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Title: Electric Bells and All About Them - A Practical Book for Practical Men
Author: Bottone, S. R.
Language: English
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Transcriber's Notes

Inconsistent spellings (e.g. depolariser & depolarizer) and hyphenation
(e.g. guttapercha & gutta-percha) are retained as in the original text.
Minor punctuation errors are corrected without comment. Changes which
have been made to the text (in the case of typographical errors) are
listed at the end of the book.

This version has been prepared using symbols from the ASCII and
Latin-1 character sets only. Italic typeface is shown with surrounding
_underscores_; small capital typeface is shown by ALL CAPS;
superscript typeface is shown by a preceding caret (^). Subscripts in
chemical formulae are shown with underscores and braces, e.g.
H_{2}SO_{4}.

The following are used to represent other special symbols:

  [Lambda]          sans-serif capital Lambda
  [rotated S]       S-like symbol rotated 90 deg.
  [box open up]     3 sides of rectangular (open side up)
  [box open down]   3 sides of rectangular (open side down)
  [oe]              oe-ligature
  [battery]         vertical lines (thick and thin)
  [L], [U], [V]     sans-serif letter shapes

       *       *       *       *       *

                         ELECTRIC BELLS AND
                           ALL ABOUT THEM.

                 A Practical Book for Practical Men.

                _WITH MORE THAN 100 ILLUSTRATIONS._

                                  BY
                            S. R. BOTTONE,

        CERTIFICATED BY SOUTH KENSINGTON (LATE OF THE COLLEGIO
               DEL CARMINE, TURIN, AND OF THE ISTITUTO
                          BELLINO, NOVARA);

          _Author of "The Dynamo," "Electrical Instruments for
                            Amateurs," &c._

                               LONDON:
              WHITTAKER & CO., PATERNOSTER SQUARE, E.C.

                                1889.

                      (_All rights reserved._)



PREFACE.


So rapidly has the use of electric bells and similiar signalling
appliances extended, in modern houses, offices, hotels, lifts, and
ships, that every bell-fitter must have felt the need of accurate
knowledge of the manner in which these instruments act and are made.

In the following pages the author has attempted to supply this need, by
giving full details as to the construction of batteries, bells, pushes,
detectors, etc., the mode of wiring, testing, connecting up, localizing
faults, and, in point of fact, by directing careful attention to every
case that can present itself to the electric-bell fitter.

  CARSHALTON, SURREY,
  _November, 1888_.



CONTENTS.


  CHAP.                                                             PAGE

    I. PRELIMINARY CONSIDERATIONS                                      1

   II. ON THE CHOICE OF BATTERIES FOR ELECTRIC BELL WORK              18

  III. ON ELECTRIC BELLS AND OTHER SIGNALLING APPLIANCES              59

   IV. ON CONTACTS, PUSHES, SWITCHES, KEYS, ALARMS, AND RELAYS       109

    V. ON WIRING, CONNECTING UP, AND LOCALISING FAULTS               144



LIST OF ILLUSTRATIONS.


