. The last tube must be large enough to be very loose
when the secondary coil is inserted in it, and it must project at least
two inches over each end of the secondary. A hard rubber division must
be placed between these primary coils. The four ends of the latter
coils are connected _C C_ to two condensers and _D D_ to two discharger
balls, the secondary wires going to the exhibitive apparatus. A further
description of these connections is to be found in Chapter XII., also
notes upon the use of the disruptive coil.

_Coils for Gas Engines._

These are either primary only or primary and secondary. Two to three
pounds of No. 14 B & S magnet wire are wound on an iron wire core eight
to ten inches in length by one inch in diameter. The contact is made
and broken in the igniter of the engine as at the wipe spring of a
ratchet gas burner. Four to eight large cells of dry battery are used,
or eight cells Edison-Lalande—iron-clad type. Number of cells varies
with size of coil needed, some classes of engines require a heavier
spark than others to ignite the vapor.

When a primary and secondary are used, the primary should be made of
two or three layers No. 14 B & S magnet wire, and a secondary of one
pound No. 34 B & S magnet wire. There can be an independent contact
breaker or the coil can be made up similar to a one-half inch spark
Ruhmkorff coil (see Chapter I.).

 [Illustration: FIG. 12.]

The method of connecting up a coil of the latter description is shown
in Fig. 12, which is self-explanatory. It shows a form of cam-shaft
switch which is operated by the engine, and which opens and closes the
primary circuit of the induction coil, the sparks from the secondary
winding passing between the points of the igniter in the engine
cylinder. As shown in Fig. 12, the igniter or ignition plug is similar
in operation to a coil discharger, the two terminals being, however,
insulated from each other by the use of porcelain. To ensure a good
insulation under the severe working conditions has been somewhat of a
task, but it seems to have been attained in the types of igniters known
as the Splitdorf and the Roche or New Standard.

The Splitdorf gas-engine coil is the result of much experiment and
careful design. It is built to stand hard usage, and the insulation
used has been adopted only after exhaustive test. In automobile work,
where a heavy strain is made upon the engine, as in climbing heavy
grades, it has been found that a stronger spark gives surer results.
This would indicate more battery current through the coil, and it is a
wise precaution to have a few extra cells attached that can be switched
on if necessary.

In constructing spark coils for gas engines particular care must
be given to the contact breaker. In most types of gas or oil vapor
engines it is absolutely necessary to have the spark pass with uniform
regularity, and immediately and surely when required. For automobiles
or where the apparatus is subject to jar, a heavy iron vibrating
armature would become unreliable by reason of its inertia and its
responding to shock. At every jolt of the vehicle it would jar and get
out of rhythm, and it certainly seems preferable to use a mechanical
contact apparatus whenever feasible. In the older type of gas engine
the spark is made by mechanism breaking contact right in the vapor. The
actual arrangement of these devices is detailed and illustrated in the
later works on gas and oil engines.


Although foreign to the title of this book, these coils will be
mentioned, being often necessary as accessories to the operation of
coils, wireless telegraphy, etc. These are coils of insulated German
silver wire, wound to a specified resistance. The main feature about
those designed for testing is that they are wound non-inductively—that
is, the wire is wound double in such manner that the current flows
both ways around the turns, and so neutralizes the inductive action.
In cases where dynamo current is to be used, as in telegraphs operated
from dynamo current, the coils are wound on tin tubes to make them
fireproof and yet radiate the heat. As the resistance of German silver
varies very largely, only approximate figures can be given. The table
(page 64) has been made up from the best averages obtainable. The
carrying capacity of resistance coils varies with their construction,
the better they can radiate heat, the more current they can safely


Ruhmkorff induction coils should always be fitted with a switch to
open, close, or reverse the power circuit, a double throw, double
pole, baby knife switch, mounted on a separate porcelain base, is very
suitable. Such a switch is open when the handle is vertical, and it
should always be left so when changing connections, fixing battery,
etc. A large, well-finished coil will have the secondary wires brought
in rubber tubes to binding posts mounted on hard rubber pillars, or
to binding posts mounted considerably above the coil cover level.
A very neat mode is shown in the frontispiece on the large 45-inch
spark coil. Here the secondary wires go to hard rubber pillars, which
also carry adjustable rod dischargers. These rods are movable towards
or away from each other by means of the large hard rubber handle to
which they are connected by a simple system of levers. In this coil
the secondary is moulded on a flexible tube, which fits loosely over
the primary tube in order to compensate for changes of temperature
and consequent expansions and contractions. All well-designed coils
should be so arranged that the primary coil and core can be readily
removed from the secondary, or _vice versa_. It is sometimes desirable
to use a different primary. This arrangement will greatly facilitate
any necessary repairs. It must be always remembered that the working
of a coil depends on the insulation between primary and secondary.
_Spare no pains to have perfect insulation_; it is a hopeless task to
reinsulate a broken-down secondary, although the sectional method of
winding facilitates repairs. In large winding rooms it is customary
to have a revolution counter connected to the spindle, so that the
number of turns can be seen at all times. A bicycle cyclometer can be
readily fitted up for this purpose, and will be found of considerable
assistance where a number of sections are needed, each with a similar
number of turns. In the commercial construction of telephone coils and
magnet spools it is often the rule to specify only the number of turns
of the requisite size wire, the ampere turns of the coils being thus


This is often necessary, and may be done in a variety of ways. When the
coil is working, and sparks be passed between fine wires mounted on the
discharger, the positive wire tip will be cold, whereas the negative
end will be quite hot. In vacuo, the positive shows a purple red when
the negative glows with a bluish violet. The decomposition of water,
which consists of oxygen and hydrogen in the formula H_{2}O, is readily
accomplished by the secondary current, and the greatest volume of gas
(hydrogen) will be evolved at the _negative pole_. For ready reference
a summary of these facts is given below:

   Positive      |  Negative
  Cold wire,     | Hot wire,
  Anode,         | Cathode,
  + sign,        | - sign,
  Purple red,    | Bluish violet,
  Zinc plate,    | Carbon plate,
  (Carbon) pole, | Zinc pole,
  Oxygen gas.    | Hydrogen gas.

Although it is customary to use bundles of fine, soft iron wire for
coil cores, very excellent results have been obtained with cores made
up of soft iron filings. These filings should be tightly packed in the
core tube and have a soft iron head at the contact breaker end. Filings
demagnetize very quickly and prevent the formation of destructive eddy
currents, which have been previously discussed (Chapter I.).

Modern practice tends towards a lengthening of the core and primary, in
some cases fully 20 per cent of the core length projects from each end
of the coil. One result must be as in electromagnets, the longer the
core, the longer it takes to magnetize or demagnetize. But even here
it is a matter of individual construction.

The common practice is to make coils to be in a horizontal position;
there is no reason why they cannot be made to stand on end. In fact,
this position to an extent takes off some of the strain on the primary.
It is mostly a matter of choice or convenience.

As to the possible output of an induction coil, it depends upon design
and construction; but S. P. Thompson gives the following law in his
work on Electricity and Magnetism: The electromotive force generated in
the secondary circuit is to that employed in the primary nearly in the
same proportion as the relative turns of the two coils.[1]

 [1] We do not attempt to reconcile this quotation with the enormous
estimates of spark potential.

In selecting a Ruhmkorff coil, it must be remembered that the rating
in spark length is subject to question. Supposing two similar coils
be operated, one with a rapid vibrator and the other with a slow
vibrator, other things being equal, the slow vibrator will give the
greatest spark length. Again, the appearance of the spark is of vast
importance. Although two coils might be sparking across the same
length air-gap, the one giving the whitest and thickest continuous
succession of sparks is the better. Fig. 13 shows a reproduction from
a photograph of a spark 32 inches long, generated by the coil shown on
the frontispiece.

 [Illustration: FIG. 13.]

It is easy to take a coil, and by snapping the vibrator contacts
together a few times a spark of thin bluish character will jump across
a gap, of length far exceeding the spark gap when vibrator is working
at normal speed. But this spark only passes at irregular intervals,
seemingly gathering strength for its forced leap. It must not be
considered in rating the coil.

In winding primary coils it is proposed to reduce the self-induction
or inductance of its adjacent coils by means of similar methods used
in winding electromagnets. The primary winding, instead of being
composed of a number of turns of one large wire, is made up of a
multiple winding of small wires, aggregating the conductivity of the
large wire. This materially reduces sparking at the contact breaker,
and certainly allows of a closer bedding of wire nearer the core, also
giving a greater percentage of ampere turns. Another scheme which uses
the Dessauer contact breaker provides two separate primary windings,
opening one when the other closes. Such schemes as these come well
within the scope of the experimenter, and it is highly possible that
valuable improvements will be made in coil design during the coming


The following are the commonest causes of coils not working to their
best limit: Contact breaker contacts dirty, burned, stuck, too small,
not in good parallel relation face to face of platinum.

Secondary wires crossed outside coil, often happens that the secondary
is quietly sparking away into or through some object touching it,
particularly when long wire connections are run from secondary to place
of desired sparking.

Condenser too small, burned out, badly insulated (see other pages on
this subject).

Battery too small—too high internal resistance or wires leading from
battery to coil too small—for ordinary coil work, distance of, perhaps,
ten feet, use No. 10 to 12 B & S flexible lamp cord or solid wire.
Ruhmkorff coils require plenty of current to produce large sparks.


                        |   ½     |   1   |   2     |   6   |  12
                        |  inch   | inch  | inches  | inches| inches
  Foil sheets           | 5½ × 4  | 6 × 4 | 6 × 6   | 10 × 5| 12 × 8
  Number                |   40    |  40   |   60    |   60  |   60
  Paper sheets          | 6½ × 5  | 9 × 5 | 8½ × 7  | 12 × 7| 14 × 10
  Number                |   60    |  60   |   80    |  80   |   80
  Core length           |    5    |   7   |    9    |  12   |   19
  Core diameter         |    ⅝    |   ¾   |    1″   |   1⅛  |    1½
  Primary size B & S    |   16    |  14   |   14    |  12   |   10
  Secondary size B & S. |   36    |  36   |   36    |  36   |   38
  Core wire size B W G. |   22    |  22   |   22    |  22   |   22
  Quantity in pounds of |         |       |         |       |
     secondary wire     |    ¾    |   1¼  |    2½   |   7   |   12
  Layers of primary     |    3    |   3   |    2    |   2   |    2
  Area of paper, sq. in.| 2,000   | 2,700 |  4,800  | 6,600 |11,000
  Area of foil, sq. in. |   880   |   960 |  2,100  | 3,000 | 5,760

As it is not always convenient to procure paper and foil in set sizes,
the area of material needed for condensers is also given. The above
table is approximate. It represents data collected from the best modern
practice. The gauge above given for copper wire is that of Brown &
Sharpe, and is used throughout these pages.


The main points of difference between coils for electrotherapeutics and
Ruhmkorff coils is that the former are devoid of condensers, are rarely
insulated to a high degree, and are arranged for current strength
regulation. The modes of regulation are many, briefly the principal
are: (_a_) In coils with independent circuit breakers, sliding both
core and primary coil out of the secondary together or independently.
(_b_) Moving a metal tube over or off the primary coil or core or both.
Many combinations of these methods are practised. Attempts have been
made to regulate battery current by rheostat, but it is not feasible,
except in large stationary outfits. Cheap medical coils are wound with
bare wire, with layers of thread between adjacent turns, or even only
bedding the wire turns in paraffined paper. It is not intended to
convey the idea that winding bare wire coils is a makeshift; far from
it. This method is being very generally adopted in telephone work. But
it requires special and delicate machinery, and is unsuited to amateur
work, where slight differences of cost or labor are insignificant.
Others for specific purposes consist of a primary coil only. The best
and most complete made are so arranged that independent secondary
coils of different sized wires can be used with the one primary, being
readily slipped on or off as required. There is another scheme of
regulation, where the coil is wound in sections and these sections cut
in or out by means of a switch, but it is not desirable.


 [Illustration: FIG. 14.]

Figure 14 shows a coil with tube mode of regulation. The core _C_
consists of a piece of iron tube, very thin, 4 inches long by ⅜ inch
diameter, and filled with soft iron wires. One end of this core is
firmly fixed in the left-hand bobbin head. The object of the iron
tube is to prevent the sliding tube from catching in the iron wires,
otherwise it can be dispensed with. Over this tube is slipped a brass
tube _T_, ending in a handle _H_ at the right-hand end; this must work
easily over the core tube. The spool for the primary is now made up by
fixing the other bobbin head on a paper or fibre tube and fastening
its free end to the left-hand bobbin head, or the spool can be made in
the usual way by glueing up two spool ends on a fibre or paper tube and
securing the iron core firmly in one end, allowing room, of course, for
the brass tube to slide in at the right-hand end. The primary winding
is three or four layers of No. 20 B & S gauge cotton-covered magnet
wire, the ends being brought out for future connection. Over this is
now laid a few layers of paraffined paper, and ten or twelve layers of
No. 36 B & S cotton-covered magnet wire is wound on for the secondary

The contact breaker _R_ is in no way different from the simple form
described in Chapter II. Its construction can be readily seen from the

A layer of cloth of the kind used in covering electromagnets is laid on
over the secondary, and the coil is ready to be attached to the base.
The base is seven inches long by three wide, and has little feet at its
four corners to elevate it from the table and prevent abrasion of the
connections underneath.

 [Illustration: FIG. 15.]

The connections are as given in Fig. 15. When in operation, the
electrode cords being attached to binding posts, Nos. 1 and 2 are in
circuit with the secondary coil only. When at Nos. 2 and 3 they receive
the induced current or extra current in the primary, caused by the
break of the battery circuit (see page 3).


This form of coil is the only one for practical medical work, and more
space will be given to its construction than to the foregoing, which
is suited only for limited use.

 [Illustration: FIG. 16.]

Fig. 16 shows side elevation of coil on base. The design can be largely
varied, also it can be used either for a wall board, a cabinet top,
or made to be carried in a case containing battery, electrodes, etc.
_S_ is one of the secondary coils, of which at least three should be
provided. The dimensions are, of course, the same—namely, four inches
long by 3½ inches wide over all. The spool ends are furnished with heel
pieces, which slide under the brass track bar _T_. This accurately
centres the coil and prevents it from working loose.


The following windings for removable or interchangeable secondary coils
are those most in use.

Coil No. 1. 4500 feet (.375 pound) No. 36 B & S, approximating 1800
ohms. This may be led out in three divisions by means of switch on coil
head. First division, 4500 feet; second division, 3000 feet; third
division, 1500 feet.

Coil No. 2. 2400 feet (.6 pound) No. 31 B & S, about 350 ohms, divided
into 2400 feet, 1500 feet, and 900 feet.

Coil No. 3. 750 feet (1 pound) No. 22 B & S in one coil, or two
divisions of 500 and 750 feet, respectively; approximate resistance of
wire, 125 ohms.

Coil No. 4. It may be necessary to obtain currents of extremely high
tension, in which case a coil may be prepared of 5000 feet No. 38 B &
S, or No. 40 B & S preferably.

The finer the wire, the less current and the most sedative effect; the
coarser the wire, the more current with corresponding increased painful

The spools, in fact as much of the framework as possible, should be
made of hard rubber, to which a fine finish can be given, although
mahogany, rosewood, or even stained oak can be used. On each side of
the right-hand spool heads a flat brass spring is screwed, making the
contact for the secondary wires on brass strips screwed on top of
the track rods. These secondary connections can be made by means of
flexible cords to binding posts, but the sliding contact is preferable.
The primary coil _P_ is firmly held in the left spool head, and
consists of a core of No. 22 B W G soft iron wires, insulated and wound
with three layers of No. 20 B & S magnet wire. The outside of this coil
is neatly enclosed in a hard rubber tube to permit of the secondary
coils sliding freely upon it. It is better, however, for the secondary
coils not to touch the primary tube. The vibrator, or contact breaker,
should be of the adjustable form shown in Fig. 17. The adjusting screw
for the contact breaker can be mounted in a brass lug carried by the
spool head.

Connections of this coil are substantially the same as those of
the first-described medical coil. This apparatus is well worthy of
elaboration; it should be fitted with a ribbon vibrator as well as an
adjustable speed slow vibrator, a switch controlling either. A great
variety of secondary coils can be made, those of coarse wire taking
the place of the current from the contact breaker. The vibrators
should be operated from an independent battery, although in the last
coil described the magnet may be wound with the same size wire as the
primary and then be in series with it. The secondary spools can be
made of stained hard wood ends fitted on to fibre tube, which latter
is easily procurable. Particular attention should always be paid to
the spools and heads; if not properly made, they may come apart, and a
disastrous unravelling of the wires ensues.

 [Illustration: FIG. 17.]


A coil much used for electric baths has a primary winding only,
regulated by the sliding in and out of the iron core, which
necessitates the use of an independent vibrator, or else by varying
the current strength with a rheostat. The general directions given
before will answer in the present case, the only data necessary being
the size of wire, which should be about six to ten layers of No. 20 B
& S. The coil with movable secondaries here comes into service. Strong
currents are needed for bath work, and any variety of winding can be
used with this make of coil. There are so many descriptions of bath and
small medical coils in the electrical magazines published for amateur
workers, that it is hardly necessary here to give more than a mention
of the principal ones.


A few remarks on medical coils and their diseases may not be amiss;
often a very little defect, if remedied in time, will prevent costly

The main care in medical electrical apparatus is the battery (see
Chapter X. for descriptions of coil batteries and their operation).
Clean, fresh solutions and clean contacts are essential. Keep zincs
well amalgamated, remove wires from binding posts, and scrape bright
the metal where the wires make connection; see no fluid is splashed on
contacts, clean all contact springs periodically. The Edison-Lalande
battery is probably the best for medical use, but even this requires
occasional attention as to contacts, new zincs, fresh solution, etc.

Poor adjustment at contact breaker, dirty or corroded contacts, loose
wires, loose binding posts, corroded binding posts, are often the only
trouble in a coil refusing to work.

Flexible cords are fruitful of trouble: the tinsel breaks, and there is
no circuit; gets wet and crosses or causes a leak; cord tips get loose
and alternately open and close a contact; one minute all is well, next
minute no current can be obtained. Another trouble in acid batteries
is caused by leaving the zincs in the fluid. It is easy to do it in
most cases, although the ingenuity of the leading medical electrical
apparatus makers to-day is directed to this point. Cleanliness and
careful inspection of all contacts is well repaid; carelessness surely
brings its evils.

It is very desirable in medical work to eliminate the noise attendant
upon the working of the coil vibrator. This jarring or humming is often
in itself a source of irritation to a nervous patient. The sound can be
deadened in various ways, for instance, by placing over the vibrator
a temporary wood cover, lined with felt, resting upon a soft rubber
gasket; or in any other manner that may suggest itself to the operator.


            |         |       || COPPER.   ||  GERMAN
            |         |       ||           ||  SILVER.
  Gauge,    |         |       ++——————————-++——————————-
  Browne    |Diameter.| Feet  ||           ||  ONLY
  & Sharpe. |         |per lb.||  Ohms     ||APPROXIMATE.
            |         |       ||  per      ++——————————-
            |         |       || 1,000 ft. ||   Ohms
            |         |       ||           ||per 1,000 ft.
        8   |  .1285  |    20 ||    .62881 ||   11.77
        9   |  .1144  |    25 ||    .79281 ||   11.83
       10   |  .1019  |    32 ||   1       ||   18.72
       11   |  .09074 |    40 ||   1.2607  ||   25.59
       12   |  .08081 |    51 ||   1.5898  ||   29.75
       13   |  .07196 |    64 ||   1.995   ||   37.51
       14   |  .06408 |    81 ||   2.504   ||   47.30
       15   |  .05707 |   102 ||   3.172   ||   59.65
       16   |  .05082 |   129 ||   4.001   ||   75.22
       17   |  .04525 |   162 ||   5.04    ||   94.84
       18   |  .0403  |   204 ||   6.36    ||  119.61
       19   |  .03539 |   264 ||   8.25    ||  155.10
       20   |  .03196 |   325 ||  10.12    ||  190.18
       21   |  .02846 |   409 ||  12.76    ||  239.81
       22   |  .02535 |   517 ||  16.25    ||  302.38
       23   |  .02257 |   660 ||  20.30    ||  381.33
       24   |  .0201  |   823 ||  25.60    ||  480.83
       25   |  .0179  |  1039 ||  32.20    ||  606.31
       26   |  .01594 |  1310 ||  40.70    ||  764.59
       27   |  .01419 |  1650 ||  51.30    ||  964.13
       28   |  .01264 |  2082 ||  64.80    || 1215.76
       29   |  .01126 |  2623 ||  81.60    || 1533.06
       30   |  .01002 |  3311 || 103       || 1933.03
       31   |  .00893 |  4165 || 130       || 2437.23
       32   |  .00795 |  5263 || 164       || 3073.77
       33   |  .00708 |  6636 || 206       || 3875.61
       34   |  .0063  |  8381 || 260       || 4888.49
       35   |  .00561 | 10560 || 328       || 6163.97
       36   |  .005   | 13306 || 414       || 7770.81



 [Illustration: FIG. 18.]