  FIG.                                                         PAGE

   1. Direction of current in cell                                9

   2.     "          "     out of cell                           10

   3. Bar and horse-shoe magnets                                 14

   4. The Dynamo                                                 16

   5.  "  Smee cell                                              28

   6.  "  Daniell cell                                           30

   7.  "  Gravity cell                                           32

   8.  "  Leclanché cell and parts                               34

   9.  "  Agglomerate cell                                       40

  10.  "  Judson cell                                            42

  11.  "  Battery in box                                         43

  12.  "  Gent cell                                              44

  13.  "  Bichromate cell                                        48

  14.  "  Fuller cell                                            50

  15.  "  Cells coupled in series                                54

  16.  "       "        "  Parallel                              57

  17. Outline of electric bell                                   61

  18. Frame of bell                                              62

  19. E-shaped frame                                             63

  20. Electro-magnet, old form                                   64

  20A.   "      "     modern form                                65

  21. Magnet frame                                               66

  21A. Winder                                                    72

  22. Mode of joining electromagnet wires                        73

  23. Armature spring                                            74

  24.    "        "   Another form                               74

  25. Platinum tipped screw                                      75

  26.     "      "    spring                                     76

  27. Binding screws                                             77

  28. Bell or gong                                               78

  29. Pillar and nuts                                            78

  30. Washers                                                    78

  31. Trembling bell                                             81

  32. Bell action enclosed in case                               88

  33. Ordinary trembling bells                                   90

  34. Single stroke bell                                         91

  35. Continuous ring bell                                       94

  36. Release action                                             95

  37. Continuous ringing with relay                              96

  38. Continuous ringing action with indicator                   97

  39. Relay and detent lever for indicator                       97

  40. Callow's attachment                                        99

  40A. Thorpe's arrangement                                     101

  41. Jensen bell, _section_                                    102

  42.    "     "   _exterior_                                   104

  43A. Circular bell                                            106

  43B. Mining bell                                              106

  44. Electric trumpet (Binswanger's)                           107

  45. Various forms of pushes                                   110

  46. Pressel                                                   111

  47. Pull                                                      112

  48. Bedroom pull                                              113

  49A.   "      "  Another form                                 114

  49B. Floor contact, ball form                                 114

  50. Burglar alarm                                             115

  51.    "      "  Another form                                 115

  52. Floor contact                                             115

  53. Door contact                                              116

  54. Sash contact                                              117

  55. Shop door contact                                         117

  56A. Closed circuit system, _single_                          119

  56B. Closed circuit system, _double_                          119

  57. Modified gravity, Daniell                                 120

  58. Contact for closed circuit                                121

  59. Thermometer alarm                                         122

  60. Fire alarm                                                123

  61A.  "    "  Another form                                    123

  61B.  "    "    "      "  in action                           123

  62. Binswanger's "watch alarm" contact                        125

  63. Watchman's electric tell-tale clock                       126

  64. Lever switch, _two-way_                                   128

  65. Morse key, _double contact_                               133

  66. Relay                                                     134

  67. Indicator, drop                                           137

  68.    "       Semaphore                                      138

  69.    "       Fall back                                      139

  70.    "       Pendulum                                       140

  71.    "       Coupled up                                     142

  72.    "       Gent's tripolar                                143

  73. Soldering iron and wires                                  148

  74. Push, interior of                                         151

  75. Bell, battery and push                                    159

  76.        "          "  And earth return                     160

  77.  " and two pushes                                         161

  78.  " two pushes and one pull                                161

  79. Two bells in parallel                                     162

  80.       "       "    Another mode                           162

  81.       "       "    with two-way switch                    163

  82. Series coupler                                            163

  83. Bell with local battery and relay                         164

  84. Continuous ringing bell with wire return                  165

  85. Bells with Morse keys for signalling                      165

  86. Bells with double contact pushes for signalling           166

  87. Bells with double contact with one battery only           167

  88. Two-way signalling with one battery only                  168

  89. Complete installation of bells, batteries, pushes, etc.   169

  90. Mode of getting out plan or design                        170

  91. Lift fitted with bells                                    173

  92. Magneto bell: generator                                   174

  93.   "      "    Receiver                                    175

  94.   "      "    Combined                                    176

  95. Detector or galvanometer                                  176



ELECTRIC BELLS.



CHAPTER I.

PRELIMINARY CONSIDERATIONS.


§ 1. ELECTRICITY.--The primary cause of all the effects which we are
about to consider resides in a force known as _electricity_, from the
Greek name of amber (electron), this being the body in which the
manifestations were first observed. The ancients were acquainted with a
few detached facts, such as the attractive power acquired by amber after
friction; the benumbing shocks given by the torpedo; the aurora
borealis; the lightning flash; and the sparks or streams of light which,
under certain conditions, are seen to issue from the human body. Thales,
a Grecian philosopher, who flourished about 600 years B.C., observed the
former of these facts, but nearly twenty centuries elapsed before it was
suspected that any connection existed between these phenomena.


§ 2. According to the present state of our knowledge, it would appear
that electricity is a mode of motion in the constituent particles (or
atoms) of bodies very similar to, if not identical with, _heat_ and
_light_. These, like _sound_, are known to be dependent on undulatory
motion; but, whilst _sound_ is elicited by the vibration of a body _as a
whole_, electricity appears to depend, in its manifestations, upon some
motion (whether rotary, oscillatory, or undulatory, it is not known) of
the atoms themselves.

However this be, it is certain that whatever tends to set up molecular
motion, tends also to call forth a display of electricity. Hence we have
several practical means at our disposal for evoking electrical effects.
These may be conveniently divided into three classes, viz.:--1st,
mechanical; 2nd, chemical; 3rd, changes of temperature. Among the
_mechanical_ may be ranged friction, percussion, vibration, trituration,
cleavage, etc. Among the _chemical_ we note the action of acids and
alkalies upon metals. Every chemical action is accompanied by electrical
effects; but not all such actions are convenient sources of electricity.
_Changes of temperature_, whether sudden or gradual, call forth
electricity, but the displays are generally more striking in the former
than in the latter case, owing to the accumulated effect being presented
in a shorter time.


§ 3. We may now proceed to study a few of these methods of evoking
electricity, so as to familiarise ourselves with the leading properties.

If we rub any resinous substance (such as amber, copal, resin,
sealing-wax, ebonite, etc.) with a piece of warm, dry flannel, we shall
find that it acquires the power of attracting light bodies, such as
small pieces of paper, straw, pith, etc. After remaining in contact with
the rubbed (or electrified) substance for a short time, the paper, etc.,
will fly off as if repelled; and this apparent repulsion will be more
evident and more quickly produced if the experiment be performed over a
metal tray. If a small pith-ball, the size of a pea, be suspended from
the ceiling by a piece of fine cotton, previously damped and then
approached by an ebonite comb which has been briskly rubbed, it will be
vigorously attracted, and never repelled; but if for the cotton there be
substituted a thread or fibre of very fine dry silk, the pith-ball will
be first _attracted_ and then _repelled_. This is owing to the fact that
the damp cotton allows the electricity to escape along it: _id est_,
damp cotton is a CONDUCTOR of electricity, while silk does not permit
its dissipation; or, in other words, silk is a NON-CONDUCTOR. All bodies
with which we are acquainted are found, on trial, to fall under one or
other of the two heads--viz., conductors and non-conductors. Nature
knows no hard lines, so that we find that even the worst conductors will
permit the escape of some electricity, while the very best conductors
oppose a measurable resistance to its passage. Between the limits of
good conductors, on the one hand, and non-conductors (or insulators) on
the other, we have bodies possessing varying degrees of conductivity.


§ 4. As a knowledge of which bodies are, and which are not, conductors
of electricity is absolutely essential to every one aspiring to apply
electricity to any practical purpose, the following table is subjoined,
giving the names of the commoner bodies, beginning with those which most
readily transmit electricity, or are _good_ conductors, and ending with
those which oppose the highest resistance to its passage, or are
insulators, or non-conductors:--


§ 5. TABLE OF CONDUCTORS AND INSULATORS.