The simple form of contact breaker already described is useful up to a
certain point, but it has disadvantages. Its rate of vibration is only
variable through narrow limits, and it is not suitable for very heavy
currents. But as it stands it has done long service, and will be used
probably wherever the requirements from it are not exacting. The most
desirable form of this simple spring break is shown in Fig. 18. _R_
is the soft iron armature; _S_, the spring; _C_, check-nut which holds
the adjusting screw _A_ from becoming loose; _T_, a second adjusting
screw used to tighten the spring and so raise its rate of vibration;
_K_ is the base to which one wire of the coil is attached; _L_, base of
adjusting device to which battery wire runs at _I_. Where tightening
screw T passes through the pillar of the adjusting screw, the hole
therein is bushed with rubber to prevent accidental contact. Both _A_
and _T_ are provided with insulating heads of rubber or ivory. At _B_
are the platinum contacts, which should be fully ⅛ inch in diameter.

One serious defect in the action of the simple spring vibrator (Fig.
19) is the tendency of the spring to vibrate, as it were, sinusoidally.
This causes an irregularity in the rate of the vibrations, which
affects the discharge of the coil very considerably. By far the better
plan is to use a very short thick spring riveted to an arm carrying
the armature at its end (Fig. 20). _R_ is the armature, _S_ the piece
of spring, and _K_ the point of attachment to the base. The actual
width of the portion of the spring which vibrates—the hinge portion, it
might be called—should not be over ⅛ inch.

 [Illustration: FIG. 19.]

 [Illustration: FIG. 20.]

The rate of motion is high; but an erroneous notion has been taken of
its performance by many persons in the knowledge of the writer. The
rate of vibration is _not_ wholly dependent on the size, or, rather,
smallness of its spring; the arm and armature considerably alter this,
although they are not pliable, by reason of their mass and the momentum
consequent on their mass.

A word here on the size of the armature. It should be somewhat larger
than the face of the electro-magnet core, and should be thick—that is,
in a circular form—say one half its diameter. Of course this does not
apply to the steel lever armature before mentioned. It is impossible to
lay down arbitrary rules where the conditions are not determined, but a
very small amount of experimenting will demonstrate the correct lines
on which to build.

When in action, all rapid rheotomes give out a definite musical note
whereby the rate of vibration can be determined. Reference to any work
on acoustics will show a table of the number of vibrations necessary to
produce any stated musical note. The foregoing style of rheotome forms
the basis of very nearly all those which are in use. The shorter and
stouter a spring the more rapidly will it vibrate, and _vice-versa_.
Carrying out this rule, we can manufacture an instrument which will
give as high as 2500 vibrations per second (Fig. 21).

 [Illustration: FIG. 21.]

The armature _A_ is a piece of flat hard steel bar ¼ × ½ inch, held
rigidly on the metal support _S_ and just clearing the upper surfaces
of the magnet cores _C_. The adjusting screw _P_ should be provided
with an arm, _B B_, whereby the rotation of it can be delicately
varied. This screw must also be firmly held or the high speed of the
armature will jar it loose. A check-nut on each side of the frame
carrying it should be provided in every case. The necessary platinum
contact can be hammered into a hole drilled before the armature is
hardened. The proper place for this contact is about one fourth of
the total length of the armature from its support, although in the
simple contact breaker it can be placed at the distance of one third
if desired. The reason is that the concussion of the adjusting screw
dampens the free vibration, and the amplitude thereof is lessened in
addition to the counter vibrations of the screw disturbing the regular
vibrationary series.

Owing to the fact that the amplitude of the armature vibration is so
small, a very delicate adjustment is necessary. The adjusting screw
can be placed nearer the free end, but for the reasons given it is not
to be desired. The metal bridge should be a solid casting, and the
armature clamped by more than one screw.

The mercury vibrator, which is applied to almost every large coil, is
as follows:

A pivoted arm carries on one end a soft iron armature, which is
attracted by the coil core. The other end is provided with a platinum
point adjustable by a set screw. This platinum point dips into a
mercury cup—a glass cup containing mercury, with a thin layer of
spirits of turpentine. The object of the spirits of turpentine, which
is a non-conductor, is to help choke off the spark which would ensue
whenever the platinum point was raised from the mercury.

A form of contact breaker which will admit of great variation of speed,
and which is adapted to carry large currents, is the wheel-break,
constructed in the following manner:

A brass or copper disk 3 inches or more in diameter and upward of ½
inch thick has its periphery divided by a number of saw cuts, which
divisions are often filled in with plugs of hard rubber or fibre. This
disk is mounted on a shaft, which latter is either the shaft of an
electro-motor, or is provided with a pulley by which it can be rapidly
rotated. A strip of spring copper on each side of the disk presses
upon the toothed surface, one strip being connected to the coil and
the other to the battery or other current source. It will now be seen
that when the disk rotates the slits or pieces of hard rubber cause the
break in the circuit through the brushes or copper strips, the rapidity
of the breaks depending upon the rate of rotation of the disk, and the
number of slits in the wheel.

The slits or rubber pieces should be one-half the width of the
intervening brass, but must be at least one sixteenth of an inch in
width, especially where a high voltage is used in the primary coil.

The shaft of the machine may serve as one point of connection in place
of one of the copper brushes; but in this event either a wide journal
must be used, or else some conducting substance, as plumbago, replace
the lubricating oil in the bearings.


Fig. 22 shows a diagram of a pole changing contact breaker which will
allow of rapid alternations of current. It is operated by an electric
motor by preference, although any motive power can be applied to it.

 [Illustration: FIG. 22.]

_W a W b_ are two brass wheels, the peripheries of which are broken by
the insertion of insulating blocks _I I_, shown black in the sketch. _S
S_ are the shafts on which the wheels are mounted, the two wheels being
necessarily insulated from each other. 1, 2, 3, 4 are four brushes of
copper pressing on the rim of the wheel and leading in the current
from the battery _B_. The primary coil is attached to the brass
body of the wheel or to the shafts. When the wheel is in the position
shown, the coil and battery are on an open circuit; but on the wheel
commencing to revolve, the brushes 1 and 2 bear on the brass, and the
current flows from the positive pole of the battery to 2 through the
wheel _W a_ to the coil _P_, up through wheel _W b_ and out at 1 back
to the battery. The next position of the brushes 1 and 2 will be on
the insulations, and 3 and 4 will come into action. Then the positive
current will reach _W b_ by means of brush 3, and after traversing
the primary coil and wheel _W a_, emerge at 4 to the battery, thus
reversing the current through _P_ as many times as there are sets of
segments, which latter can be multiplied according to requirements. The
main point to be considered after that of good connections is that the
brushes 1 and 3 and 2 and 4 do not at any time touch any part of the
brass wheel at the same time, as this would short circuit the battery.
This is avoided by making the insulating space longer than the brass
surface, and adjusting the brushes as in the sketch, that each pair of
them is a fraction further apart than the length of the brass tooth.

Accordingly, a wheel may be constructed with many segments and rotated
at a high speed and rapid reversals of current produced, the uses of
which are manifold.

As will be described in the notes on the Tesla effects, an
electro-magnet, the poles of which are brought near the sparking point
of the contact breaker, will help wipe out the spark, and so assist the
suddenness of the break.

An extremely successful expedient in operating contact breakers is
to employ a high-pressure air blast directed point blank against the
contact point. The effect of this air blast when the contact is made is
of course null, but on the platinum surfaces becoming separated, the
high air pressure produced forms a path of extremely high resistance,
and tends to blow off the spark as soon as it is generated. The stream
of air should issue from an insulated nozzle of glass or rubber, and
should not contain moisture.


One of the most important inventions in coil work is the electrolytic
interrupter of Wehnelt. Briefly, the apparatus consists of a vessel
containing a solution of acid, into which dip two electrodes connected
in series with the source of power and the primary of the coil. Upon
passing a current through the combination the fluid becomes agitated at
the electrodes and a rapid make and break of the current ensues (Fig.

 [Illustration: FIG. 23.]

It requires considerable electromotive force for operation, a minimum
of 40 volts being desirable. Its rapidity of action varies up to and at
times exceeding 4000 interruptions per second. A Wehnelt interrupter
can be made as follows: Procure a glass jar _J_ holding about one
quart or a little less, also a cover for same _C_, a piece of sheet
lead _L_ large enough to fit loosely across the jar and yet not touch
the bottom, eight inches of one-quarter-inch glass tube _M_, a few
inches of No. 20 platinum wire _P_, and two ounces of mercury. Heat
the end of the glass tube in a gas flame, and bend an inch or less at
a right angle; at the same time seal in the platinum wire by means of
a blowpipe, so that the tip just projects from the bent end of the
tube. This sealing can be accomplished readily by one unused to working
glass, but almost any philosophical instrument maker will have it done
at small cost. Holes being bored through the cover, the lead plate
and the glass tube are fitted in, the platinum point almost touching
the lead. Adjustment is, however, easy, as the tube, being turned,
will retract or advance the platinum point from or towards the lead
electrode. Nearly fill the jar with a solution composed of one part
sulphuric acid to eight parts water, and fill up the glass tube with
mercury. The connections can then be made by means of a clamp on the
lead and a wire dipping into the mercury. Connect the lead plate _L_ to
one pole of the battery or source of energy, and the platinum-mercury
electrode _F_ to one post of primary. The other side of battery and
coil being closed, the apparatus will begin to work. No condenser is
needed with this interrupter.


This is a modification of the spring hammer-head type, but has a
platinum contact on both sides of the spring. It thus obtains double
vibrations, but is liable to stick. The elasticity of the spring
normally prevents the circuit remaining closed on the forward movement
of the hammer head, but this combination requires attention.


For light currents and rapid vibrations, such as are employed in
electrotherapy, the steel ribbon interrupter is suitable. It consists
of a steel ribbon _V_ one-half inch wide by six or eight inches long
and the thickness of a stout visiting-card. Near the end is riveted
a platinum contact. One end of the ribbon is held by a brass upright
_R_, to which connection is made to circuit; the other end is riveted
to a threaded rod, which passes through a brass pillar, and is held
by a thumb-screw and check nut _S_. Turning the thumb-screw either
way tightens or loosens the ribbon and so raises or lowers the rate of
vibration (Fig. 24).


Contact breakers in vacuo, as applied to Ruhmkorff coils, are by no
means of recent date. Poggendorff made use of such prior to 1859, and
noted the diminished sparking at the contact breaker and increased
effect in the secondary circuit.

 [Illustration: FIG. 24.]

 [Illustration: FIG. 25.]

Mr. D. McFarlan Moore, whose experiments in vacuum tube lighting
have proven so interesting, was granted patents upon various forms
of contact breakers, in which the chief merit was that the contacts
were broken in a vacuum. The sparking was almost eliminated, and the
suddenness of the break of contact so accentuated as to materially
improve the output of an induction coil. A perusal of his patents,
copies of which may be procured through almost any bookseller, will
prove profitable to the coil constructor.


The most important advantage of this arrangement is the abrupt break,
owing to a collar in the vibrator striking a movable contact while at
full speed. Reference to Fig. 25 will show that the movable platinum
contact is carried on a small vertical spring behind the vibrator
spring, and projects through a collar on the vibrator spring. When
the contact is made, the movement of the vibrator is not arrested,
but continues at its full amplitude, thus allowing a long "make."
The vibrator is kept moving at a constant amplitude by means of the
small coil shown in the illustration, which is in shunt with the main
circuit. In the old forms there has always been a liability of the
platinum contacts sticking (or welding together). In the new form, as
the break is made when the vibrator is in the middle of its swing, the
sudden blow with the entire momentum of the iron hammer head is always
sufficient to break the platinums apart. This form of contact breaker
is very efficient on electric-light circuits, and operates with the
utmost regularity.


This is a device where the actual break is made in alcohol between
large studs of platinum nearly one-quarter inch in diameter. The bottom
contact can be raised or lowered by means of an adjusting screw. The
top contact is secured into the bottom end of a rod passing down a
guide tube into the alcohol to meet the lower contact. By means of
an electric motor and a cam motion, the top contact and plunger are
made to work up and down in the alcohol, thus making and breaking the
current flow. One of the commendable features of this contact breaker
is that the platinum studs are caused to revolve while in operation,
thus presenting new faces to each other after each blow. The apparatus
is not adapted for rapid action, but for the handling of heavy currents.


An adjustable contact breaker for medical coils is shown in Fig. 26.
_M M_ are the magnet coils, _A_ is the armature, carrying a platinum
contact, which vibrates against the adjusting screw _P_. The armature
is pivoted at _J_, but is held at a distance from the magnets by the
springs _S S_. The other end of the armature carries a ball _B_, which
can be slid up and down on the rod and set at any point by a set-screw.
When the ball is at the end of the armature rod most remote from the
magnets, the vibrations are slowest; when moved towards the magnets,
the vibrations become more rapid. Adjustment of the two springs _S S_
at _R R_ enables the contact breaker to operate on varying current
strength, and also tends to lessen the jerkiness of gravity contact
breakers. A flat spring, however, can be substituted for the spiral
springs, in which case the pivot would be dispensed with and the spring
riveted, as in the hammer form of vibrator. The illustration shows this
arranged for a wall board, but it can readily be adapted for table work.

 [Illustration: FIG. 26.]


Fig. 27 shows a form of contact breaker much used in portable medical
coils for slow speeds. It consists of a cone of iron _H_, mounted on
the vibrator spring, and furnished with adjustable contact spring
and screw _A_. Its amplitude of vibration is limited by the two pins
mounted on the disc, between which the cone vibrates. The disc is
turned by hand, thus moving the pins, and so varying the travel of
the cone _H_ to and from the core _C_. It does not give good results
from the fact that the rhythmical movements are disturbed every time
the cone strikes against the pins, also at the contact spring striking
the contact screw. As we showed before, a really satisfactory contact
breaker should have a spring, which allows of no sinusoidal movement.
Where a pivoted armature is governed by a spiral spring, the result is
a series of steady, rhythmical shocks, provided the adjustments are

 [Illustration: FIG. 27.]


Fig. 28 shows the details of a contact breaker to be attached to
the coil head direct. It is often used on very small coils, which,
together with a miniature dry cell, is slipped into a pocket case.
An important detail in small coils is to use a contact breaker of
sufficient size. Most of them are not large enough to stand ordinary
usage, the adjusting screw is not of sufficient diameter and the thread
soon strips. There is no reason why the adjusting screw, its platinum
tip, and the pillar or lug which holds it should not be solidly built,
it would certainly require less adjustment. Either single or double
check-nuts can be fitted to the adjustment screws of nearly all the
forms of contact breakers described.

 [Illustration: FIG. 28.]


It is absolutely essential that the _diameter_ of contacts for all
contact breakers should be as large as possible and their faces filed
truly parallel to enable them to easily carry all the current required.
One of the main causes of failure of coil is burning of the platinum
point and platinum burr, the current being then materially reduced.
Large sparks at point of rupture are often indications that the
condenser is not working properly—perhaps has broken down or is not
large enough. The contacts will sometimes fuse together; at any rate,
the excessive sparking is an evidence of waste as much as in a dynamo

The adjustable method of arranging condensers (see Chapter IV.) is
here of great value, but it is easy to attach more condenser sections
to the contact screw pillar and vibrator pillar and notice result.
In the construction of Ruhmkorff coils it is a good plan to make all
connections possible on the coil base, instead of inside the condenser
chamber. This is done either by means of rubber-covered wires or neat
strips of brass, screwed down on the base from points of connection,
and, of course, carefully bent over or well insulated from all other
leads which they have to cross.

The best makers of induction coils construct their instruments so
that they can be readily taken apart with as little detachment of
connections as possible.



In selecting an insulating compound for apparatus designed to be
under the influence of high tension currents, a glance at some of the
peculiarities of such currents will not be out of place. Mineral oil is
used in many of the converters employed to transform the high voltage
currents on the mains of the alternating electric-light systems to the
comparatively low voltage used at the points of consumption. Professor
Elihu Thomson, in a series of experiments, noticed some interesting
facts in the sparking distances of high potentials in oils.

He found that discharges of low frequencies, as 125 alternations per
second, were capable of puncturing mineral oils at one third to one
half the thickness of an air layer sufficient to just resist punctures
by the same discharge; but with frequencies of 50,000 to 100,000
per second, an oil thickness of one thirtieth to one sixtieth was a
sufficient barrier.

At a frequency of 125 per second, a half-inch spark in the air
penetrated one third to one fourth inch of oil; but at frequencies of
50,000 to 100,000 per second, a layer of oil one fourth of an inch
successfully resisted the passage of a spark which freely passed
through 8 inches of air.

The effect of drying an oil improved its insulating qualities. (Tesla
uses boiled-out linseed-oil.)

He also noted that pointed electrodes could be brought nearer together
under oil than balls without allowing a discharge. Flat plates allowed
of still greater sparking distances. Tesla notes that oil through which
sparks have passed must be discarded, probably owing to particles of
carbon being formed.

Paraffin wax has a higher resistance than oil, providing it has not
been heated over 135° C. It will stand alternate heating up to 100°
C. and cooling, being of lower resistance when hot than when cold.
But a serious permanent deterioration takes place when it has been
heated over 100°C.; its color, from the normal pure white, changes to a
yellowish tint when its insulation is impaired. Paraffin also undergoes
a deterioration when heated for a long time even at 100° C., and should
never be used for fine work when it is at all yellow. It is always best
to melt it in a hot-water bath, not permitting, however, any steam or
moisture to come near it. In this climate (United States) it is not
so necessary to mix in any tallow to obviate brittleness, the average
temperature of most workshops being sufficiently high to keep it from
becoming brittle.

Resin oils do not suffer permanent injury from being heated, as does
paraffin, but their insulating properties diminish much more rapidly
on becoming even warm, the initial resistance of resin oils being lower
than that of paraffin.

Paraffin has a fault—its tendency to absorb a slight degree of
moisture. It has been found in telephone and telegraph cables
saturated with paraffin that this is a very important cause of their
deterioration. In Ruhmkorff coils, however, which are intended for
operation in enclosed places free from damp atmospheres, the absorption
of moisture would be probably reduced to its minimum.

There is one substance which, were it not for its cost, would be far
preferable to paraffin for coil work, and that is beeswax. Its cost,
however, is generally five times that of paraffin, even when purchased
in quantities. It never becomes brittle enough to be damaged in careful
handling, its melting point is low, and it does not absorb moisture.
But it must be unquestionably pure and clear.

In foreign practice a variety of resinous mixtures are used to
insulate the turns of the wire in Ruhmkorff coils.

Equal parts of resin and beeswax used hot, paraffin, resin and tallow,
and shellac and resin are employed.

Shellac—that is, the yellow lac—is much used as a varnish for
electrical instruments, being dissolved in alcohol to saturation. For
dynamo armatures and similar apparatus the shellac varnish is of great
service, and many good compounds of shellac, such as insullac and
armalac, have been prepared for ready use. But (excluding beeswax) for
our purposes paraffin stands pre-eminently at the head of the list.