  -----------------+------------------------------+---------------------
     Quality.      |      Name of Substance.      | Relative Resistance.
  -----------------+------------------------------+---------------------
       Good       {|Silver, annealed              |           1.
    Conductors    {|Copper, annealed              |           1.063
                  {|Silver, hard drawn            |           1.086
                  {|Copper, hard drawn            |           1.086
                  {|Gold, annealed                |           1.369
                  {|Gold, hard drawn              |           1.393
                  {|Aluminium, annealed           |           1.935
                  {|Zinc, pressed                 |           3.741
                  {|Brass (variable)              |           5.000
                  {|Platinum, annealed            |           6.022
                  {|Iron                          |           6.450
                  {|Steel, soft                   |           6.500
                  {|Gold and silver alloy, 2 to 1 |           7.228
                  {|Nickel, annealed              |           8.285
                  {|Tin, pressed                  |           8.784
                  {|Lead, pressed                 |          13.050
                  {|German silver (variable)      |          13.920
                  {|Platinum-silver alloy, 1 to 2 |          16.210
                  {|Steel, hard                   |          25.000
                  {|Antimony, pressed             |          23.600
                  {|Mercury                       |          62.730
                  {|Bismuth                       |          87.230
                  {|Graphite                      |         145.000
                  {|Nitric Acid                   |      976000.000
                   |                              |
    Imperfect     {|Hydrochloric acid             | [1]
    Conductors    {|Sulphuric acid                |     1032020.000
                  {|Solutions of metallic salts   | varies with strength
                  {|Metallic sulphides            | [1]
                  {|Distilled water               | [1] 6754208.000
                   |                              |
     Inferior     {| Metallic salts, solid        | [1]
    Conductors.   {| Linen  }                     |
                  {| Cotton } and other forms of  | [1]
                  {| Hemp   }    cellulose        |
                  {| Paper  }                     |
                  {| Alcohol                      | [1]
                  {| Ether                        | [1]
                  {| Dry Wood                     | [1]
                  {| Dry Ice                      | [1]
                  {| Metallic Oxides              | [1]
                   |                              |
  Non-conductors, {| Ice, at 25 c.                | [1]
       or         {| Fats and oils                | [1]
    Insulators.   {| Caoutchouc                   |    1000000000000.
                  {| Guttapercha                  |    1000000000000.
                  {| Dry air, gases, and vapours  | [1]
                  {| Wool                         | [1]
                  {| Ebonite                      |    1300000000000.
                  {| Diamond                      | [1]
                  {| Silk                         | [1]
                  {| Glass                        | [1]
                  {| Wax                          | [1]
                  {| Sulphur                      | [1]
                  {| Resin                        | [1]
                  {| Amber                        | [1]
                  {| Shellac                      | [1]
                  {| Paraffin                     |    1500000000000.
  -----------------+------------------------------+---------------------

[Footnote 1: These have not been accurately measured.]

The figures given as indicating the relative resistance of the above
bodies to the passage of electricity must be taken as approximate only,
since the conductivity of all these bodies varies very largely with
their purity, and with the temperature. Metals become worse conductors
when heated; liquids and non-metals, on the contrary, become better
conductors.

It must be borne in mind that _dry air_ is one of the _best
insulators_, or worst _conductors_, with which we are acquainted; while
damp air, on the contrary, owing to the facility with which it deposits
_water_ on the surface of bodies, is highly conducive to the escape of
electricity.


§ 6. If the experiment described at § 3 be repeated, substituting a
glass rod for the ebonite comb, it will be found that the pith-ball will
be first attracted and then repelled, as in the case with the ebonite;
and if of two similar pith-balls, each suspended by a fibre of silk, one
be treated with the excited ebonite and the other with the glass rod,
until repulsion occurs, and then approached to each other, the two balls
will be found to attract each other. This proves that the electrical
condition of the excited ebonite and of the excited glass must be
different; for had it been the same, the two balls would have repelled
one another. Farther, it will be found that the _rubber_ with which the
ebonite or the glass rod have been excited has also acquired electrical
properties, attracting the pith-ball, previously repelled by the rod.
From this we may gather that when one body acting on another, either
mechanically or chemically, sets up an electrical condition in one of
the two bodies, a similar electrical condition, but in the opposite
sense, is produced in the other: in point of fact, that it is impossible
to excite any one body without exciting a corresponding but opposite
state in the other. (We may take, as a rough mechanical illustration of
this, the effect which is produced on the pile of two pieces of plush or
fur, on being drawn across one another in opposite directions. On
examination we shall find that both the piles have been laid down, the
upper in the one direction, the lower in the other.) For a long time
these two electrical states were held to depend upon two distinct
electricities, which were called respectively _vitreous_ and _resinous_,
to indicate the nature of the bodies from which they were derived. Later
on (when it was found that the theory of a single electricity could be
made to account for all the phenomena, provided it was granted that some
electrified bodies acquired more, while others acquired less than their
natural share of electricity), the two states were known as _positive_
and _negative_; and these names are still retained, although it is
pretty generally conceded that electricity is not an entity in itself,
but simply a mode of motion.


§ 7. It is usual, in treatises on electricity, to give a long list of
the substances which acquire a positive or a negative condition when
rubbed against one another. Such a table is of very little use, since
the slightest modification in physical condition will influence very
considerably the result. For example: if two similar sheets of glass be
rubbed over one another, no change in electrical condition is produced;
but if one be roughed while the other is left polished, this latter
becomes positively, while the former becomes negatively, electrified.
So, also, if one sheet of glass be warmed, while the other be left cold,
the colder becomes positively, and the latter negatively, excited. As a
general law, _that body, the particles of which are more easily
displaced, becomes negatively electrified_.


§ 8. As, however, the electricity set up by friction has not hitherto
found any practical application in electric bell-ringing or signalling,
we need not to go more deeply into this portion of the subject, but pass
at once to the electricity elicited by the action of acids, or their
salts, on metals.

Here, as might be expected from the law enunciated above, the metal more
acted on by the acid becomes negatively electrified, while the one less
acted on becomes positive.[2] The following table, copied from Ganot,
gives an idea of the electrical condition which the commoner metals and
graphite assume when two of them are immersed at the same time in dilute
acid:--

                   {  v    Zinc.        ^  }
                   {  v   Cadmium.      |  }
                   {  |    Tin.         |  }
                   {  |    Lead.        |  }
                   {  |    Iron.        |  }
    The portion    {  |    Nickel.      |  }   The portion out
  immersed in the  {  |    Bismuth.     |  }  of the acid fluid
    acid fluid.    {  |    Antimony.    |  }
                   {  |    Copper.      |  }
                   {  |    Silver.      |  }
                   {  |    Gold.        |  }
                   {  |    Platinum.    ^  }
                   {  v    Graphite.    ^  }

[Footnote 2: This refers, of course, to those portions of the metals
which are out of the acid. For reasons which will be explained farther
on, the condition of the metals in the acid is just the opposite to
this.]

The meaning of the above table is, that if we test the electrical
condition of any two of its members when immersed in an acid fluid, we
shall find that the ones at the head of the list are _positive_ to those
below them, but negative to those above them, if the test have reference
to the condition of the parts _within_ the fluid. On the contrary, we
shall find that any member of the list will be found to be _negative_ to
any one below it, or _positive_ to any above it, if tested from the
portion NOT immersed in the acid fluid.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]


§ 9. A very simple experiment will make this quite clear. Two strips,
one of copper and the other of zinc, 1" wide by 4" long, have a 12"
length of copper wire soldered to one extremity of each. A small flat
piece of cork, about 1" long by 1" square section, is placed between the
two plates, at the end where the wires have been soldered, this portion
being then lashed together by a few turns of waxed string. (The plates
should not touch each other at any point.) If this combination (which
constitutes a very primitive galvanic couple) be immersed in a tumbler
three-parts filled with water, rendered just sour by the addition of a
few drops of sulphuric or hydrochloric acid, we shall get a
manifestation of electrical effects. If a delicately poised magnetic
needle be allowed to take up its natural position of north and south,
and then the wires proceeding from the two metal strips twisted in
contact, so as to be parallel to and over the needle, as shown in Fig.
1, the needle will be impelled out of its normal position, and be
deflected more or less out of the line of the wire. If the needle be
again allowed to come to rest N. and S. (the battery or couple having
been removed), and then the tumbler be held close over the needle, as in
Fig. 2, so that the needle points from the copper to the zinc strip, the
needle will be again impelled or deflected out of its natural position,
but in this case in the opposite direction.


§ 10. It is a well-known fact that if a wire, or any other conductor,
along which the electric undulation (or, as is usually said, the
electric current) is passing, be brought over and parallel to a
suspended magnetic needle, pointing north and south, the needle is
immediately deflected from this north and south position, and assumes a
new direction, more or less east and west, according to the amplitude of
the current and the nearness of the conductor to the needle. Moreover,
the direction in which the north pole of the needle is impelled is found
to be dependent upon the direction in which the electric waves (or
current) enter the conducting body or wire. The law which regulates the
direction of these deflections, and which is known, from the name of its
originator, as Ampère's law, is briefly as follows:--


§ 11. "If a current be caused to flow _over_ and parallel to a freely
suspended magnetic needle, previously pointing north and south, the
north pole will be impelled to the LEFT of the _entering_ current. If,
on the contrary, the wire, or conductor, be placed _below_ the needle,
the deflection will, under similar circumstances, be in the opposite
direction, viz.: the north pole will be impelled to the RIGHT of the
_entering_ current." In both these cases the observer is supposed to be
looking along the needle, with its N. seeking pole pointing at him.