In using shellac varnish, in high tension work more particularly, care
must be taken that the moisture has entirely evaporated. Although a
piece of shellacked apparatus may appear perfectly dry, yet when the
current is allowed to flow unlooked-for results may appear—it takes
hours in a dry atmosphere for shellac varnish to dry. Baking the
apparatus in a warm oven is a necessary expedient whenever feasible,
care being taken not to burn or decompose the shellac. The proportions
most generally used are 1 ounce shellac to 5 ounces alcohol. Stand the
vessel containing the mixture in a warm place, and shake it frequently;
filtration improves the varnish somewhat.

A ready and efficient varnish for silk is prepared by mixing 6 ounces
of boiled linseed-oil and 2 ounces of rectified spirits of turpentine.
For paper, 1 part of Canada balsam and 2 parts of spirits of turpentine
dissolved in a warm place and filtered before being used. A good
insulating cement for Leyden jars and insulating stands is prepared
from sulphur, 100 parts; tallow, 2 parts, and resin, 2 parts, melted
together until of the consistence of syrup, and sufficient powdered
glass added to make a paste. To be heated when applied, this will
resist most acids. The resin and beeswax compound is handy when making
experimental mercurial air pumps of glass tubes, as it has a fair
tenacity, is not too brittle, and is easily used.



A condenser is an apparatus whereby a charge of electrical energy
may be temporarily stored, the amount of energy it will hold
determining its "capacity." The capacity of a condenser is measured
in micro-farads, the commercial unit representing one millionth of a
farad. A farad equals the capacity of a body raised to the potential of
one volt by a charge of one ampere for one second at one volt—_i.e._ =
one coulomb.

The measurement of the capacity of a condenser is accomplished by
the use of a ballistic galvanometer. The latter instrument has a
bell-shaped magnet suspended in a coil of fine wire. When a momentary
current is passed through this coil the magnet hardly commences to
rotate until the current has practically ceased. A beam of light is
reflected from a mirror fixed to the magnet on to a scale. The degree
of deflection is compared with that obtained by the discharge of a
condenser of known capacity, and the capacity of the condenser being
measured is deduced by a simple rule. The farad, which is the unit of
capacity requiring a condenser of an immense size, is replaced by a
commercial unit, the micro-farad—that is, one millionth of a farad.

The original form of the condenser was the Leyden jar, which owes its
name from the town of Leyden in Europe.

 [Illustration: FIG. 29.]

The Leyden jar is made as follows (Fig. 29): A clean uncracked glass
jar with a wide mouth is coated on the inside and outside with tinfoil;
sometimes loose tinfoil is filled inside, the tinfoil, however, not
reaching more than two thirds of the jar's length from the bottom.
A cork is fitted, and through the middle of it a wire is passed
touching the inside coating of tinfoil and terminating in a metal
sphere outside. A simple Leyden jar can be made in a few moments by
half filling a glass bottle with water and wetting the lower half of
the outside; a wire run through the cork into the water finishes the
job. But this is at least only a makeshift, although a fair amount of
current has been collected from a leather engine belt in motion in one
thus made.

A condenser can be easily made as follows (Fig. 30):

 [Illustration: FIG. 30.]

Procure a clear glass plate, _G_, free from flaws, 11 inches square
by 3∕32 inch thick. Give this a good coating of shellac varnish all
over, sides and edges. Cut out of smooth tinfoil two sheets, _T_, 8
inches square, and round off the corners with a pair of shears. There
must be no sharp corners, projections, or angles to induce leakage.
Lay the glass plate on a sheet of paper, and mark its outline thereon
with a pencil; then remove it and substitute a sheet of the tinfoil,
and mark that. This will enable you to centre the foil. Give one side
of the glass plate another coat of varnish, and so lay it on the paper
that its outline coincides with the pencil outline. When the varnish
has partly dried take a sheet of the trimmed foil, and by observing
the pencilled marks you can lay it on the varnished plate exactly in
the centre. Lay down the top edge first along this line, and carefully
deposit the remainder of the foil in place. Next, with a flat brush
full of varnish go over the plate, pressing out any air bubbles, and
ensuring both a flat and a well-varnished surface. When this is dry,
turn over the plate and repeat the operation on the other side.

If desired, a metal hemisphere of at least an inch in diameter may be
attached with varnish, first scraping the foil to make a contact. The
whole plate can be swung in a cradle of two silk threads, laid on a
glass tumbler, or mounted on end in a shellacked block of wood.

A strip of tinfoil, _S_, attached at the corner can be used as a
connector. The plates must be joined in the following manner when two
or more are used in conjunction, and a quantity of current is desired.
They should be placed so the connecting strips project alternately from
each side (Fig. 31), and all on each side joined so as to leave two
terminals, one to the 1, 3, 5 plates, the other to the 2, 4, 6 plates,
and so on, which, when joined, will have the same effect as would
result from the use of two large plates of the same total area. The
nearer the plates are together the greater capacity they will have,
always supposing the insulation is good, the insulation being known
as the dielectric. Another good method, when a high quality of glass
can be procured, is to lay the tinfoil on the plates without varnish,
piling one on top of the other, tinfoil and glass alternately, and
clamping the whole securely, laying a piece of cloth top and bottom
to avoid cracking the glass from the pressure. This must be kept from
moisture; a strip of paraffined paper stuck along the edges and extra
paraffin run on will answer very well.

 [Illustration: FIG. 31.]

In constructing these glass condensers, they must be designed to
correspond with the coil with which they are to be charged. In the
foregoing description we have allowed a margin of 1½ inches of glass
around the foil coatings. This will make 3 inches as the maximum
distance between the coatings. Although a 2-inch spark from the coil
would not jump this interval, a certain discharge will take place,
and the less this occurs, the more serviceable the condenser will be.
Therefore a greater margin should be allowed for a longer spark than 2

In the commercial condenser for telephone and telegraph use, paraffin
and paper are substituted for glass, as will be described later.
Heavy paraffin oil gives excellent results, but its fluidity is

There is no valid reason why paraffin could not be used on the glass
plate condensers, care being observed that it is free from dirt and
metallic chips. In fact, the space between the glass plates of the
multiplate condenser may be filled in with paraffin, and thereby
exclude the air. Only a condenser so built up is not convenient to take
apart for experimental purposes.

The foregoing description of a glass insulated condenser was written
with the assumption that a good quality of glass be used. But the
ordinary window glass is generally useless, and paraffined paper is
preferable. The quality of glass known as "hard flint glass" is best,
the superior qualities being imported from Europe. This latter is used
in the manufacture of the standard Leyden jar for lecture purposes.

Were it not for its cost, the finest dielectric we could use would be
sheet mica. Unfortunately sheet mica over 3 inches square is expensive,
and becomes rapidly more so as it becomes larger.

Standard condensers for testing are made with mica carefully selected,
and retain the charge for the maximum length of time. The built-up mica
condenser is immersed in molten paraffin until the same has permeated
the sheets, and then the complete mass is put under a pressure until
the paraffin is well set.


The paper used in the manufacture of the commercial form is a special
thin, tough linen paper carefully selected, sheet by sheet, to avoid
pin-holes or flaws, and kept in an oven until used to ensure absolute

When this cannot be procured, use thin unsized writing paper of a
good quality, well dried, and absolutely clean. As an example of
the necessity of cleanliness, a light lead-pencil mark would serve
to conduct the current entirely from a charged sheet to wherever it
terminated, and if suitably located, utterly destroy the usefulness
of the apparatus. Ink, which most generally contains iron, will cause
trouble, and although some cheap foreign condensers are built up of old
ledger pages, yet their efficiency is very uncertain.

The paper used in commercial condensers is from four to seven
thousandths of an inch in thickness.


The smaller the amount of surface the less will be the capacity, but
the quicker the discharge. The apparatus heretofore mentioned has had
the alternate plates connected together in two series, presenting a
large surface and rendering a large amount of current. A condenser
so made will have a low voltage or potential, but is not so liable
to leakage as one made to render a high potential. The multiple
condenser of a large capacity will hardly discharge and spark over an
air gap requiring a contact of the two electrodes. But a smaller one,
consisting only of a single pair of small plates, will spark across
quite a considerable air gap.

A number of charged condensers may be put in series, and the resultant
potential thereby increased. Cut a number of pieces of paper of the
desired size, say 6 inches square, and a number of sheets of foil 3
inches square. Round off the corners of the foil and build up first a
sheet of paper, then a sheet of foil in its centre, then another paper
and another foil sheet, and so on. There is to be no connection from
sheet to sheet, only the inductive action of one on its neighbor. The
foil must be considerably smaller than the paper in this construction,
owing to the greater tendency to discharge round the edges of the
sheets, owing to the greater potential of the current.

When the requisite number of sheets have been built up, leave a sheet
of foil top and bottom for connection, tie between two pieces of stout
card or board, and immerse in the molten paraffin. When thoroughly
soaked, remove and put under pressure until cold. It will be found
undesirable to make these with more than a dozen pairs of sheets, but
to make a number of blocks of that number for ready service.

 [Illustration: FIG. 32.]

Fig. 32 shows the arrangement of the apparatus to charge a Leyden
jar, the plate form being connected in a similar manner. The jar is
stood upon an insulating support—a dry tumbler will answer—with the
ball _B_ connected to one pole of the coil. From the outside tinfoil
coating _T_ a wire runs to the discharger _D D_, which is in circuit
with the secondary coil, _S_. The discharger balls _D D_ are carefully
approximated until the spark just passes, this latter point being
of great importance. Were the discharger balls too near the spark
would probably pierce the dielectric of the condenser, therefore the
balls should be carefully _brought near_ to each other until the
exact distance is found. Even if the insulation of the condenser were
not pierced, yet a path would probably be opened through which some
succeeding discharge would pass, and ruin the instrument.

Another method of charging is to leave an air gap at _B_; then there
is not much liability of the condenser discharging back through the
coil—an undesirable event, as it would most likely perforate the
insulation of the coil.

 [Illustration: FIG. 33.]

In designing or using any apparatus intended to hold a charge of high
potential, it must be kept in mind how readily points or sharp edges
serve to allow the current to pass off—we might almost say evaporate.
Given two bodies, one a globe and the other a rectangular block, each
well insulated from the earth or any other large body, and the globe
would be found to hold its charge long after the block had dissipated
all trace of the charge given to it. Therefore round off every edge and
angle, projection or point.

In making handles, supports, or any work requiring an intervening high
insulation, hard rubber is preferable to glass where there is liability
to moisture. When the apparatus is as shown in Fig. 33, the condenser
is alternately charged and discharged with a loud noise, the vivid
sparks passing across the discharger balls _D D_ possessing great
deflagratory powers.

In experimenting with a Ruhmkorff coil it is not advisable to leave the
instrument working while the secondary terminals are beyond sparking
distance, as there is a great strain on the secondary insulation. Nor
is it wise to use only one electrode in an experiment, unless the other
is connected to some apparatus of an approximate capacity to that at
the other, for the foregoing reason.


Now that the condenser has become so important a factor in telephone
work, many schemes for cheapening and facilitating their manufacture
have been devised. One in particular merits description, the
"rolled-up" condenser having come largely into use. The tin-foil is
supplied in rolls containing many yards of foil of the requisite width
for the condenser to be made. Likewise rolls of paper are provided,
exceeding in width, however, those of tin-foil. These rolls are
arranged upon horizontal spindles in front of an empty spindle, or
mandrel, upon which the condenser is to be formed. A few turns of the
paper ribbon are made around the mandrel, then the foil is brought
forward and a few turns made, then follows a turn of paper ribbon
and another of foil, and finally a paper layer; and the mandrel being
rotated, the alternate layers of foil and paper are laid on and rolled
around each other on the mandrel until the requisite quantity is
obtained. It then becomes an easy matter to cut the paper ends so no
contact is possible between the layers of foil. The whole thing is
slipped off the mandrel, secured by a rubber band or two, placed in a
hot paraffin bath, and left to become saturated while still warm and
before the paraffin has time to harden; the cylinder is put under a
press and squeezed flat, driving out excess paraffin, and leaving the
condenser in a convenient shape to handle. Connections are then made to
the foil leaves, and a case of wood or metal completes the work.

There is no reason why aluminum foil or lead foil, or, in fact, any
thin sheet metal should not be used in condensers. In telephone work,
paper covered with gilt paint was tried, and worked fairly well, but
was ultimately rejected in favor of tin-foil. In some cases, when
it is desired to construct a condenser for high potential work, the
oil-tank apparatus can be used. This is readily made of any desired
dimensions, as follows: Procure a square glass jar, such as is made
for storage batteries, a few pieces of sheet metal cut to fit loosely
in the jar, some glass rods and sufficient clean "transformer oil"
or heavy paraffin oil to nearly fill the jar. The sheets of metal
can then be hung from the glass rods into the jar, being separated
one-half inch, and the oil poured in. Two plates, about 8 inches by 6
inches, will hang nicely into a type D^3 Chloride Battery jar, which
is 7⅞ inches long by 9½ inches high by 3¼ inches wide. Altering the
relative distances between the plates will give considerable adjustment
to this simple condenser, or, if desired, more plates may be inserted
and connected up, as in the tin-foil condensers. This type can be made
portable, but it is not to be recommended unless no objection is had
to emptying and refilling the jar with oil.


In operating large coils, it is convenient to be able to vary the
capacity of the condenser on the primary circuit. To make an adjustable
condenser presents no more difficulty than a non-adjustable one, simply
more labor. For example, the large condenser used with the 6-inch
spark coil might be divided into four sections, containing 2000 square
inches, 500 square inches, 300 square inches, and 200 square inches of
surface (see Fig. 34). Wires leading from the ends of the foil sheets
_C C_ are to be brought to the brass plates _G G_. The brass rods
_B B_ are connected by binding posts to the coil, each strip being
well insulated from its neighbor. Any combination is possible by the
insertion of brass plugs in holes drilled between the strips. The plugs
must be fully large enough to make good contact on each of the two
strips between which they are inserted, and should be turned taper.
With the largest coils the condenser and contact breaker are generally
mounted separately, and are fully adjustable.

 [Illustration: FIG. 34.]


  Dry air        1.000
  Sulphur        2.590
  Hard rubber    2.290
  Paraffin       1.996
  Shellac        2.750
  Kerosene       2.225
  Paraffin oil   2.710
  Castor oil     4.962
  Olive oil      3.575

Condensers made with dielectric of high inductive capacity (insulation
being equal) will retain greater charge than those made with
dielectrics of low inductive capacity. Thus, one made with shellac
would be nearly half as great again as with paraffin.

Capacity of a condenser increases with area of foil surface, with
diminished distance between foil plates and with increase of



The luminous effects that can be obtained by means of a Ruhmkorff coil
are exceedingly beautiful and instructive. The simplest experiment
of this nature is the production of the spark consequent on the
approximation of the electrodes attached to the secondary coil. This
spark can be varied in both length, intensity, or shape by the form
and nature of the substances between which it is permitted to pass.
Attach to each end of the discharger a fine steel needle, and bring
them together until the spark jumps from one to the other. A long thin
snapping spark will pass, which, however, appears to be trying to take
any but a straight path across the air gap. The peculiar crookedness
of this, as in a lightning flash, is credited to the fact of particles
of matter floating in the air conducting the current better than the
pure air. The curious odor noticed in these discharges, as, in fact, in
the working of all high-tension apparatus, is ozone—O_{3}, triatomic
oxygen. This gas, so noticeable after a thunderstorm, has a powerful
effect on the mucous membranes of the throat and nasal passages,
and must be inhaled with caution. It is being used by the medical
profession for the destruction of germs and for general therapeutic

Substitute pieces of fine iron wire for the needles, and bring the ends
together about one quarter the distance through which the normal spark
will pass. The spark will be found to have changed its appearance, now
being thick and redder, or, rather, of a deep yellow, and to possess
vast heating qualities.

The iron wire will melt at one electrode, and if the other be examined
it will be perceived that it has not even become warm. The cold wire
will be the one connected to the positive pole of the coil.

Connecting the poles together with a piece of very fine iron wire will
result in the deflagration of the wire in a vivid light.

The short thick spark is termed the calorific spark, and believed to
possess its yellow color from the combustion of the sodium in the air.
This spark will easily ignite a piece of paper held in its path.

Take a sheet of hard rubber and breathe on its surface; lay a wire from
each pole of the secondary to points on the sheet, about twice as far
apart as the spark would pass over in the air. The electric current
will strive to complete its circuit; streams of violet light forming a
perfect network will issue from each pole, until, provided the rubber
is sufficiently damp, they will unite in a spark far exceeding its
normal length in the air. It is curious to watch how the streams branch
out from these two points, and how persistently they strive to meet
each other. Scatter some finely powdered carbon on this sheet (crushed
lead-pencil or electric light carbon is good material). The points may
now be removed to still further distant places, and yet the current
will work across. Each particle of carbon seems to be provided with
innumerable scintillating diamonds, so sparkling is this effect.

Hard rubber is not absolutely necessary for these experiments; glass
will do, but the black background of the rubber intensifies the
luminosity of the discharges. Take a teaspoonful of powdered carbon and
scatter it between the points on the rubber, so that the spark can find
a ready path, evidenced by but little visible light. It will be seen
that this powder is blown away from one electrode after a few minutes,
leaving the latter in the centre of a clear space, but at the other
electrode not much disturbed.

Bring the points so close to one another that the spark becomes short
and fat; soon the carbon will commence to burn, forming a veritable arc
light. Take two pointed lead-pencils and wrap a few turns of wire from
the electrodes round the blunt ends of them; bring the pointed ends
together, and an arc will soon be established; but at various points
where the wire is wrapped the current will burn through the wood, and a
number of incandescent points will ensue.

In these experiments on the rubber sheet it will be noticed that the
spark acts as it does in the air, inasmuch as it does not take a direct
path, but jumps in an irregular track from point to point.

If two small metal balls be substituted (Fig. 35) for the points
between which the sparks be passing, it will be noted that the sparks
do not pass through so great an air gap as before, or even as rapidly.

The spark between two balls is much noisier than that passing between
points, and if the balls be of about 1 inch in diameter, a curious
effect ensues on the passage of the current (Fig. 36). This effect has
been likened to a stream of water issuing from a horizontal nozzle into
a cavity when the nozzle is moved up and down slowly in the space of a
few inches.

 [Illustration: FIG. 35.]

 [Illustration: FIG. 36.]


 [Illustration: FIG. 37.]

This easily made exhibit (Fig. 37) is one that is susceptible of quite
a number of applications. In its simple form it is but an enlarged
version of the rubber sheet scattered with carbon dust. The old way
to make it was to take a plate of glass and cement on one face of it
a sheet of tinfoil, using shellac varnish preferably. When dry, the
tinfoil was scored across and across in such manner as to divide
it up into little squares or diamonds. When the current was applied
to each end of the plate, the spark divided into innumerable little
ones; between each bit of tinfoil and its neighbors there would be
many little sparks, and the effect was very pretty, somewhat as
was described before when the carbon dust was strewn between the
electrodes. It is more easily and quickly prepared by giving a sheet
of glass a coating of shellac varnish, and then sparingly dusting any
powdered conductor over its surface, using perhaps carbon dust or
filings of metal. By cutting out a stencil from a piece of thin card
and laying it over the sparkling plate, the design shows out very
strikingly, and various designs in stencils can be prepared, different
powdered conductors giving different colored sparks.

A long glass tube moistened inside with mucilage or shellac varnish and
then having some conducting dust shaken through will also give quite a
pleasing effect.


Coat one side of a glass plate with tinfoil, leaving an attached strip
for connection. Shellac a piece of paper of a size corresponding to the
design to be rendered luminous. When the shellac has dried so far as to
become "tacky," lay a sheet of foil on it and press it down evenly all

Then draw on the paper a design that can be readily cut out. Use a pair
of scissors or a very sharp knife. If the latter, lay the sheet on a
piece of glass; but there is a greater tendency to tear the design when
a knife is used if an unpractised hand wields it.