§ 12. From a consideration of the above law, in connection with the
experiments performed at § 9, it will be evident that inside the tumbler
the zinc is _positive_ to the copper strip; while, viewed from the
outside conductor, the copper is positive to the zinc strip.[3]

[Footnote 3: From some recent investigations, it would appear that what
we usually term the negative is really the point at which the undulation
takes its rise.]


§ 13. A property of current electricity, which is the fundamental basis
of electric bell-ringing, is that of conferring upon iron and steel the
power of attracting iron and similar bodies, or, as it is usually said,
of rendering iron magnetic. If a soft iron rod, say about 4" long by
1/2" diameter, be wound evenly from end to end with three or four layers
of cotton-covered copper wire, say No. 20 gauge, and placed in proximity
to a few iron nails, etc., no attractive power will be evinced; but let
the two free ends of the wire be placed in metallic contact with the
wires leading from the simple battery described at § 9, and it will be
found that the iron has become powerfully magnetic, capable of
sustaining several ounces weight of iron and steel, so long as the wires
from the battery are in contact with the wire encircling the iron; or,
in other words, "_the soft iron is a magnet, so long as an electric
current flows round it_." If contact between the battery wires and the
coiled wires be broken, the iron loses all magnetic power, and the
nails, etc., drop off immediately. A piece of soft iron thus coiled with
covered or "insulated" wire, no matter what its shape may be, is termed
an "electro-magnet." Their chief peculiarities, as compared with the
ordinary permanent steel magnets or lodestones, are, first, their great
attractive and sustaining power; secondly, the rapidity, nay,
instantaneity, with which they lose all attractive force on the
cessation of the electric flow around them. It is on these two
properties that their usefulness in bell-ringing depends.


§ 14. If, instead of using a _soft_ iron bar in the above experiment, we
had substituted one of _hard_ iron, or steel, we should have found two
remarkable differences in the results. In the first place, the bar would
have been found to retain its magnetism instead of losing it immediately
on contact with the battery being broken; and, in the second place, the
attractive power elicited would have been much less than in the case of
soft iron. It is therefore of the highest importance, in all cases where
rapid and powerful magnetisation is desired, that the _cores_ of the
electro-magnets should be of the very softest iron. Long annealing and
gradual cooling conduce greatly to the softness of iron.

[Illustration: Fig. 3.

MAGNETS, showing Lines of Force.]


§ 15. There is yet another source of electricity which must be noticed
here, as it has already found application in some forms of electric
bells and signalling, and which promises to enter into more extended
use. If we sprinkle some iron filings over a bar magnet, or a horse-shoe
magnet, we shall find that the filings arrange themselves in a definite
position along the lines of greatest attractive force; or, as scientists
usually say, the iron filings arrange themselves in the direction of the
lines of force. The entire space acted on by the magnet is usually known
as its "field." Fig. 3 gives an idea of the distribution of the iron
filings, and also of the general direction of the lines of force. It is
found that if a body be moved before the poles of a magnet in such a
direction as to cut the lines of force, electricity is excited in that
body, and also around the magnet. The ordinary magneto-electric machines
of the shops are illustrations of the application of this property of
magnets. They consist essentially in a horse-shoe magnet, in front of
which is caused to rotate, by means of appropriate gearing, or wheel and
band, an iron bobbin, or pair of bobbins, coiled with wire. The ends of
the wire on the bobbins are brought out and fastened to insulated
portions of the spindle, and revolve with it. Two springs press against
the spindle, and pick up the current generated by the motion of the iron
bobbins before the poles of the magnet. It is quite indifferent whether
we use permanent steel magnets or electro-magnets to produce this
effect. If we use the latter, and more especially if we cause a portion
of the current set up to circulate round the electro-magnet to maintain
its power, we designate the apparatus by the name of DYNAMO.

[Illustration: Fig. 4.

TYPICAL DYNAMO, showing essential portions.]