This design may either be stuck on to the plain side of the glass plate
with varnish or simply laid on (Fig. 38). Connect one secondary wire to
the foil coating of the plate and the other to the design. This must be
shown in the dark, and the luminosity will not be strikingly apparent
until the eyes become accustomed to the darkness—that is, when the
room has been previously lighted.

One of the most beautiful and easily obtained phenomena of the
high-tension discharge is the "electric brush" (Fig. 39). This occurs
when the secondary electrodes of the coil are too far apart to allow
of the free passage of the spark, and can only be seen at its best in
a perfectly dark place. The ball tips before mentioned show this brush
very plainly, or two sheets of tinfoil in circuit hung far enough apart
to prevent vivid sparking will cause this so-called "silent" discharge.
This latter arrangement should not be used for over fifteen minutes,
as the ozone which is liberated in large quantities will affect those
persons in the vicinity.

 [Illustration: FIG. 38.]

 [Illustration: FIG. 39.]

In fact, when a rapid vibrator is being used with the coil, the leading
wires from the secondary terminals present this brush appearance, the
curious threads of light resembling luminous hairs waving in the air.
The more rapid the vibrations the more prominent the brush effect, as
will be seen in the Tesla coils. The positive ball of the discharger
shows the brush as a spreading mass of luminous threads reaching out
toward the negative ball, which latter resembles a star, as in the

The intensely disruptive power of the long spark is readily shown by
its power to perforate substances, but great care must be taken that
the secondary wires of a coil are led away from the body of the coil. A
good plan is to hang two silk cords or stout threads from the ceiling,
to which the secondary wires may be attached and kept in sight when
experimenting at any distance from the coil.

To pierce a piece of thin glass, take two lumps of paraffin about the
size of a walnut, and, warming them and the glass sheet, stick them on
opposite sides of the glass facing each other. Then warm the ends of
the two pointed wires and thrust them into the lumps of paraffin, that
they terminate on the glass surface directly opposite each other. On
connecting these to the secondary coil a few impulses to the contact
breaker will start an electric discharge sufficient to pierce the glass
if the thickness be proportioned to the power of the apparatus. The
great Spottiswood coil pierced a block of glass 6 inches in thickness.

There is, however, a certain element of danger to the secondary
insulation in performing this experiment.



If a metal or the salt of a metal be burned in a flame it imparts to
the flame a distinctive color; table salt thrown into the fire burns
with a yellowish flame, denoting the presence of sodium, and a greenish
tint, indicating the combustion of chlorine. Violet flames accompany
the burning of the salts of potassium, and barium burns green. Lithium
and strontium give a red hue. But to be ordinarily perceptible,
the salts require for the most part to be present in considerable
quantities. By the use of the spectroscope, however, extremely small
proportions of these metals and salts can be readily detected and

 [Illustration: FIG. 40.]

If a beam of light be transmitted through a prism of glass the rays
are decomposed, and what is known as a spectrum is formed (Fig.
40). The most generally observed spectrum is the rainbow. When the
light from a flame in which is burning some suitable substance be
transmitted through the prism, the color which predominates in the
flame will predominate in its spectrum. The combination of a prism and
tubes for observing these effects is a spectroscope (Fig. 41). The
short fat spark from the Ruhmkorff coil is most useful in this work.
The electrodes are provided with a portion of the substance to be
examined, and the spark is passed and viewed through the spectroscope.

 [Illustration: FIG. 41.]

The spectroscope is shown in connection with the coil in Fig. 41. _A_
is the aperture in the screen through which the rays from the metal
burning at the discharger balls _D D_ passes. The lens at _L_ is used
to view these rays after they have been decomposed by the prism _P_,
which, as well as the lens, can be rotated. _I_ is the coil, _P P_ the
primary and _S S_ the secondary wires, _C_ being a condenser bridged
across the circuit.

The screen should be pierced by a very narrow aperture, _A_, and be
placed at a considerable distance from the prism _P_, that the rays
issuing through the aperture may not strike the prism until they have
widely diverged and become separated from each other. The aperture is
practically formed of perfectly parallel knife edges, forming a slit
not exceeding one hundredth of an inch in width.

The colored spaces in the solar spectrum do not occupy an equal extent
of area; the violet is the most extended, the orange the least. The
proportion is in three hundred parts: Violet, 80; green, 60; yellow,
48; red, 45; indigo, 40; orange, 27.

The solar rays exhibit on careful examination dark lines crossing
the spectrum at right angles to the order of the colors, and always
occupying the same relative positions. These are called Fraunhofer's

If, however, the spectra of metals, gases, and other elements be
examined they will be found to present certain characteristic _bright_
lines, the body of the spectrum being often feeble or entirely dark.
The spectrum of hydrogen gives two very bright lines of red and orange.

An extremely minute quantity of an element is necessary to give
distinct lines. Sodium gives a single or double line of yellow light in
a position agreeing with that of the orange rays in the solar spectrum.

Potassium gives a red line in the red end and a violet line in the
violet end of the solar spectrum. Strontium presents eight bright
lines; calcium gives mainly one broad green band and one bright orange

In practical work with the spectroscope a solar spectrum is often
arranged that it can be used as a comparison with the spectrum being
investigated, one spectrum being formed above the other, and the
observation made as to which lines coincide. Iron gives nearly sixty
bright lines coinciding with the same number of dark lines of the solar

The violet rays of the solar spectrum are the rays which possess the
maximum chemical action, the yellow the maximum light effect, the red
the maximum heating effect. Beyond the violet band of the spectrum
exist certain rays termed the invisible rays or ultra-violet rays,
which in themselves are not luminous. Their vibratory rate is higher
and their wave length shorter than the violet rays, according to the
most generally accepted theory of light. These rays, when passed
through certain substances, suffer a change and become visible
in a luminous state of the substance, which luminosity is termed

The bright yellow line of sodium in the orange rays is found in nearly
all spectra, owing to its extensive diffusion in the atmosphere.

Tesla has succeeded in producing electric waves of length approximating
to those of white light, which appear to have very little heat. The
ideal light is that which shows no heat and does not liberate noxious
gases in the air, and were it not for its feeble luminosity, the light
of the electric spark passing through a carbonic acid vacuum would
approximate this most nearly.

The present mode of obtaining light—that of raising to a high
temperature some substance or collection of particles—seems certainly
somewhat antiquated. The following notes may be of interest and
assistance in researches bearing on the lighting question.

Solid bodies, when heated, show a red glow in daylight at an elevation
of temperature corresponding to 1000° Fahr.

  Temperature,    Color of
  degrees F.      Substance.

    1000           Red.
    1200           Orange.
    1300           Yellow.
    1500           Blue.
    1700           Indigo.
    2000           Violet.
    2130           All colors—_i.e._, white.

The number of vibrations per second necessary for the production of
light, and the velocity of light being determined, the calculation of
the wave lengths of the colored rays becomes possible.

The following table (Sprague) shows this in ten-millionths of a
millimetre (a millimetre = .039 inch) measured in the dark lines of the
solar spectrum, from red to violet:

  Orange =            6.88
  Orange, Higher =    6.56
  Yellow =            5.89
  Green =             5.26
  Blue =              4.84
  Blue, Higher =      4.29
  Violet =            3.93



Notwithstanding it requires an intensely high potential to enable the
current to jump an air gap of 1 inch, the same potential will produce a
luminous discharge through exhausted glass tubes aggregating 8 feet or
even more.

But the exhaustion can be carried so far that there is no apparent
discharge; and, on the contrary, air at as high a pressure as 600
pounds per square inch will resist the passage of the spark over an
extremely short space. If the tubes be filled with various gases
and then partially exhausted, the length of tube through which the
luminous discharge will pass varies with the gas, becoming shorter in
the following order: Hydrogen, nitrogen, air, oxygen, and carbonic
acid—the shortest.

 [Illustration: FIG. 42.]

Before detailing some of the more striking phenomena connected with
high-tension discharges in vacuo, a description of a few forms of
simple mercurial air pumps will be serviceable.

Fig. 42: If a glass tube, _F_, stopped at one end, 3 feet long or
over, be filled with mercury and the open end immersed in a vessel
of mercury, _T_, the column of metal in the tube will sink until it
attains a height, _M_, of about 30 inches, varying according to the
condition of the atmosphere.

The space between the mercury column and the top of the tube will be
a fairly good vacuum. This fact was noted many years ago, and the
gradual evolution of the mercurial air-pump based on this result can
be followed in the articles on the mercurial air-pump by Silvanus P.
Thompson, read before the Society of Arts, England, some years ago.

Geissler, the first manufacturer of the "Geissler" or vacuum tube for
electrical research, seeing the inconvenience of the above-described
operation and the meagre results obtained, invented the pump called by
his name (Fig. 43).

_F E_ is a stout glass tube some 3 feet long, having a bulb, _B_, at
its upper extremity, and a rubber tube, _S_, attached to the curved
end. A reservoir of mercury, _R_, connects with this rubber tube, and a
special glass tap is fixed in the upper end of the glass tube at _E_,
beyond which tap being the point of attachment for the object to be
exhausted. The operation is as follows: On turning the tap a part of
the way it allows a passage between the tube _F E_ and the atmosphere.
The reservoir _R_ is then raised until the mercury flows into the bulb
and up the tube to the tap. The tap is then turned a fraction, and the
communication with the air is shut off and opened between the object to
be exhausted and the tube _F E_. The reservoir is then lowered and the
mercury falls, drawing down the air from the object into the tube. The
tap is then turned as in the first place, and the reservoir _R_ raised,
when the air drawn into the tube is forced out by the rising column of
metal. This operation being repeated many times, withdraws nearly all
the air from the object—in fact, makes a fairly good vacuum. This pump
has been much modified from the simple form described.

The form of pump most used in the United States lamp factories is based
on the application of the piston-like action of a quantity of mercury
dropping down a tube. This is known as the Sprengel pump, after the

 [Illustration: FIG. 43.]

 [Illustration: FIG. 44.]

Fig. 44: _F_ is a stout glass tube about 40 inches long by
one-twelfth of an inch internal diameter, carrying the reservoir
funnel _R_ at the top, a piece of soft rubber tubing, _S_, nipped by a
pinch-cock being interposed to admit of the regulation of the mercurial
drops. The lower end of this "fall tube," as it is called, is immersed
in mercury contained in a suitable vessel, _V_, a branch tube being
blown or cemented into the fall tube to admit of the connection of the
object to be exhausted at _E_. _S_ is another piece of rubber tubing
with a pinch-cock regulation. The point _H_ is the normal barometric
height of the mercury—about 30 inches. On attaching a bulb, for
example, at _E_, and regulating the pinch-cock at the top of the fall
tube _F_, a succession of drops of mercury falls down the tube, each
drop acting as a piston to drive the air before it, sucking the same
from the bulb, and forcing it down through the tube and vessel out into
the atmosphere.

On its first being set into operation, the cushions of air between the
drops silence their fall; but as a higher degree of rarefaction occurs,
the air cushions become insufficient, and the drops fall with a sharp
click on the top of the barometric column.

One great disadvantage in this form of pump is the tendency to fracture
of the glass tube that is manifested by the concussion of the drops of
mercury at the barometric height. However, this has to a certain extent
been obviated in later forms of this useful and efficient pump.

For many electrical experiments, the simple exhaust tube (Fig.
42) mentioned at the beginning of the article will be found very
satisfactory. The top end need not necessarily be sealed off with
glass, a cork having a wire, _W_, run through for connection being
driven in, and a coat of paraffin or one of the cements mentioned in a
later chapter be laid on.

The second electrical connection is made by a wire dipping in the
tumbler of mercury.


In a simple glass tube having two wires carrying balls inserted
through its ends, from which the air has been partially exhausted, the
study of the changes shown by the passage of the spark is extremely
interesting. Before the commencement of exhaustion no luminous effect
can be discerned; at a low degree of exhaustion a luminosity appears
between the ends of the wires, the negative pole being surrounded by a
violet glow and a larger pear-shaped red discharge from the positive.
An interval near the negative electrode is in darkness, widening as the
exhaustion progresses. When the degree of exhaustion is very high, a
series of arches concentric with the positive ball appear and become
broader and more distinct as the rarefaction progresses. The arches or
bands are called striæ, and are most distinct when the tube is made in
the form of a narrow cylinder, with a bulb at each end. Carbonic acid
gas vacua give the best results. If the finger be placed on the bulb at
either end a luminous spot appears, and by using a very rapid contact
breaker in the primary circuit, the luminous discharges become highly
sensitive, being diverted from their regular path on the approach of
the hand, a magnet, or a grounded wire. An extended treatment of these
phenomena would be out of place here, but can be found in nearly all
comprehensive works on electricity.

If an incandescent-lamp bulb be held in the hand and one end be brought
near to a terminal of the coil, a beautiful bluish light appears.[2]
The carbon filament, if long, and not held by its loop, becomes
electrified and oscillates, often giving out a clear, high, bell-like
sound as it strikes the glass. Particles of carbon deposited on the
glass during the burning of the lamp, shown in daylight as a blackening
deposit, generally show little sparks, like stars scattered over the
inside of the globe.

 [2] This depends on the degree of exhaustion.

A vacuum tube will phosphoresce if held in the hand near a secondary
terminal, or even if laid on the table near the coil, and will light
quite brilliantly if one end be held against a terminal. This latter
method, however, is generally inconvenient, as a certain amount of
physical pain ensues from the discharge into the skin.

Different gases in the tubes give characteristic colors. In carbonic
acid gas the whitish green hue prevails; in hydrogen, white and red; in
nitrogen, orange yellow. The characteristic spectra are given by the
gases in the tubes, and can be readily examined in the spectroscope.
But there is sometimes a slight variation in these colors, dependent
upon changes in the current.

In many Geissler tubes, a portion of the bulbs is made of uranium
glass. On the passage of the spark in the tube this glass glows with
a magnificent emerald green hue. Other tubes are constructed with an
outside enveloping glass tube fitted with a corked orifice into which
can be poured different solutions.

 [Illustration: FIG. 45.]

 [Illustration: FIG. 46.]

 [Illustration: FIG. 47.]

Fig. 45 shows a solution tube to be filled with solution of sulphate of
quinine, etc.

Fig. 46 shows three exhausted tubes arranged in series.

_A_ is of uranium glass, and glows dark green; _B_ of English glass,
showing a blue hue, and _C_ of soft German glass, glowing with a bright
apple-green tint.

Crystals of nitrate of calcium, nitrate of silver, benzoic acid,
tungstate of calcium, lithia benzoate, sodium salicylate, zinc
sulphide, and acetate of zinc fluoresce.

Fig. 47 is a highly exhausted tube, having at its lowest part a few
pieces of ruby. When the secondary current is turned on at _P_ and _N_
the rubies shine with a brilliant rich red, as if they were glowing hot.

 [Illustration: FIG. 48.]

Fig. 48 shows the tube to exhibit the effect resulting from focussing
the electric rays on a piece of iridio-platinum at _B_.

The cup _A_ forms the negative pole; the metal disk _C_, the positive.

On increasing the intensity of the spark, the metal at _B_ glows with
extreme brilliancy, and melts if the intensity be carried too far.



Although the luminous discharges in the exhausted tubes are extremely
beautiful, yet the effect is indescribably enhanced when the tubes are
rotated. Gassiot's star was the name given to the earliest exhibit of
a rotating tube carrying a luminous discharge, owing to the curious
phenomenon ensuing from the interruptions of the spark. As the human
retina is only capable of retaining an impression for a fraction of a
second, and as the tube is only momentarily luminous during the passage
of the spark, the effect of the revolving tube is that of a series of
such arranged as the radii of a circle, the number apparent, being
governed by the rapidity of rotation and the rate of interruption of
the current.

 [Illustration: FIG. 49.]

Fig. 49 represents a form of rotating wheel which is easily made, and
yet susceptible of many novel and attractive effects. Such a wheel,
placed in a store window, would undoubtedly attract many persons by
the beautiful variations of colored figures which it presents while
in motion. And once a crowd is collected and its attention attracted
to one spot, the capabilities of advertising the goods on sale are

A pasteboard or light wooden disk _D_, 3 feet in diameter or over, is
mounted on a shaft, _S_, operated by an electric motor or such power
as may be attainable. Upon its surface are mounted the tube-holders _T
T T T_, connected, as shown, by wires leading from the secondary of
the Ruhmkorff coil. Starting at the shaft _S_, the circuit runs to the
first tube-holder, where the continuity of the wire is broken to allow
of the attachment of the vacuum tube. From the first tube-holder the
wire runs in turn to each of the other three tube-holders, terminating
at _R_, where it passes through a hole to a metal ring on the back of
the disk shown by the dotted circle. This ring and the shaft are in
connection with the secondary coil, by reason of its electrodes being
attached to two brushes or strips of metal pressing, one on the ring,
the other on the shaft; or the bearing in which the shaft turns may
displace one of the brushes. _W W_ are two counterbalance weights,
that the wheel may run smoothly and be not affected by the irregular
distribution of the tubes or its surface. _E E_ are elastic bands,
looped over the wire and through rings in the disk, that the wires may
not be liable to touch or short circuit.

At Fig. 50 is an enlarged view of a tube-holder, although, as it
is meant only as a diagram, considerable variation of design is
permissible. The springs at _H H_, to which the wires run, being bent
back, the metal pins _P P_ may be thrust through the rings on the ends
of the tube, and the elasticity and pressure of the spring will hold it
in place and make the necessary contact. A wooden block, _B_, secured
to the face of the disk, is provided with a thumb-screw, _S_, securing
the tube-holder to it, by means of which the tube-holders may be
turned a trifle upon their axes and so vary the effect of the wheel.

 [Illustration: FIG. 50.]

 [Illustration: FIG. 51.]

Fig. 51 is a side view of the wheel, showing one manner of mounting
the disk and its connections. The same figures apply to the parts as
in the preceding figure. _M N_ are the wires leading to the coil, _P_
is a pulley on the shaft whereby the rotary power may be applied. The
wires on the face of the disk are not shown, as they would impair the
clearness of the diagram unnecessarily.

The greatest danger in the operation of such a piece of apparatus
will be the tendency of the high tension spark to wander where it is
not wanted, and to take short but forbidden paths back to the coil.
However, care and perhaps experiment will prove the remedy. It will be
noticed by reference to Fig. 49 that a circle has been drawn almost
bisecting two of the tube-holders. This circle represents a circle of
danger, and where a thin material has been used for the disk, the disk
may very well be reinforced by a piece of stouter card cemented on its

The disk, whether of wood or of pasteboard, must have a liberal coating
of insulation, either shellac varnish, paraffin, or beeswax, and be
absolutely free from unnecessary holes. Moreover, the ring _R_ must be
of such a distance from the support _F_, if the latter be metal, as
will preclude any jumping of the spark. A Ruhmkorff coil giving upward
of three quarters of an inch of spark will be large enough to operate a
wheel carrying four 8-inch tubes.

The wheel may be set back in a window and surrounded by dark fabrics,
or built in, as it were, in a cave of such. The judicious use of
pieces of looking-glass scattered on the sides of the cave, in such
manner as to reflect the light of the tubes, will enhance the effect.
There is no danger of fire where ordinary care is used, as the _long_
spark necessary to the production of the luminosity will hardly ignite
anything but gas, unless specially arranged to do so.

Fig. 52 is a triangle formed of three Geissler tubes, and intended for
rotation as a whole. _M M_ are two pieces of mica or glass, to prevent
any possibility of the spark jumping and short circuiting, in which
event the tubes would fail to light.

 [Illustration: FIG. 52.]

This triangle is shown diagrammatically at _A B C_, Fig. 53, mounted
on an insulated rotating disk. Before commencement of rotation, and
upon the current being turned on to the tubes, a simple triangle will
result, but at a certain stage of rotation the Maltese cross shown
is formed. A still higher rate of rotation will produce the double
star, Fig. 54, and as the rotation and rate of vibration of the coil
contact-breaker is varied, an apparently endless succession of stars or
triangles appears to grow out into view.

 [Illustration: FIG. 53.]

 [Illustration: FIG. 54.]

Although Figs. 53 and 54 serve to illustrate a triangle of tubes and
its variations, a very pretty and simple effect can be obtained with it
as follows: Three strips of looking-glass are cut and scratched across
their silvered surface, as described for the luminous pane, Fig. 37.
The current then being allowed to pass, and the wheel being rotated,
the triangle acts as in the preceding paragraphs, multiplying and
forming figures, which are extremely interesting to watch.

While treating on the subject of store-window attractions, a few
suggestions on a display of stationary Geissler tubes may be made.
Starting with the assumption that the platform on which the goods
would be displayed is of wood, a very small amount of preparation is
necessary. The platform is covered with a dark material free from
gloss, such as canton flannel, on which the tubes are laid in any fancy
pattern, or may be scattered haphazard. Fine bare wire (No. 36 B. & S.
is not any too small) is run from tube to tube, using care that it does
not touch itself in such manner as to short circuit the current. There
is not much necessity to cover the wires, unless the rate of vibration
of the contact be so rapid as to show the brush discharge from the wire
strands. In a jewelry store the cylindrical portions of the tubes may
be covered with strips of dark cloth, concealing all but the bulbs.
The Uranium bulbs will resemble emeralds; the yellow bulbs, topaz; and
the blue, turquoise—certainly a very striking collection of gems. A few
diamond-shaped pieces of the foil-coated glass scratched across, by the
whiteness of the tiny sparks will aid to set off the whole. The outfit
is not expensive: a coil giving a one half inch spark will light from
four to six tubes to great brilliancy. Cloths with metallic threads
woven in them must not be used, nor any of the metallic powders known
in the trade as "glitters."



When it is desired to light clusters of gas jets situated in
inaccessible places, or a number of them simultaneously, this method
finds ready application. It operates in the division of a long spark
among a number of burners, the gas being turned on at the main and
the primary circuit of a Ruhmkorff coil closed and opened until the
succession of sparks ignites the gas, Fig. 55. There are various
commercial forms of these burners, prominent among which is the "Smith
jump spark" burner.

 [Illustration: FIG. 55.]

A lava tip is provided with a mica or isinglass flange midway between
the tip and the lower end of the burner. This flange isolates the
electrodes from any possibility of the spark straying away to the
metallic pillar in which the burner is inserted. The multiple lava tip
burner is intended for use where a very short burner is needed, also
for flash rings multiple lights. Here the tips are placed close enough
together to ignite by contagion. In this case one of the common tips
is removed from the ring and a multiple lava tip substituted. It is
customary to allow sixteen burners to one inch of spark. Any number of
series can be operated alternately by means of a suitable switch.

The wire used to connect the burners is generally bare copper, and as
small in diameter as will sustain its own weight without injury, the
amount of the current being infinitesimal. It is supported on porcelain
or glass knobs screwed to the wall or ceiling, being carefully planned
to avoid any metallic substances to which the spark might be tempted
to escape. In wiring chandeliers, the wire is run through glass tubes
wherever there is any liability of its coming near the metal pipes.
There is a very great danger of this jumping of the spark where it is
not wanted, and the utmost care must be taken in planning the course
the wires shall take. Even a damp wall will cause trouble or a gilt
cornice, although the latter may be entirely insulated from the ground.
The switch bases for the groups of circuits must be of hard rubber, and
the switch points and levers be placed so far apart that there is no
liability of the spark jumping, which it certainly will do if it gets
a chance. Ordinary insulated wires are ineffectually protected by the
rubber compounds used. Glass, mica, and better still, a large air gap
are the only insulations that will serve, for the tremendous potential
or voltage of the current must be carefully considered whenever
insulation is necessary. The coil is better provided with a spring key
in the primary circuit than a vibrator, it gives better control of the
circuit and probably a larger and better spark.


The spark which occurs at the contact breaker of a Ruhmkorff coil
is held in check by the condenser; were no condenser used, it would
possess considerable powers of combustion. Using a large primary coil
and a few cells of open circuit battery, this spark is made to pass
across the path of a gas jet, which it instantly ignites. The contact
breaker consists of a platinum point, fixed on the gas tip, and a
German silver spring, carried on a lever, which latter is pulled across
the tip so as to make and break the circuit at the burner orifice. Some
burners are provided with a ratchet arrangement, by which pulling the
lever once turns on and lights the gas, pulling again turns it off;
others require the gas to be turned on first.

 [Illustration: FIG. 56.]

Reference to Fig. 56 will show the connections to two burners _P P_ and
an automatic burner _A_, to be described later. The coil _C_ is a core
of soft iron, about ¾ inch diameter and eight to ten inches long, wound
with from two to four pounds of magnet wire, Nos. 12 or 14 B & S. One
side of the battery goes to ground or to the gas pipe, thus forming the
return circuit. The wiring on the fixtures is done with No. 20 to 24 B
& S gas fixture wire, insulated with four windings of silk or cotton.
This is fastened to the lacquered brass work by means of thick shellac
varnish, it being tied on first with thread, which can readily be
removed when shellac is dry and hard. The wire is held on the insulated
collar of the burner by a small nut and screw, and great care must be
taken to ensure no grounding. The setting up of a gas-lighting outfit
is extremely simple, but it often fails for want of care. There must be
the best possible insulation between wire and metal work.


There are several forms of these burners, but the principle of all is
the same. A gas burner protrudes from the top of a brass case which
encloses the actuating mechanism. This mechanism consists of two
electromagnets, the armature of one opening a valve and allowing the
gas to flow, at the same time vibrating a platinum-tipped rod, which
produces a series of sparks at the burner tip. These sparks ignite the
gas, and a second magnet is provided to shut off the flow of gas, thus
extinguishing the light. Some devices use one electromagnet for both
lighting or extinguishing, but the majority are with double magnets.
The circuit is worked from a push button situated at any desired
location, and having a white and black button, one for lighting and
the other for extinguishing. The principal automatic burners are the
Holtzer, the Boston, and the Bartholdi, between which there lies little
choice, so admirably are they constructed.


Instead of a rotating stop-cock, as in other automatics, a gravity
valve is employed in the Bartholdi, which is held to its seat by the
weight of the armature and connecting stem, as shown in Fig. 57. When
the gas is turned off the valve rests upon its seat, as indicated in
the cut. By a closure of the electric circuit at the turn-on button,
two of the helices _M P_ are energized, causing the armature _J_ to
be lifted, thus, by means of the stem _H_, raising the valve _G_ from
its seat into the dotted position, and opening the gas way so that
the gas may issue to the tip, as shown by the arrows. At the same
time, the top of the valve strikes against the end of the lever _W_,
causing the circuit to be broken at the spark points _T U_, resulting
in a continuous sparking as long as the finger presses the button.
The magnet when raising the armature has also twisted or partially
revolved it, so as to bring the notch _d_ in the armature over the end
of the hook _e_, as shown in dotted lines. When the circuit is broken
by lifting the finger from the button the notch falls into the hook and
the valve is locked open.

 [Illustration: FIG. 57.]

To extinguish the flame the turn-off button is pressed, when a second
magnet (not shown in cut) lifts the armature and twists it in the
opposite direction, so that when the circuit is broken the armature
falls free to its normal position, closing the valve.

 [Illustration: FIG. 58.]

In wiring up an automatic burner it is necessary to run two wires to
it, one from the white button and another from black button on push
plate _S_. Reference to Fig. 58 will make this clear. Most burners
are provided with two binding posts inside the brass case, and the
wires are run through a rubber-bushed hole in the base. If the push
has already been set in position and wired up, as per Fig. 58, have
the buttons pressed alternately, when on touching the binding posts
on automatic with the wires, the lighting or extinguishing connection
is easily selected. The lighting armature in most automatic burners
buzzes violently, while the extinguishing one only strikes once on
contact being made. Fig. 58 shows how to connect up two pushes to one
automatic, one push, perhaps, being located downstairs and the other
upstairs in the case of a hall lamp. In setting up these burners care
must be taken not to bend contacts or alter adjustment, and absolute
precaution is necessary that no crosses or weakly insulated places
are in circuit. After burning for some time it often happens that the
burner refuses to light, only buzzing feebly or not at all. If feebly,
the trouble is in battery, which should consist of, at least, four or
six cells of open circuit battery with low internal resistance, such as
Samson-Law carbon cylinder, or for occasional use large, dry cells.

If no click is heard on pressing white button, examine all connections;
if still no trouble is found, examine the platinum break. The platinum
tip may be bent by the continual hammering against the platinum tip on
vibrating rod, preventing contact on collar, or that soot has formed
there. These are the commonest maladies of automatic burners, and can
be easily remedied by readjusting platinum tip and cleaning. Contacts
here must be clean. In general wiring use waterproof office wire or,
better still, rubber-covered wire; for fixtures use the fixture wire
before described. When shellacking the wire to the fixture don't
attempt to connect up batteries until the shellac is dry and hard,
say for half a day. Electric gas-lighting is fruitful of trouble if
the work is not well done. Another cause of trouble may arise from a
dirty burner not allowing the gas to strike near the contact (clean
the burner), or the collar carrying contact may have shifted, perhaps
short-circuited; it should be insulated with a thin strip of asbestos.
Although white lead at the joints makes a fairly good contact, some
persons prefer to use tin-foil, a piece of foil being worked around
screw thread and the burner screwed on; it prevents leaks as well as
lead if well done, and makes better contact. As a short circuit on
the wires will cause all the burners to fail, many devices have been
invented to open the circuit upon such an occurrence. These will be
found described in the catalogues of electrical stores; they do not
come within the province of this book for description.



In selecting a battery to operate the coil, one is needed which will
supply a large steady current for a considerable period. Although the
primary circuit is opened and closed rapidly, yet the class known
as open circuit cells is not suitable, even though they have a low
internal resistance, and thereby render a large current. Such cells
are only suitable for the uses for which they are mostly designed,
bell-ringing or annunciator work. There is one case, however, where an
open circuit cell may be used with an induction coil, and that is in
gas lighting as previously described; but here a dozen or so impulses
of current are generally sufficient, followed by long periods of rest.
For the latter work the cells in common use are the Samson, Champion,
and Monarch, all of which are of low internal resistance and great
recuperative power.

The reason that such cells will not work for long periods, is that they
polarize. This latter action takes place in these open circuit cells,
which are of the Leclanché type as follows: A positive plate of zinc
is immersed in a solution of ammonium chloride (or salammoniac), and a
negative plate of carbon and peroxide of manganese, contained either in
a porous cup or compressed into a block also stands in the solution.
Care is taken that these two plates do not touch each other. When the
outside circuit is closed the zinc combines with the chlorine of the
solution liberating free hydrogen and ammonia. The hydrogen appears
at the negative plate, where it is acted upon by the oxygen of the
peroxide of manganese to form water.

But when the circuit is of too low resistance, the oxidizing action of
the peroxide of manganese is not rapid enough, and a film of hydrogen,
which is a poor conductor, forms over the negative plate, increasing
the internal resistance of the cell and setting up local action. In
the best class of these open circuit cells, this hydrogen is absorbed
after a rest, and the battery recuperates and is ready for work again.
The circuit of the Ruhmkorff coil is low, and this polarization always
occurs a few minutes after the contact-breaker is started.

 [Illustration: FIG. 59.]

In the class of closed circuit cells, chosen for the present purpose,
the Grenet or bottle bichromate is one of the handiest for occasional
use. A glass bottle-shaped jar, _J_, Fig. 59, is provided with a hard
rubber cap, _G_, on which are mounted the binding posts _A B_. To
the underside of this cap are attached two carbon plates _C C_, which
reach nearly to the bottom of the jar, being connected together on the
cap by a varnished copper strip, the latter being in turn connected to
one binding post. Through the centre of the cap passes a brass rod,
_R_, having attached to its lower end a piece of sheet zinc, _Z_, well
amalgamated with mercury. This process of amalgamation consists in
cleaning the zinc, then rubbing its surface with a rag dipped in dilute
sulphuric acid, and pouring a few drops of mercury on the wet zinc.
The mercury will spread readily over the zinc, provided it has been
well cleaned, and if properly done should give the zinc plate a bright,
shining appearance.

When the cell is not in use, the zinc is drawn up into the neck of
the bottle and clamped by a set screw against the brass rod. A copper
spring pressing on the rod serves to carry the current to the second
binding post.

This cell originated in France, whence its name, but a cheaper form is
now made in the United States known as the Novelty Grenet. The shape of
the jar is somewhat different, and the carbon is moulded, whereas the
French carbon is sawed from the carbon deposited in the gas retort; but
the American form is practically of as great utility as the French, and
the cost recommends it.

The bichromate solutions are affected by light, and deteriorate less it
kept in stoneware jugs. The Grenet battery can very well be fitted into
a neat wood case, which will serve the further purpose of preventing
chance knocks from fracturing the glass jar.

Carbons which are used in batteries containing the foregoing solution
should be well washed in warm water whenever the solution is changed,
and especially when it is intended to put the battery out of active
service. When the solution acquires a decidedly green hue it should
be replaced with fresh. The electromotive force of this cell varies
from 1.90 to 2 volts, and the amperage is dependent on the size of the
plates, running from 5 amperes upward.

The glass jar is filled up to the commencement of the neck with a
solution of bichromate of potash or sodium, called electropoion fluid,
and prepared as follows: To 1 gallon of water add 1 pound of bichromate
of sodium, mixing in a stoneware vessel. When dissolved add 3 pounds
of sulphuric acid in a thin stream, stirring slowly. As the mixture
heats on the introduction of the acid, care must be used to pour in the
latter slowly. This solution should not be used until quite cold.

The sodium salt is preferable to the potassium, owing to its not
forming the crystals of chrome alum, and also on account of its lower
cost and greater solubility, the latter being four times greater than
that of the potassium salt. The commercial acid used should contain at
least 90 per cent pure acid and should be free from impurities. On
filling the battery use utmost care not to splash the solution on any
of the metal work, or it will cause corrosion. Although the salts in
the solution will most likely make a stain, the corrosive action of the
acid can be arrested if the solution be splashed on the clothes by the
prompt application of ammonia solution.

 [Illustration: FIG. 60.]

The "Fuller" cell, Fig. 60, which is another type of the bichromate
cell, is one from which a steady current can be obtained for a
longer interval than from the Grenet, but the current is less. The
electromotive force is the same, but the current is only 3 amperes,
except in certain modifications.

In the porous cup is a cone-shaped zinc having a stout copper wire cast
in. This wire is occasionally covered with rubber insulation, but,
as a rule, is bare. The porous cup is of unglazed porcelain, thick,
but very porous. This sets in the glass jar, a wooden cover fitting
_loosely_ over the whole to exclude dust. Through this cover passes
the wire leading from the zinc, and also the carbon plate carrying a
machine screw and check nuts for connection. The cover is dipped in
melted paraffin, as is also the upper end of the carbon and the rim
of the glass jar. This is to prevent the creeping of the salts in the
solutions and the corrosion of the brass work.

Into the porous cup is poured a solution composed of 18 parts by weight
of common salt and 72 parts by weight of water. Electropoion fluid is
held by the glass jar, the two solutions reaching a level of two thirds
the height of the jar. One ounce of mercury is added to the porous cup
solution to ensure the complete and continuous amalgamation of the
zinc. The salt can be more readily dissolved in warm water, but _all_
solutions must be used _cold_. It is not always necessary to renew the
solutions when the battery fails to give out its accustomed strength,
but several ounces of water can be substituted for a similar amount of
fluid in the porous cup. Stir the solution by moving the zinc up and
down, and a temporary improvement will be noticed.

To obtain a greater current from this cell, use a larger zinc, such as
a well-amalgamated zinc plate, and add a teaspoonful of sulphuric acid
to clean water for the porous cup solution. Additional carbon plates
connected together and placed round the porous cup will lower the
resistance of the cell and increase the current, and also tend to keep
down the polarization.

A new form of this battery was described by M. Morisot a short time ago.

The positive pole is of retort carbon in the outer cell in a
depolarizing mixture made of 1 part sulphuric acid, 3 parts saturated
solution bichromate of potash, crystals of the latter salt being
suspended in the cell to keep up the saturation. A porous cup contains
a solution of caustic soda. The zinc is in a second porous cup placed
within the first, which holds a caustic soda solution of greater
density. The electromotive force is 2½ volts when the cell is first
placed in circuit, and will remain at 2.4 for some hours. The internal
resistance is low, but varies with the thickness of the porous cups.
This cell is not suitable for any but use for a few hours at one time.

The Dun cell has a negative electrode of a carbon porous cup filled
with broken carbon. The zinc is in the form of a heavy ring, and hangs
at the top of the solution in the outer jar. Permanganate of potash
crystals are placed in the porous cup, and the entire cell filled with
a solution of caustic potash 1 part to water 5 parts. The voltage is
1.8, and the internal resistance being low the resultant current is

A cell with an electrode of aluminum in a solution of caustic potash
and carbon in strong nitric acid in porous cup is claimed to have an
electromotive force of 2.8, but the nitric acid is not a desirable acid
to handle.

Metallic magnesium in a salammoniac solution with a copper plate in a
hydrochloric acid and sulphate of copper mixture is of high voltage,
nearly 3 volts being obtained, and the current is large, but it is a
new combination and has not as yet stood the test of time.

There are other formulæ for solutions to be used in Fuller or Grenet
cells which may be useful to the experimenter. Trouvé's is as follows:
Water, 36 parts; bichromate of potash, 3 parts; sulphuric acid, 15
parts, all by weight. Bottone's: Chromic acid, 6 parts; water, 20
parts; chlorate of potassium (increases electromotive force), ⅓ part;
sulphuric acid, 3½ parts, all by weight. A convenient "red salt" or
"electric sand": Sulphate of soda, 14 parts; sulphuric acid, 68 parts;
bichromate of potash, 29 parts; soda dissolved in heated acid, and
potash stirred in slowly. When cold can be broken up and prepared when
required by dissolving in five times its weight of water.

The chromic acid used in Bottone's solution is very soluble in water,
it being possible to dissolve five or six times the amount in the same
quantity of water as of bichromate of potash. The simple solution of
chromic acid is 1 pound to 1 pint of water, to which is added 6 ounces
of sulphuric acid.

When it becomes necessary to cut zinc plates, it may be readily done by
making a deep scratch on the surface, filling the scratch first with
dilute sulphuric acid, and then with mercury. The mercury will quickly
eat into the metal, and the plate may be easily broken across or cut
with a saw. Zinc plates can be bent into shape by the application of
heat. Hold the plate in front of a hot fire until it cannot be touched
by the bare hand: it will be found that it has softened so that it
can be bent around a suitable wooden form. As zinc plates are most
attacked at the surface of the acid solution, it is advisable to coat
the extreme upper portion of them with varnish or paraffin. Rolled
zinc is always preferable to cast, especially so when immersed in acid

To avoid confusion, it may be stated here that it is the rule to speak
of the zinc element as the positive plate and the negative electrode or
pole, and the carbon _vice versa_. The portion of the element immersed
in the solution is the plate, the part outside, the pole or electrode.
In diagrams and also in formulæ positive is shown by a + (plus) sign
and negative by a-(minus) sign.

The relation of cost of the materials most used is shown in the
subjoined table, which cost, however, varies with the market:

  Sulphuric acid, chemically pure    18
      "       "   commercial          1.5
  Muriatic    "                       1.12
  Nitric      "                       3.5
  Electropoion fluid                  2
  Bichromate of potash               10.5
       "     "  soda                  8.5
  Caustic soda                        9
  Salammoniac                         7
  Chromic acid                       19
  Blue vitriol                        4
  Litharge                            5.75
  Mercury bisulphate                 94
  Paraffin                            9
  Beeswax                      35 to 45
  Shellac varnish                    87
  Tinfoil                            35


A cheap modification of the Daniell cell. A glass jar has at the bottom
a copper plate consisting of 4 to 6 leaves of thin sheet copper, set
on their edges in a starlike shape, a copper wire being attached to
the copper rivet which holds the leaves together. A mass of crystals
of sulphate of copper is filled in and laid on the top of the copper
electrode an inch or so above its top. The negative plate is a
variously shaped plate of cast zinc hung from the edge of the jar and
reaching about 2 inches from the top into the fluid. Water is poured in
until it covers the zinc, and the battery is complete. The sulphate of
copper deposits its metallic copper on the copper leaves and liberates
sulphuric acid, which rises and attacks the zinc, setting free sulphate
of zinc. The sulphate of zinc solution being of greater density remains
near the bottom, and the sulphate of zinc solution stays near the zinc.
When the cell is left too long on an open circuit the two solutions
tend to mix, and copper is deposited on the zinc. The sulphate of zinc
finally saturates the top solution, which has to be partly drawn off
and replaced by fresh water and crystals of sulphate of copper dropped
into the jar to take the place of that which has been decomposed.
Electromotive force 1 volt, current from 3∕10 to 5∕10 of an ampere.
The practical working of this cell will be treated of later on in
these pages.

 [Illustration: FIG. 61.]

The Gethins (Fig. 61) and the Hussey bluestone cells both have the
zincs standing in porous cups (shown by dotted lines), which in turn
are supported half-way down the jar, generally resting on the copper
strip acting as a porous partition between the fluids. The zinc stands
in a solution of zinc sulphate, or a weak sulphuric acid solution.
The internal resistance is low, and the current large, being from 1
to 5 amperes. These cells are the ideal bluestone cells for charging
storage batteries requiring very little attention. The special Gethins
cell shown in the figure has the copper made with a collar, which
encircles the porous cup, and thereby lowers the internal resistance
of the battery. The voltage not being over 1 volt, however, renders
these cells hardly suitable for direct connection. Five cells connected
in multiple would give all of 10 amperes of current, and 1 volt, and
a number of these multiple groups could be connected in series for a
higher voltage.


is similar in operation to the Edison-Lalande, but differs in details
of construction. The zinc is a heavy ring suspended outside, but not
touching a perforated tin cylinder closed at the bottom, containing the
oxide of copper in flakes. Its internal resistance is slightly higher
than the Edison-Lalande cell, otherwise there is little choice. The 6 ×
8 size is excellent for coil work, giving 250 actual ampere hours and
remaining on open circuit for long periods without deterioration.


This is a practical form of the old Lalande-Chaperon cell, and gives
a steady, large current, being of low internal resistance, but is of
low electromotive force, being less than .70 volt on closed circuit
of medium resistance. Being of low internal resistance, however, its
output is large—three cells of the type _S_; internal resistance, 0.025
ohm. Capacity, 300 ampere hours, will about equal one cell type E 5
of the Chloride Storage Battery. The elements of this cell consist of
positive plates of amalgamated zinc, suspended on each side of negative
plates of the black oxide of copper in an electrolyte solution of
caustic potash. In action the decomposition of water forms an oxide of
zinc from the positive element, which with the potash in combination
leaves a soluble salt of zinc and potash. The hydrogen of the water
acts on the oxide plates to form metallic copper, thus really reducing,
instead of increasing, the internal resistance of the cell. A layer
of heavy paraffin oil is poured on top of the solution to prevent the
action of air.


or Roche dry cell. This cell possesses remarkable recuperative powers
and low internal resistance. Made in many sizes, the best suited for
medical coils is No. 2; dimensions, 5-7∕8 × 2-7∕16 inches. For heavier
work the No. 5, 6 × 2-9∕16 inches, and known as the Navy Standard, is
recommended. A convenient size for portable medical coils is No. 3, 3¾
× 1⅞ inches, taking up very little room, yet giving a large output.
Two of these latter cells enclosed in the coil case will give with a
suitably wound primary (No. 18 to 20 B & S) as strong a current as can
be used in electrotherapy. For Ruhmkorff coils cells Nos. 6 and 7 (6
× 3 inches and 7 × 3 inches) furnish a most desirable battery for all
work not needing the constant operation of the contact breaker, such
as wireless telegraphy, gas-lighting, etc. They will do service on
X-ray work, but the writer prefers a storage cell or the copper oxide
types. The E. M. F. of the above cells is one and six-tenths volts, and
current from 9 amperes to the No. 7 size, which gives 24 amperes on
short circuit.


As a matter of practice, there is no really dry cell; all so-called
cells contain liquid held in suspension, and their output is limited
to the amount of fluid. One of this type can easily be made in the
following manner: A containing jar is made up of first-quality sheet
zinc, the edges being joined by a turned seam and then soldered, the
bottom of zinc being also soldered in. In soldering here, as actually
in all such operations, be _absolutely sure_ the edges of the metal
are clean. The jar is partially filled with the following composition:
Oxide of zinc, 1 part; sal ammoniac, 1 part; plaster of paris, 3
parts; chloride of zinc, 1 part; water, 2 parts, all by weight; or sal
ammoniac, 1 part; chloride of calcium, 5 parts; calcined magnesia, 5
parts; water, 2 parts, or enough water to make a thin paste. A brass
binding post is soldered to the zinc case and a carbon plate having a
binding post is inserted in the centre of the cell, care being taken
that it does not touch the zinc. A small disc of wood laid in the
bottom of the cell will prevent contact at the bottom. Molten pitch
or a composition of pitch and rosin in the proportion of 6 to 1 is
poured on top, so as to seal the cell. As gas is generated in the cell,
a safety valve should be provided, either a piece of porous cane or
a short length of hard rubber tube, inside of which have been placed
a few strands of woollen thread. This class of cell is so cheap and
so many forms are available for choice that it is rarely desirable
to make one's own. They will not do for steady current, but only for
intermittent work. The large sizes being of low internal resistance,
can be used for signalling in wireless telegraphy, where it is not
possible to use wet (or free fluid) cells. The principal dry cells on
the market are the Mesco, the O. K., the Nungesser, and the Samson
semi-dry cell.



The development of the storage or secondary cell has been one of the
most important electrical advances of the century. For purposes of
experiment or work, where a large or steady current is required from
compact and readily tended apparatus, the storage cell proves its
utility. The simplest form was that used by the early experimenters,
and as it is easy to make, a form of it may very well be described.

 [Illustration: FIG. 62.]

 [Illustration: FIG. 63.]

From a sheet of lead ⅛ inch thick two or more pieces are cut of
the requisite size, say, 5 inches square. In making these plates,
they should be cut so as to leave a strip 1 inch wide and 3 inches
long, projecting from one corner, _A_ (Fig. 62), for the purpose of
connection. This is for the reason that the fumes of the sulphuric
acid solution would quickly corrode any wires or screws in the plates,
and also to give a better connection. The number of plates cut must
be an odd one, as it is general to make the two outside plates of the
same polarity—viz., negative. These plates are then scored with a steel
point across and across on both sides to perhaps a depth of 1∕64 of an
inch. This scoring is not absolutely necessary; it somewhat hastens the
formation of the plates. The plates are then laid face to face, being
separated by pieces of wood, rubber, or, still better, by a piece of
grooved wood, Fig. 63 having a thin piece of asbestos on each side.
These grooves are to carry off the gas, and should run up and down the
board, as in the figure. The wood is ⅛ of an inch thick or thereabouts,
and preferably perforated with holes ¼ of an inch or larger. When laid
together, a few strong rubber bands hold the plates from coming apart.
To prevent lateral motion, a few rubber pins may be thrust through
the plates. The alternate strips are to be connected together in two
series, as in a condenser, and the complete series placed in a jar
containing a mixture of seven parts of water to one of sulphuric acid.
The terminal of the strips connected to the smallest number of plates
is to be marked _P_ or +, for positive.

This terminal is now to be connected to a charging current (not over 1
ampere), as described in the directions for charging batteries, for
eight hours, and then discharged at a rate not over 1 ampere for six
hours. Then the connections are to be reversed and the cell charged
backward, as it were, and discharged. This has to be repeated for a
long period, perhaps a month, before the cell is in good condition; on
the final charge it is to be connected positive to positive of charging
source. This operation is called "forming," and the result is to change
the metallic lead of the positive plate into red-brown peroxide of
lead, and the lead negative plates into spongy lead.

In modern commercial cells this operation is no longer pursued, the
plates are variously constructed of lead frameworks holding plugs of
litharge or lead oxide, which is "formed" with great facility. For many
purposes other than operating Ruhmkorff coils, a few simple cells made,
as described, are handy to have and easy to make. In sealing the cells
up for portability, care must always be taken to leave a small hole in
the cover for the escape of the sulphurous acid gas.


Although the charging of a storage or secondary battery is by no means
a difficult operation, yet it requires care, and one unaccustomed
to the work will meet many slight difficulties which may seriously
affect the results. Pre-eminently the best charging source is a direct
current, constant potential electric-light circuit. The amount of
current required varies according to the type and make of the cell, but
we will select one of a capacity of 50 ampere hours for illustration.

By 50 ampere hours is meant a delivery of 1 ampere per hour for fifty
hours, or a rate of discharge equal to the above, as 2 amperes per hour
for twenty-five hours. In practice a secondary cell will not be found
to act exactly as above, the total amount of current decreasing as
the discharge is greater. Each cell is constructed to discharge at a
certain rate, above which it is not safe to go. Five amperes per hour
is a suitable rate for a fifty-hour cell, and should not be greatly
exceeded. The Chloride type, however, is one which can be discharged
at a higher rate than normal without any serious results, the latter
being generally a bulging or "buckling," as it is called, of the plates
whereby they short circuit or fall apart. The voltage of the charging
source should be at least 10 per cent over that of the battery when
fully charged. The voltage of a cell of storage battery varies from
about 2.3 at commencement of discharge to 1.7, at which latter voltage
discharge must be stopped and charging recommenced.

Fig. 64 shows the connections to charge a storage battery from an
electric-light circuit. The latter must be direct current and of low
tension. The circuit from the negative lead runs to the rheostat handle
_R_, thence through as many coils as are in circuit (varied by moving
the handle over the contact pieces in connection with the resistance
coils). The positive of the cell is connected to the positive main.

In connecting storage cells to the mains the utmost care must be taken
that the terminals are correctly attached. It happens in isolated
plants that some change is made in the wiring or the switchboard,
which reverses the current without warning being given to the battery
charger. It is the safest way to test the polarity of the terminals of
_both_ battery and mains each time charging is commenced. For polarity
tests see Chapter I. It is immaterial on which side of the battery the
rheostat or similar device is placed.

 [Illustration: FIG. 64.]

 [Illustration: FIG. 65.]

Fig. 65 shows the employment of lamps instead of the rheostat. The
lamps _L L_ regulate the current flow by the manner in which the
circuit is arranged. If only one lamp be turned on, the current
necessary for only one lamp circulates through the battery. Each
additional lighted lamp adds to the current by decreasing the
resistance of the circuit. _S_ is a switch which must always be left
open when the dynamos are to be stopped.


In many instances an electric-light circuit is not available for
charging purposes, in which event recourse must be made to a primary
battery. The one most suited for the work is the modified Daniell,
or copper and zinc combination in solutions of sulphate of copper
(bluestone) and sulphate of zinc respectively.

There are many good forms of this cell on the market, chief of which
are the simple gravity, the Gethins, and the Hussey, which have
been previously described. An example will now be described of the
operations necessary with the gravity cell, charging one 50-ampere
hour storage cell. At least six cells of gravity will be required, as
the voltage of each cell is never over 1 volt, and is dependent on
the resistance in the external circuit falling as the resistance is
lowered. Place the six clean glass jars on a firm foundation, where
there is no liability of shaking and no dust likely to settle. Unfold
the copper strips into the form of a star, bending the corners for half
an inch so as to give an anchorage in the bluestone. Place them into
the bottom of the jars and pour in water enough to cover them at least
3 inches below the surface. Now carefully drop in 4 pounds of clean
bluestone, which will fill in the angles between the copper wings, at
the same time holding the element down to the bottom of the jar. Hang
the zincs from the top edge of the jar, and fill up with water to 1
inch from the top. The addition of 5 ounces of sulphate of zinc per
cell will render the cells immediately available, and for the further
hastening of the chemical action, the copper wire from each copper may
be inserted in the binding post-hole of the zinc belonging to its own
cell and screwed tight for a few hours; or the cells may be connected
together in series, and the wire from the last copper be screwed to the
zinc of the first, thus putting the whole series on short circuit. The
only advantage of the first method being a saving of time when a number
of cells is being set up. This saving of time is often of consequence,
as the longer the newly set-up cell is on open circuit, the more
copper will be deposited on the zinc, which is highly undesirable.
This is shown by the blackening of the zinc as soon as it is put in
the solution, which blackening it is hard to prevent entirely. When
the cell is working satisfactorily it will show a clearly defined line
between the colorless solution above and the deep blue solution beneath.

Gravity cells should never be moved. If no sulphate of zinc is
available, half a teaspoonful of sulphuric acid may be poured in over
the zinc, which will tend to form the sulphate of zinc. Without any of
these helps the cell will take at least twenty-four hours on a short
circuit before it will give its normal current. This current should
be from 4∕10 to 5∕10 of an ampere. Five cells set up by the writer
varied after the addition of the zinc sulphate from 200 milli-amperes
(thousandths of an ampere) to 300 milli-amperes, although they were
apparently all set up alike; but after twelve hours' short circuiting
they all gave a fairly uniform current of from 470 to 500 milli-amperes.

 [Illustration: FIG. 66.]

From time to time on storage battery work, say, every week, the
specific gravity of the top solution must be tested with a hydrometer
(see Fig. 66), which should be put into the solution and allowed to
come to rest. The indicated number at the level of the liquid should
be 25°. If the number is higher some solution should be drawn off
and clear water added, until the hydrometer settles down to 25° or
thereabouts. The inside of the glass jar for 1 inch from the top may be
greased to prevent the salts of zinc creeping over the edge, or half an
inch of heavy paraffin oil be poured on the top to prevent evaporation
and creeping. When the zinc gets very much coated with the dark deposit
it must be taken out and scraped and washed. When the bluestone needs
replenishing, drop in carefully and be sure none lodges on the zinc


Each manufacturer of storage cells issues specific directions for the
charging of his own make, but generally the method is as follows:
The acid solution is prepared by mixing one volume of sulphuric acid
to from four to seven volumes of water, according to the make of the
cell. The sulphuric acid should have a specific gravity of 1.82 and be
chemically pure. _The acid must always be poured into the water, and
slowly, stirring all the time, then set aside for the mixture to cool._
It is best to mix the solution in a separate earthenware vessel, and
when two or more cells are to be set up, to mix all the solution at
one time, to ensure the same strength, unless a hydrometer is used to
determine this.

A good method to ascertain the exact quantity of solution required
is to place the elements in the jar and cover 1 inch deep at least
with water, then remove the elements and pour off the volume of water
corresponding to the proportion of acid to be added, and lastly pouring
the remaining water into the mixing vessel, prepare the solution, or
electrolyte, as it is called. New elements should be wetted with pure
water before being immersed in the solution. An ordinary charge of
the electrolyte requires from six to ten hours to cool thoroughly, as
considerable heat is evolved in the mixing.

Having now prepared the storage battery solution and set up the primary
cells, the charging can be proceeded with. The current must be turned
on the storage cell immediately the elements are placed in the acid.
Connect the wire from the zinc of the primary battery to the negative
of the storage cell and the copper wire to the positive. As the current
from a gravity cell is but small, it will take quite a time to charge a
storage cell of 50 ampere hours' capacity fully; it is a good scheme to
get the cell charged up from a dynamo source, and use the gravity cells
to keep it charged; but this cannot always be done, and the gravity
battery will do the work in time. As the best storage cells render but
90 per cent of the current put into them, they must be charged over the
number of hours for which they are required to deliver current.

When the cell is fully charged the solution will become milky and
give off gas freely. This gas in large quantities is detrimental to
health, and on no account should a storage cell be _charged_ in a
sleeping apartment. It affects the throat and lungs, and renders them
susceptible to take cold under suitable circumstances. The average
voltage of storage cells, when tested with the charging current on, is
2.4 volts, and the lowest they should be allowed to reach is 1.9 volts,
unless otherwise specified by the manufacturers.

Cells in poor condition are liable to form a _white_ deposit of
sulphate of lead, this fault being known as "sulphating." This trouble
requires much careful nursing, and the cells must be charged for a long
time at a very low rate until the plates of the positive element regain
their normal gray color. Chips of straw or excelsior, etc., falling in
between the plates will carbonize and cause trouble.

Most portable cells are sealed, but all cells can be easily sealed with
paraffin wax for amateur use. Cover the elements fully ½ inch above
the normal height of the electrolyte with water before pouring in the
electrolyte. Melt some paraffin in an earthenware jar and pour it on
top of the water, about the middle of the surface, when it will spread,
and care having been taken to have the jar sides dry, will cake solid
and form a good seal. Then bore a hole with a brace and bit or some
such tool through the wax and pour out the water. The cell can then
be set up as usual, the hole being only partly closed to allow of the
escape of the generated gas. A glass or rubber tube can be sealed into
the hole in the wax, and makes a more finished job.

While on the subject of primary batteries for charging storage cells, a
few remarks on their electromotive force may not be amiss. Although the
specifications issued by the manufacturers specify an excess charging
voltage of 10 per cent over the total voltage of the storage cells,
this does not apply to primary cells in its entirety. The voltage of
five gravity cells in series would aggregate 5 volts, and the voltage
of one storage cell but 2 volts, but there would not be 5 volts
available to force the charging current through the latter. In the
first place there is the counter electromotive force of the storage
cell working against the gravity battery. Simple subtraction would show
only 3 volts excess in favor of the primary electromotive force; but
the working voltage of a galvanic cell varies according to external
resistance of the cell and the external resistance of the circuit.
When the internal resistance is high, as in the gravity cell, and the
circuit resistance is low, in this case being the storage cell, the
available electromotive force of the primary is low also.

In many cases it is desired to operate a Ruhmkorff coil from an
electric-light main direct. This can readily be done if the circuit be
of the constant potential class—that is, one constructed to furnish
current for incandescent lamps in multiple. With the direct current,
such as the Edison, all that is necessary is either to interpose a
rheostat, as in Fig. 64, or to use the lamps, as in Fig. 65. The
manner of connecting up is the same as if the storage cell B be
replaced by the coil. Using the formula _C_ = _E_∕_R_, for example,
if the circuit be at 110 volts and the coil require 10 amperes, a
resistance of 11 ohms will be required. Or using the lamps in the
diagram, Fig. 65, about 20 lamps are to be put in circuit. If the
current be an alternating one, the contact-breaker will have to be
screwed down or short circuited.


This cell is of the lead-zinc type, being the practical form of the
Reynier cell. It is to be recommended for working Ruhmkorff coils, its
output weight for weight being far in excess of the lead-lead types.
This cell is readily portable and easy of operation, the zinc electrode
being the only one needing renewal, and that at very infrequent

The lead electrode consists of plates of peroxide clamped together,
and presents quite a large surface. The zinc in most types is of
the circular sheet form, and encloses the lead block, being kept
amalgamated by mercury lying in the bottom of the cell. The E. M. F.
on open circuit is about 2.5 volts, which is higher than any lead-lead
combination. On closed-circuit work this drops to from 2.35 volts
downwards. During action, when a large amount of current is being drawn
from the cell, a white sulphate appears, but this disappears upon
the cell being recharged or even left to rest. Bubbles of gas, which
sometimes form under the peroxide block, should be removed by gently
tilting the cell or hitting the table or shelf upon which it stands
a smart blow. The large type No. 3 is suitable for X-ray work, and a
still larger cell is made, which is preferable for heavy or continuous
discharges of current.


The No. 1 cell recently put upon the market has given excellent results
for open circuit work. It consists of a negative element with lead
peroxide as a depolarizer. The positive element is self-amalgamating
zinc, which is free from local action. The electrolyte is dilute
pure sulphuric acid. The potential is high, being 2.5 volts, and
the internal resistance is 0.14 ohm. This cell belongs to a group
which is midway between primary and storage, or secondary cells. Its
construction is similar to the lead-zinc secondary cell, in place of
which it may be used, it being easy to recharge an exhausted cell by
passing a weak current through it in reverse direction, thus recharging
the peroxide of lead grid and renewing the zinc and electrolyte.

The large size, or type No. 3, which the manufacturers are producing,
differs from the No. 1 cell in that it has a larger negative element,
or grid, and has two zincs, instead of one; consequently, it has a
lower internal resistance—0.07 ohm—and a higher discharge rate with
a capacity of 150 ampere hours. The potential is 2.5 volts. It is
suitable for coil work or for sparking gas engines, and for ease of
manipulation and convenience is to be highly recommended.

 [Illustration: FIG. 67.]

The elements are shown in Fig. 67, lead grid _L_, which is filled
in with paste of peroxide of lead, and which neither buckles nor
disintegrates. The zinc _Z_, however, possesses a novel feature.
A cavity is cast in the zinc element and filled with an amalgam of
mercury, the copper electrode passing through this amalgam into the
solid zinc, as shown in the cut. As the action of the battery proceeds,
this amalgam forces its way into the pores of the element and keeps
up so good an amalgamation of both copper rod and zinc that zincs can
be used up to a point when the rising internal resistance makes it
economy to throw them away, and absolutely no perceptible local action
takes place in the cell upon continued open circuit. A preparation
is furnished if desired, which forms a jelly of the electrolyte,
making the cell readily portable. Like all of these combinations, its
electromotive force exceeds two volts, and its internal resistance is
low enough to advise its employment in coil work.

When a storage battery is to remain unused for a long time it must
first be fully charged, and then every week or so the charging current
passed through it until it bubbles. Where it is to be laid away
for a long period of time, and weekly charging is not feasible, the
following operations are necessary: First, fully charge battery, remove
electrolyte, and replace by water immediately. Discharge at normal rate
until voltage runs down to 1.7 per cell. Gradually decrease resistance
until battery is almost on short circuit. Let it stand for a day, then
pour off the water, and keep elements in a dry, clean place.



The currents of high frequency used by Tesla in his researches are
produced by electrical rather than mechanical means. The alternating
current dynamo used by him renders a current of 10,000 alternations
per second, but the actual current necessary to the performance of
the luminous effects has a frequency of millions of oscillations per
second, produced by the discharge of Leyden jars or condensers.

Dr. Oliver J. Lodge, in his "Modern Views of Electricity," shows
that the discharge of the Leyden jar is in general oscillatory, the
apparently single and momentary spark, when analyzed in a very rapidly
rotating mirror, is shown to consist of a series of alternating
flashes, rapidly succeeding one another and lasting individually
less than one hundred thousandth of a second. The capacity of the
condenser and inertia of the circuit regulate the rapidity of these
oscillations. A 1 microfarad condenser discharging through a coil of
large self-induction, such as one having an iron core, may oscillate
only a few hundred times per second. On the other hand, a Leyden jar
of the 1 pint size discharging through a short circuit will set up
oscillations, perhaps ten million per second; and a still smaller jar
would give oscillations away up in the billions. But these small jars
are quickly discharged, and require a constant replenishing.

The discharge actually consists of a principal discharge in one
direction, and then several reflex actions back and forth, becoming
feebler until their cessation. In their vibration they generate waves
in the surrounding medium, similar in many respects to sound waves, but
of infinitely higher velocity. Their length depends on the rate of
vibration of the source and their velocity. The microfarad discharge
before mentioned will have a wave length of perhaps 1200 miles, the
small jar not over 70 feet; and yet the true light wave has only an
average length of one fifty thousandth of 1 inch. These waves travel
into space until they either die out from exhaustion or are absorbed
by some suitable body; but they possess the quality of resonance in a
degree like those of sound. Two tuning forks of the same pitch will
influence one another—that is, one on being vibrated will start the
other in vibration, even at a considerable distance, but the electric
waves far surpass them in this respect.

 [Illustration: FIG. 68.]

Dr. Hertz made the first practical experiments in this direction with
his electric resonator (Fig. 68). This apparatus consisted of a 3-inch
spark induction coil, _I_, the secondary wires _S S_ being connected
to the copper rods _R R_, provided with metal balls _B B_, nearly 11
inches in diameter. The discharging balls _D D_ were approximated
until a satisfactory discharge passed between them. A large wire ring
having a spark gap in its circuit was so influenced by the resonance
as to show minute sparks passing across this gap even when the ring
was situated in a distant room. In many experiments with a rapidly
vibrating induction coil current, a sparking has been noticed in
metallic objects in the same room, in one instance it being discovered
in the metallic designs on a wall-paper.


In exploring the comparatively new field opened up by Professor
Crookes, Nikola Tesla has stimulated research into the mysteries of
high tension and frequency currents and their effects. In the majority
of his experiments Tesla uses alternating currents generated by
machinery of his own design, but in a large number of cases his effects
can be duplicated with an induction coil suitably energized. In the
latter case the apparatus consists of a battery, a Ruhmkorff coil, two
condensers, and a second specially constructed induction or disruptive
coil, with some few subsidiary implements. The contact-breaker or
rheotome must be one giving interruptions of very rapid sequence.

 [Illustration: FIG. 69.]

Fig. 69 shows a diagram of the Tesla arrangement with a Ruhmkorff
coil. The terminals of the secondary coil of the Ruhmkorff coil _I_
terminate at the condensers _C C_. Bridged across the wires before
they reach the condensers is the discharger _D_. The second terminals
of the condensers are led through the split primary of the disruptive
coil, terminating at the points _B B_ of the second discharger. The
secondary of the disruptive coil is either outside or inside the
primary coil. The condensers are of special design, being small, but of
high insulation. They each consist of two plates of metal a few inches
square immersed in oil and arranged so they can be brought nearer
together or further apart, as necessary. Within limits, the smaller
these plates are the more frequent will be the oscillations of their
discharge. They also fulfil another purpose, they help nullify the high
self-induction of the secondary coil by adding capacity to it.

 [Illustration: FIG. 70.]

The discharger tips are preferably metal balls under 1 inch
in diameter. Tesla uses various forms of dischargers, but for
experimental purposes the two metal balls will answer. They are
adjusted when the whole apparatus is working according to the results
desired. The mica plates serve to establish an air current up through
the gap, making the discharge more abrupt, an air blast being also
used at times for the furtherance of this object. The device (Fig. 70)
consists of an electro-magnet, _C_, set with its poles _P_ across the
air gap, helping to wipe out the spark, as in a well-known form of
lightning arrester. This form, described by Tesla, has the pole pieces
_P_ shielded by mica plates _M_, to prevent the sparks jumping into the
magnets. Fig. 70 is an elevation and Fig. 71 a plan of this device.

 [Illustration: FIG. 71.]

The resonance effects obtained during the operation of a Tesla coil
are very marked, and their study may lead to the solution of the
problems of communication between distant points without the use of
other conducting media than the atmosphere. But the main use to which
the Tesla currents have been put is that of artificial illumination.
These currents have enabled experimenters to obtain a high luminosity
in vacua by the aid of only one conducting wire—in fact, in some cases
without the utilization of any conductor than the air. An ordinary
incandescent lamp connected to one terminal of the coil will show in
a fair degree some of the luminescent phenomena. The brush effects
from the terminals of the secondary coil are extremely marked and
interesting; but to detail the experiments that can be performed with
the Tesla disruptive coil would be an impossibility here. Reference is
recommended to the published works of Nikola Tesla, which happily are
readily procurable.

These currents of high frequency have of late been turned to account
in electrotherapeutics, principally for the stimulation they exert on
the nutritive process. They also exert a very great influence on the
vasomotor centres, as is evidenced by the reddening of the skin and
exudation of perspiration. This result is readily obtainable by placing
the patient in connection with one electrode on an insulating stool,
and terminating the other electrode at a large metal plate situated a
few feet distant; or the patient may be surrounded by a coil of wire in
connection with the coil of sufficient diameter, however, to prevent



Although the remarkable discovery that it was possible by electrical
means to depict an image of an object on a photographic sensitized
plate, despite the intervention of solid bodies, was first given to the
world at large by Professor Roentgen, yet he was undoubtedly led to
the results by consideration of the works of previous experimenters in
electrical discharges through vacua.

It is not intended here to trace the previous work of Professor
Crookes, the inventor of the radiometer, which is actuated by the
heat rays of light, nor of Hertz, who found that gold leaf was
transparent to rays emanating from certain vacuum tubes carrying
a luminous electrical discharge. It is mainly the purpose of these
pages to give directions for practical work, and not deal in theories,
interesting though they be. At the beginning of X-ray investigation
many claims were made which have since been disproven, but the
fundamental operations remain the same. A Crookes tube of special
design is energized from a coil or similar electrical distributor,
and by means of the resultant rays otherwise opaque objects appear
partially transparent, their shadows being cast upon the screen of a
fluoroscope, or these shadows are allowed to act upon a sensitized
photographic plate, and subsequent development reveals outlines or
shadowgraphs. The general arrangement of apparatus is shown in Fig. 72.
_C_ is a Ruhmkorff coil, giving not less than 2 inches of spark; _B_
the battery operating same; _T_ the modified form of Crookes tube used
most generally; _X_ the object under observation; _F_ the fluoroscope
or the sensitized photographic plate. The usual precautions are
taken to avoid the leakage of current from the secondary wires, the
tube _T_ being best mounted in a wooden stand (Fig. 72), and the wire
connections brought to it as direct as possible. No condenser, stand,
etc., are shown in drawing, to avoid unnecessary complication.

 [Illustration: FIG. 72.]


This is a funnel-shaped cardboard box with an opening at the smaller
end for the eyes and a piece of card across the larger end. The inside
surface of this card is covered with crystals of barium platino
cyanide, the most satisfactory fluorescent substance obtainable. The
earlier fluoroscopes were made with tungstate of calcium, but the above
salt has proven far more satisfactory. The operation of the fluoroscope
is simple. It is held in the hand by a convenient handle, the open
end pressed close to the eyes, so as to exclude outside light, and
with the hand or other object held against the outside of the big end,
or screen, it is directed towards the Crookes tube. The screen then
appears to glow with a bluish light, and the shadow of the object is
distinctly seen on the screen. Different adjustments of the coil give
results which will be treated upon later.


Messrs. Siemens and Halske manufactured a tube which allowed of a
slight variation of vacuum by using the vapor of phosphorus. An
auxiliary tube containing phosphorus was added to the main tube, and
upon heat being applied to it by means of a lamp, vapor is given off,
which materially reduces the vacuum of the main tube. When the opposite
result is desired part of the current is diverted through the auxiliary
tube, and the vapor is caused to solidify itself upon the walls of the

 [Illustration: FIG. 73.]


The most satisfactory tube for X-ray work is one where the vacuum is
readily adjustable. Reference to Fig. 73 shows the Queen form. A small
bulb, containing a chemical which gives off vapor when heated and
reabsorbs it when cooled, is directly connected to the main tube and
surrounded by an auxiliary tube, which is exhausted to a low vacuum. In
the auxiliary tube the cathode is opposite to the above-mentioned bulb,
so that any discharge through it will heat the bulb by the bombardment
of the cathode rays. The cathode is connected to a spark point, which
can be adjusted to any distance from the cathode of the main tube. The
anode of the small tube is directly connected to that of the main tube.
When the tube is put into operation the vacuum and, consequently, the
resistance of the main tube being high, the current preferably passes
by the spark point and auxiliary tube, heating the chemical for a
few seconds until sufficient vapor has been driven into the main tube
to permit the current to pass through the latter. After this only an
occasional spark will jump across the gap to counteract the tendency of
the reabsorption of the vapor and consequent raising in resistance of
the main tube.

This device presents easy means of adjusting the vacuum in the main
tube. With the spark point at a considerable distance from cathode the
vacuum will be high. When the spark gap is short the vacuum will become
low. The main bulb is about 4½ inches in diameter, and at the place
where the X-rays pass only 1∕64 of an inch in thickness. The cathode is
of aluminum, the anode of platinum. In starting this tube, it is best
to make the spark gap about one inch in width. When connected up and
working properly the main bulb will be filled with a green striated
luminosity between anode and cathode, and the tip of the chemical bulb
will have the shadow of the little platinum tip thrown upon it. The
green light is not always brilliant; at times it is quite weak, but yet
does its work well. A brilliant green light is often one of the signs
of wrong connection, and particularly so when the little shadow on the
chemical bulb is absent. Never run these or any other tubes backwards,
but be sure the current is flowing in correct direction at first

Other forms of Crookes tubes differ only in form, or are devoid of
adjustment, and the connections of coil, tube, etc., are the same.


A high vacuum gives greater penetrative power than a low vacuum. Where
the operator has not an adjustable tube it is imperative that he have
at least two tubes, one high and one low. It is the contrasts which
render the X-ray practical, and these contrasts are largely governed
by the vacuum. In locating a metallic substance in the human body a
high vacuum tube would be needed, that the bones and dense tissue be
rendered more transparent. On the other hand, to make a radiograph
of the bones, a lower vacuum is necessary in order to get a contrast
between the bones and the tissues. In general, a high vacuum is
best for fluoroscope work and a low vacuum for making pictures on a
photographic plate. Short exposures in radiography are obtained by
powerful rays and consequently by coils operating at considerable
energy. In extended examinations or where a subject is under the X-rays
for more than a minute or so, a screen should be interposed between
the subject and the tube to avoid the burning effect which is often
noticeable. This screen consists of a piece of cardboard well covered
with gold leaf, and should be grounded—that is, a connection be run
from the gold surface to a water-pipe or other ground connection.
Sheet lead is an efficient screen to the rays, and, if desired, a lead
screen can be made, partially enclosing the apparatus, to protect the
operator. But it must be large enough and far enough distant from the
coil and tube to avoid any possibility of leakage of current or even
inductive influence. In operating X-ray machines never attempt to alter
connections or make adjustments other than at coil platinum screw or
Crookes tube spark gap without first shutting off current. Remember
that a very unpleasant shock can be easily obtained from touching the
apparatus with only one hand. It is often advisable to remove one's
watch, particularly when using Ruhmkorff coils of large size.

The tube may be worked until it shows a slight redness in the centre of
the platinum, but care must then be taken not to increase current, or
the platinum will melt. Never allow the tube to come in contact with
any object other than its stand and connections while working, and be
sure the wires from secondary do not come near tube until they reach
places of attachment, or they may spark through glass and ruin the tube.

In making radiographs on sensitized plates the unused plates should
be kept at a considerable distance from the coil while working. Better
still if they are in another room. Plates for X-ray work are made by
most photographic supply dealers; in fact, almost any good brand of
sensitized plates or even films will answer. When making a radiograph,
the plate can either be left in the holder or well wrapped in black
paper, but current should never be turned on coil before the plate
and subject are in position. In photographing the chest, neck, etc.,
the plate can be strapped on to the part; but the subject must remain
absolutely still. The time of exposure varies considerably with the
size of coil, thickness of object, etc. Radiographs of the hand have
been taken by simply laying the hand on top of the plateholder and
operating tube for 100 seconds. But, as a rule, longer exposures are
necessary. Most radiographs will generally require that the plate be
"intensified" and a developer used that gives great detail, such as
metol quinol, etc. At any rate, great care should be exercised in
developing the plate, as many a good radiograph has been spoiled by
undue haste.



In Chapter XII. we showed how Dr. Hertz caused electric waves to pass
through space and become visible by sparks across an air gap in a wire
ring situated at a distance from the source of energy. The apparatus
used, and termed an electric resonator, is in principle similar to that
of the wireless telegraph. The minute sparks instead of idly passing
across the air gap are made to traverse a "coherer" (to be afterwards
more fully described). This "coherer" substantially consists of a
resistance, preferably metal filings placed in series, with a battery
and relay. Normally, the resistance is so adjusted that the battery
current is not strong enough to operate the relay. A wire is led from
one side of this coherer up into the air to intercept the Hertzian
waves, the other side of the coherer is put to earth, or "grounded."
When a wave strikes the air wire it sends a current through the coherer
to ground (as before it sent a spark across the air gap), and this wave
acts on the filings in its passage through them; in effect, to lower
their resistance, so that the current is increased through the relay
circuit and the relay armature is attracted to its magnet. The relay
makes contact in the usual manner at the platinum points, and in its
turn causes the local circuit, sounder, bell, or pen register to record
the wave (or signal). After each wave the filings are in such state
that to restore them to their former high resistance it is necessary
to give the coherer a smart tap. This is generally accomplished
automatically by means of an arm extending from the sounder lever,
which strikes against the coherer each time the sounder armature is

 [Illustration: FIG. 74.]

Figures 74 and 75 are diagrams of a simple circuit, Fig. 74 being the
transmitting apparatus and Fig. 75 the receiving apparatus.

In Fig. 74 _P P_ and _S S_ are the primary and secondary of a Ruhmkorff
coil, _D_ two brass balls on the discharger, _B_ the battery, _K_ a
key, in place of the usual contact breaker, which is either absent or
screwed down; _V_ a wire leading from one arm of the discharger up into
the air, of a height varying with the results desired; _G_ a ground
plate in connection with the other discharger arm.

The coil condenser is left out of the diagram for sake of clearness;
but, of course, is necessary to the operation of the apparatus.

In Fig. 75, _C_ is the coherer, also called the Branly tube, or radio
conductor; _S_ a telegraph sounder, or electric bell; _R_ a relay;
_R B_ and _L B_ the relay battery and local battery, respectively;
_G_ a ground connection; _M_ a resistance, or choke coil, and _V_ a
vertical wire, as in the transmitter; in fact, in the station set the
same vertical wire answers for both transmitter and receiver.

 [Illustration: FIG. 75.]

The coil to be used may be from two inches of spark upwards, dependent
upon the distance the signals have to travel. The relay battery may
be two cells of dry battery, the local battery as much as is desired
to operate the bell, sounder, or pen register receiving the signals.
Presuming the apparatus set up and adjusted, and designating the
transmitter as Station A and the receiver as Station B, the operation
will be as follows: A pressure and release of key _K_ sends an impulse
of current through the primary _P_, inducing a current in _S_, which
manifests itself by a spark between the discharger balls at _D_. An
electric wave is released, which, starting from _V_, Station A, meets
in its passage _V_ of Station B. Travelling along this wire to the
ground, it finds two paths—through _C_ or _R_. As the choke coil deters
it from passing through the relay, it finds passage through _C_ and so
to ground.


Many forms of this apparatus are in use, but as yet no definite design
can be recommended for specific purposes. The most general mode of
construction is that of the Branley Coherer, as shown in Fig. 76.

 [Illustration: FIG. 76.]

It consists of a glass tube, 2 inches long by ¼ inch inside diameter,
furnished with well-fitted metal plugs at each end, to which
connections are made. These plugs can be slid in and out of tube for
adjustment, the gap between them being loosely filled with fine metal
filings. The metal used varies, according to the operator's preference,
the most generally adopted being pure nickel for both plugs and
filings. Another mode of construction for purely experimental use is
to merely cork the ends of the tube and pass the wires through these
corks into the filings, ensuring, however, good contact between wires
and filings. Marconi's favorite form is a glass tube two inches long
with silver plugs, each one-quarter inch long, in each end, intervening
space being partially filled with a mixture of nickel and silver
filings. These plugs are then adjusted to as close as one-twenty-fifth
of an inch, and the whole apparatus exhausted of air either by means of
a leading-in tube or by placing coherer in a vessel from which the air
can be drawn. As a rule, coherers containing air become less sensitive
after continued use.


Pointed carbon rods can be inserted in the tube instead of metal,
and carbon dust substituted for the metal filings; but this form is
suitable only for special purposes. It is very delicate in its action,
but somewhat uncertain.


Were it not for reasons, such as difficulty of decoherence, the metal
filings might be dispensed with and two rods of metal placed in light
contact. The construction of the coherer reminds one very much of the
microphone, a satisfactory coherer having been made out of the old
"nail microphone," four wire nails being placed crossing one another in
the battery circuit, in one case acting as a sound transmitter, whence
the name; in the other as a coherer.


Aluminium, a metal which has steadily grown into favor, and which is
now readily obtainable, can be made to serve in the present apparatus
in place of nickel both as to electrodes and filings. It is advisable,
however, to use aluminium electrodes of slightly larger diameter than
those of other metals.


A recent writer has recommended the use of balls of steel, such as are
used in ball bearings, such, however, not to exceed ⅜ inch diameter.
Such a coherer would take the form of an upright glass tube, with
electrodes exerting pressure on a series of four or more steel balls.
Decoherence here becomes difficult, and mention is but made of it to
show the variety of forms which this important little article may

Coherers are adjusted by advancing or receding the electrodes,
altering the quantity of the filings, etc. There exists but little
difficulty in operating coherers; considerable latitude is permissible
as to adjustment, size, character, etc. There does not seem so much
difficulty in obtaining sensitiveness as in guarding against external
electrical disturbances. Wings or vanes of thin sheet metal are
sometimes attached to the metal ends or electrodes of the coherer for
purposes of adjustment, their size and capacity being determined by
experiment. It is best that they present no sharp angles, but be of a
disc, or spherical, form, the better not to dissipate energy.


This is the name given the contrivance at the ends of the discharger,
_D_ being the point at which the electrical oscillations, or waves, are


This consists of two brass spheres, generally 3 inches in diameter,
and mounted on a stand or sometimes on top of the induction coil. The
distance between the balls is readily adjustable by either attaching
the balls on the ends of two sliding rods, or causing the balls
themselves to slide on the rods (Fig. 77).

 [Illustration: FIG. 77.]

 [Illustration: FIG. 78.]


Here three balls are used, two outside ones connected to the circuit,
being one-half inch diameter, and the middle one, isolated from all
connection, of three inches in diameter. This form is best mounted on
a separate stand, the balls either being on glass or hard rubber legs
(Fig. 78). Connecting wires from the secondary of the coil must in all
cases be run with the greatest precautions against crosses, as directed
in Chapter V.

It is possible to make many different designs in oscillators. Some
experimenters use the simple Clarke form, others prefer the triple
balls; yet, again, others vary the sizes and the relative sizes of the
balls. One form of oscillator prescribes the balls to be immersed in
oil or vaseline. Such methods all have their adherents. Even the plain
points of an induction coil discharger will serve for short-distance

Oscillators are adjusted by altering their proximity to one another,
and should have care given to keep the spheres bright. It is easy to
alter capacity of an oscillator by connecting its spheres to other
insulated spheres.


The coil for wireless telegraphy does not differ from the regular
Ruhmkorff, except that in place of the contact breaker a signal or
Morse telegraph key is substituted. Of course, the contact breaker can
be made to perform the same duty by retracting the adjusting screw out
of reach of the platinum on spring, and then operating the hammer and
spring in same manner as key.


Under this head are included relay sounder, bell, or register, which
are at receiving set. They do not differ from the regular telegraphic
apparatus. The sounder may be of the Western Union pattern, wound to
4 ohms; the relay also Western Union pattern, and wound to 150 or 250
ohms, as best suits the individual case.

In order to protect the receiver from the action of the transmitter
belonging to the same set of instruments, particularly when powerful
waves are generated, it has been found at times necessary to enclose
the receiver in a metal case. Marconi has patents on such devices,
particularly on a movable shutter in the case, which opens when the
transmitter is not in operation. Edouard Branly placed his receiving
set in a metal case with a vertical slit eight inches by one-tenth of
an inch.


The vertical wire extending from the coherer up into the air must be
insulated from all other objects in the best possible manner. A bare
copper wire of No. 14 B & S gauge can be suspended from porcelain
insulating knobs, which in turn can be strung from each other by means
of stout silk cord or even wire. There is a special form of insulator
used in electric construction work, and known as a circuit breaker,
which will answer and which is easy of attachment; reference to Fig.
79 will show manner of using.

 [Illustration: FIG. 79.]

Temporary grounds can be made to water pipes, but it is better to use
regular telephone copper ground-plates sunk deep in moist earth.

At South Foreland, England, a mast has been erected, 150 feet in height
for transmission across Channel, a distance of nearly thirty miles. At
Notre Dame University, Illinois, Professor Green used a wire 150 feet
in length, suspended from top of a high church tower, but was unable
to transmit much over three miles, owing, presumably, to fact that
the intervening country was well supplied with overhead wires, which
probably intercepted the waves.

It has been claimed that earthed or grounded air wires are necessary,
but balls or similar "capacities" are not of service on the top of the
wire. A theory has been advanced that the currents do not pass from
air wire tip to air wire tip, but are conducted by the varying strata
of the earth. No general confirmation is obtainable, however, and
the experimental reader will find a wide field for research in this
direction. Marconi, on the other hand, has accomplished much with zinc
cylinders under six feet high, _not grounded in any respect_, indeed,
and he also finds it impossible to assume a proportion between distance
of effect and height of air wire. The following investigations and
experiments are of interest in this connection:

At a meeting of the Institution of Electrical Engineers, in December,
1898, Dr. Oliver Lodge showed that there must be a certain relative
position between the receiving and transmitting circuits.

He placed on one side of a room a box, containing a battery, bell,
relay, and coherer properly connected up. On the other side he had
an induction coil and pair of parallel discharger rods, with a spark
gap to transmit waves across the room. When the rods of the receiver
and transmitter were placed parallel to each other the receiving
bell was operated; when the receiving rods of the transmitter were
at right angles to those of the receiver the bell either failed to
work, or weakened very considerably. He also told of an experiment
made to determine the influence of different methods of grounding the
apparatus. He found that when the apparatus was connected by a wire
laid on the ground, there was the required response at the receiving
station; but when the two stations were situated each side of a lake,
and the ground wires immersed in the water, the receiving instrument
failed to work. It seemed to him that the conductivity and power
absorption of ether wave energy by water was too great to allow of the
transmission of Hertz waves. This would seem to bear out the results
obtained by Marconi in dispensing with ground wires.



  Acid, Chromic, 189.

   "    Sulphuric, 212.

  Air pump, Geissler, 142.

   "   "    Simple, 141.

   "   "    Sprengel, 143.

   "  blast, 76.

   "  wire, 262.

  Amalgamation, 180.

  Assembly of coil, 22.

  Attraction, Window, 154, 163.

  Automobile coil, 40.

  Automatic burners, 170.

  Automatic burners, Adjustment, 176.


  Ballistic galvanometer, 99.

  Barium platino cyanide, 238.

  Base for coil, 30.

  Bath coil, 61.

  Battery, Bichromate, 180.

     "     Champion, 179.

     "     Daniell, 191.

     "     Dun, 187.

     "     Edison-Lalande, 195.

     "     Fuller, 184.

     "     Gas-lighting, 179.

     "     Gethins, 193.

     "     Gordon, 194.

     "     Gravity, 191.

     "     Grenet, 180.

     "     Harrison, 219.

     "     Monarch, 179.

     "     Morisot, 188.

     "     Novelty, 182.

     "     Open circuit, 178.

     "     Polarization, 179.

     "     Samson, 179.

     "     solutions, 185, 188, 189.

     "     Standard dry, 196.

     "     Storage, 200.

     "     Storage, to charge, 208.

     "     Storage, to make, 101.

     "     Storage, to seal, 215.

     "     U. S. storage, 218.

  Beeswax, 95.

  Brush, Electric, 128.


  Capacity of condenser, 100.

  Carbons for battery, 182.

  Cements, 97.

  Charging condenser, 110.

  Chromic acid, 189.

  Closed magnetic circuit, 6.

  Coherer, Aluminium, 251.

     "     Branly, 254.

     "     Carbon, 256.

     "     Steel ball, 258.

  Coil, Failure to work, 49.

   "    for gas engine, 37.

   "    general remarks, 42.

   "    in series, 32.

   "    Medical, 51.

   "    Oil immersed, 33.

   "    Output of, 46.

   "    Primary, 7.

   "    Resistance, 40.

   "    Secondary, 10.

   "    Table of dimensions, 50.

   "    Tesla, 35.

   "    Testing, 44.

   "    To select, 46.

   "    Winding, 20.

  Condensers, Aluminium, 115.

      "       Adjustable, 117.

      "       Charging, 110.

      "       capacity, 100, 119.

      "       Discharge of, 225.

      "       Glass, 101.

      "       Mica, 108.

      "       Oil, 116.

      "       Paper, 105, 107.

      "       Rolled up, 115.

      "       Series, 108.

  Cone vibrator, 88.

  Contact breaker, 26.

     "       "     Adjustable medical, 85.

     "       "     Adjustable cone, 88.

     "       "     Dessauer, 80.

     "       "     Electrolytic, 77.

     "       "     Highspeed, 69.

     "       "     in vacuo, 81.

     "       "     Polechanging, 73.

     "       "     Queen, 83.

     "       "     Queen, large form, 84.

     "       "     Steel ribbon, 80.

  Contacts, Care of, 90.

  Core, 4.

    "   Iron filing, 46.

  Crookes tube, 241.


  Dessauer contact breaker, 80.

  Dielectric, 104.

  Discharger, 26.

  Dry cell, 196.

  Dun cell, 187.


  Eddy currents, 6.

  Edison-Lalande cell, 195.

  Electric sand, 189.

  Electrode, 190.

  Electrolyte, 213.

  Electrolytic interrupter, 77.

  Ends for coil, 25.

  Extra current, 3.


  Farad, 100.

  Fluoroscope, 239.

  Fluorescence, 137.

  Foucault currents, 6.

  Frauenhofer lines, 135.

  Frontispiece, Notes on, 42, 47.


  Galvanometer, 99.

  Gas burners, 170.

   "  engine coil, 39.

   "  from water, 44.

   "  lighting, 164.

  Gassiot star, 153.

  Geissler tube, 159.

  Glass, To pierce, 130.

  Gordon battery, 194.


  Harrison cell, 219.

  Hertz resonator, 226.

  Hydrometer, 211.

  Hysteresis, 6.


  Induction, 1.

      "      Self, 8.

  Insulations, 97.


  Leyden jars, 99.

  Lighting gas, 164.


  Magnetic circuit, Closed, 6.

  Medical coils, 51.

     "      "    Care of, 62.

  Mercury contact breaker, 71.

  Mica condenser, 61.


  Noise of contact breaker, 63.


  Oil, Capacity of, 119.

   "   Coil immersion, 33.

   "   for oscillator, 260.

   "   Linseed, 93.

   "   Resin, 96.

   "   Spark through, 95.

  Oscillator, 258.

      "       Clarkes, 259.

      "       Triple, 259.

  Output of coil, 46.


  Paper condenser, 107.

  Paraffin, 94.

  Phosphorus tube, 239.

  Photography, X-Ray, 245.

  Pocket coil, 89.

  Polarity tests, 45.

  Pole, 190.

  Polechanging switch, 32.

  Polechanging contact breaker, 73.

  Primary coil, 7.


  Queen contact breaker, 83.

    "   Crookes tube, 241.


  Radiography, 245.

  Reel ends, 25.

  Resistance coils, 40.

  Resonance, Electric, 226.

  Resonator, Hertz, 226.

  Rheotome, 2.

  Roentgen Ray apparatus, 236.

  Rotating wheel, 154.


  Series, Coils in, 32.

  Selection of coil, 46.

  Shellac, 96.

  Signs, Battery, 190.

  Soda, Bichromate of, 183.

  Spark, Electric, 120.

    "    Choice of, 47.

  Spectroscope, 132.

  Spectrum, Solar, 132.

  Standard dry cell, 196.

  Sulphating, 215.

  Switch, Polechanging, 32.


  Table of cost, 191.

  Tesla coil, 35.

    "    "    disruptive, 36.

  Testing polarity, 46.

  Transformer, 5.

  Tube, Insulating, 9.


  U. S. storage cell, 218.


  Vacuum, Adjusting, 242.

     "    Choice of, 243.

     "    Contact breaker, 81.

     "    To procure, 141.

     "    pumps, 140.


  Water, Decomposition of, 44.

  Wax, 95.

  Wehnelt interrupter, 77.

  Wheel, Rotating, 154.

  Winder, Coil, 16.

  Winding coils, 20.

     "    Secondary, 10, 14.

     "    Sectional, 11.

  Wire for secondary coil, 24.

   "    "  primary coil, 9.

  Wires, Air, 262.

  Wireless telegraphy, 248.

  Wireless telegraphy circuit, 250.


  X-Ray apparatus, 236.

    "   Remarks, 243.

    "   photographs, 246.



 =Electricity, Its Theory, Sources and Applications=, by JOHN T.
 SPRAGUE. 3rd edition.

 =Induction Coils and Coil Making=, by F. C. ALLSOP.

 =The Construction of Large Induction Coils=, a Workshop Handbook, by
 A. THARE. Illustrated.

 =A Manual of Electricity=, by H. M. NOAD, Ph.D. London, 1859.

 =Practical Electrics.=

 =Sloane's Electrical Dictionary.=

 =Houston's Electrical Dictionary.=

 =Electricity and Magnetism=, by PROF. SILVANUS P. THOMPSON.


 =Small Accumulators and How to Make Them=, by P. MARSHALL.

 =Primary Batteries=, by H. S. CARHART.

 =Practical Electrics.=

 =Electric Batteries, How to Make Them=, by P. MARSHALL.


 =A History of Wireless Telegraphy=, by J. J. FAHIE.

 =Improvements in Magnetic Space Telegraphy, Telegraphing by Magnetic
 Induction, and Aetheric Telegraphy=, by SIR W. H. PREECE, S. EVERSHED,

 =Science Abstracts, Physics and Electrical Engineering.=

 =The Model Engineer and Amateur Electrician.=

Queen Instruments

_Induction Coils_ capable of producing thick, heavy sparks from 60"
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J. G. GRAY, President

  59 Fifth Avenue
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Mesco Dry Battery

 [Illustration: The Battery]


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Translated with permission of the Author by _GEORGE RICHMOND, M.E._



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A Handbook on the Care of Boilers



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loss of heat.

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_Uniform in One Volume. Cloth Cover; Price, $1.00._

Table of Contents.

CHAPTER I.—Introductory and Historical; Steam Jacketing. CHAPTER
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CHAPTER V.—Valve Setting continued, with diagrams of same; Table for
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CHAPTER XII.—Foundations continued; Materials for same, etc.

Third Edition, with an Appendix.



Engines and Boilers

Practical Instruction for Young Engineers and Steam Users.



Synopsis of Contents

Cleaning the boiler, removing scale, scale preventers, oil in boilers,
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150 pages, illustrated, 16mo, cloth, $1.00








A first class American Book for young Engineers and all those wishing
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1. Elementary Thermodynamics. 2. Theory of the Steam Engine. 3, Types
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Third edition, (1901), thoroughly revised to date and considerably

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  VII. The Effect of Alterations to Valve and

  VIII. Note on Link Motions.

  IX. Note on Very Early Cut-Off, and on Reversing
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  _88 Pages._      _41 Illustrations._      _12mo, Cloth, $1.00._







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_The Design and Construction_



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Pistons, Connecting Rods, Fly-Wheels, Air and Exhaust Cams, Valves,
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Different Kinds of Engines, etc. 3. Testing the Engine, Faults and
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                           EVERYONE'S GUIDE




                         OF PHOTOGRAPHIC WORK.

                         E. J. WALL, F.R.P.S.
         Author of _The Dictionary of Photography_, etc., etc.

                            SECOND EDITION


                               NEW YORK:
                          SPON & CHAMBERLAIN,
                         12 CORTLANDT STREET.




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Per dozen pads, $2.50.


Made from this paper but printed on both sides. Size of book 5 × 8
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  SPON & CHAMBERLAIN, 12 Cortlandt St.,


Manual of Instruction in

Hard Soldering


Repair of Bicycle Frames

Notes on Alloys and a Chapter on Soft Soldering


The flame, lamp, charcoal, mats, blow-pipes, wash-bottle, binding wire,
chemicals, borax, spelter, silver solder, gold solder, oxidation of
metals, fluxes, anti-oxidisers, oxidation of cases, the cone, oxidising
flame, reducing flame, heat transmission, conduction, capacity of
metals, radiation, application, the work table, the joint, applying
solder, applying heat, the use of the blow-pipe, joints, making a
ferrule, to repair a spoon, to repair a watch case, hard soldering
with a forge or hearth, hard soldering with tongs, preserving thin
edges, silversmith's pickle, restoring color to gold, chromic acid,
to mend steel springs, sweating metals together, retaining work in
position, making joints, applying heat, preventing the loss of heat,
effect of sulphur lead and zinc, to preserve precious stones, annealing
and hardening, burnt iron, to hard solder after soft solder. Tables
of—specific gravity, tenacity, fusibility, alloys.

66 pages, illustrated, cloth, 75 cents.

 For Soldering Receipts, Cements and Lutes, Pastes, Glues and such



How Made and Used

_A Practical Handbook for Students and Young Electricians_


Contents of Chapters

I.—The Theory of the Accumulator.

II.—How to make a 4-Volt Pocket Accumulator.

III.—How to make a 32-Ampere-Hour Accumulator.

IV.—Types of Small Accumulators.

V.—How to Charge and Use Accumulators.

VI.—Applications of Small Accumulators, Electrical Novelties, etc.
Useful Receipts. Glossary of Technical Terms.

80 pages, 40 illustrations, 12mo, cloth, 50c.




Fitting Up and Adaptability to Every-Day Use



Some electrical considerations: I.—Introductory. II.—Construction.
III.—Lines, Indoor Lines. IV.—Signalling Apparatus. V.—Batteries. Open
Circuit Batteries. Closed Circuit Batteries. VI.—Practical Operations.
Circuit with Magneto Bells and Lightning Arresters. How to Test the
Line. Push-Button Magneto Circuit. Two Stations with Battery Bells.
VII.—Battery Telephone. Battery Telephone Circuit. Three Instruments on
one Line. VIII.—General remarks. Index.

80 pages, 23 illustrations, 12mo, cloth, $1.00. In paper, 50c.









_Alarms._—Doors and Windows; Cisterns; Low Water in Boilers; Time
Signals; Clocks. _Batteries._—Making; Cells; Bichromate; Bunsen;
Callan's; Copper-oxide; Cruikshank's; Daniel's; Granule carbon;
Groves; Insulite; Leclanché; Lime Chromate; Silver Chloride; Smee;
Thermo-electric. _Bells._—Annunciator System; Double System;
and Telephone; Making; Magnet for; Bobbins or Coils; Trembling;
Single Stroke; Continuous Ringing. _Connections._ _Carbons._
_Coils._—Induction; Primary; Secondary; Contact-breakers; Resistance.
_Intensity_ Coils.—Reel; Primary; Secondary; Core; Contact-breaker;
Condenser; Pedestal; Commutator; Connections. _Dynamo-electric
Machines._—Field-Magnets; Pole-pieces; Field-magnet Coils; Armature
Cores and Coils; Commutator Collectors and Brushes; Relation of size
to efficiency; Methods of exciting Field-Magnets; Magneto-Dynamos;
Separately excited Dynamos; Shunt Dynamos; Field-Magnets; Armatures;
Collectors; Brush Dynamo; Alternate Currents. _Fire Risks._—Wires;
Lamps; Danger to persons. _Measuring._—Non-Registering Instruments;
Registering Instruments. _Microphones._ _Motors._ _Phonographs._
_Photophones._ _Storage._ _Telephones._—Forms; Circuits and Calls;
Transmitter and Switch; Switch for Simplex; etc., etc.

  135 PAGES.       126 ILLUSTRATIONS.       8VO.
  Cloth, 75 cents




  Engineers Tables]

Bound in roan, round corners, gilt edges in celluloid case, 50c.


Bound in roan, round corners, gilt edges, in celluloid case, 50c.
_Copies mailed post-paid on receipt of price._

       *       *       *       *       *

Transcriber's Notes

The Errata have been implemented.

Obvious typographical errors have been silently corrected. All other
spelling, hyphenation and punctuation remains unchanged.

Italics are represented thus _italic_. Bold is indicated thus =bold=,
except in the catalogue of books where, for the sake of clarity, it has
not been indicated.

*** End of this Doctrine Publishing Corporation Digital Book "Induction Coils, How to Make, Use, and Repair Them. - Including Ruhmkorff, Tesla, and medical coils, Roentgen - Radiography, etc. etc." ***

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