§ 16. Our space will not permit of a very extended description of the
dynamo, but the following brief outline of its constructive details will
be found useful to the student. A mass of soft iron (shape immaterial)
is wound with many turns of insulated copper wire, in such a manner
that, were an electrical current sent along the wire, the mass of iron
would become strongly north at one extremity, and south at the other. As
prolongations of the electro-magnet thus produced are affixed two masses
of iron facing one another, and so fashioned or bored out as to allow a
ring, or cylinder of soft iron, to rotate between them. This cylinder,
or ring of iron, is also wound with insulated wire, two or more ends of
which are brought out in a line with the spindle on which it rotates,
and fastened down to as many insulated sections of brass cylinder placed
around the circumference of the spindle. Two metallic springs,
connected to binding screws which form the "terminals" of the machine,
serve to collect the electrical wave set up by the rotation of the
coiled cylinder (or "armature") before the poles of the electro-magnet.
The annexed cut (Fig. 4) will assist the student in getting a clear idea
of the essential portions in a dynamo:--E is the mass of wrought iron
wound with insulated wire, and known as the _field-magnet_. N and S are
cast-iron prolongations of the same, and are usually bolted to the
field-magnet. When current is passing these become powerfully magnetic.
A is the rotating iron ring, or cylinder, known as the _armature_, which
is also wound with insulated wire, B, the ends of which are brought out
and connected to the insulated brass segments known as the
_commutator_, C. Upon this commutator press the two springs D and D',
known as the _brushes_, which serve to collect the electricity set up by
the rotation of the armature. These _brushes_ are in electrical
connection with the two terminals of the machine F F', whence the
electric current is transmitted where required; the latter being also
connected with the wire encircling the field-magnet, E.

When the iron mass stands in the direction of the earth's magnetic
meridian, even if it have not previously acquired a little magnetism
from the hammering, etc., to which it was subjected during fitting, it
becomes weakly magnetic. On causing the armature to rotate by connecting
up the pulley at the back of the shaft (not shown in cut) with any
source of power, a very small current is set up in the wires of the
armature, due to the weak magnetism of the iron mass of the
field-magnet. As this current (or a portion of it) is caused to
circulate around this iron mass, through the coils of wire surrounding
the field-magnet, this latter becomes more powerfully magnetic (§ 13),
and, being more magnetically active, sets up a more powerful electrical
disturbance in the armature.

This increased electrical activity in the armature increases the
magnetism of this field-magnet as before, and this again reacts on the
armature; and these cumulative effects rapidly increase, until a limit
is reached, dependent partly on the speed of rotation, partly on the
magnetic saturation of the iron of which the dynamo is built up, and
partly on the amount of resistance in the circuit.



CHAPTER II.

ON THE CHOICE OF BATTERIES FOR ELECTRIC BELL WORK.


§ 17. If we immerse a strip of ordinary commercial sheet zinc in dilute
acid (say sulphuric acid 1 part by measure, water 16 parts by
measure[4]), we shall find that the zinc is immediately acted on by the
acid, being rapidly corroded and dissolved, while at the same time a
quantity of bubbles of gas are seen to collect around, and finally to be
evolved at the surface of the fluid in contact with the plate.
Accompanying this chemical action, and varying in a degree proportionate
to the intensity of the action of the acid on the zinc, we find a marked
development of _heat_ and _electricity_. If, while the bubbling due to
the extrication of gas be still proceeding, we immerse in the same
vessel a strip of silver, or copper, or a rod of graphite, taking care
that contact _does not_ take place between the two elements, no
perceptible change takes place in the condition of things; but if we
cause the two strips to touch, either by inclining the upper extremities
so as to bring them in contact out of the fluid like a letter [Lambda],
or by connecting the upper extremities together by means of a piece of
wire (or other conductor of electricity), or by causing their lower
extremities in the fluid to touch, we notice a very peculiar change. The
extrication of bubbles around the zinc strip ceases entirely or almost
entirely, while the other strip (silver, copper, or graphite) becomes
immediately the seat of the evolution of the gaseous bubbles. Had these
experiments been performed with chemically pure metallic zinc, instead
of the ordinary impure commercial metal, we should have found some
noteworthy differences in behaviour. In the first place, the zinc would
have been absolutely unattacked by the acid before the immersion of the
other strip; and, secondly, all evolution of gas would entirely cease
when contact between the two strips was broken.

[Footnote 4: In mixing sulphuric acid with water, the acid should be
added in a fine stream, with constant stirring, to the water, and not
the water to the acid, lest the great heat evolved should cause the acid
to be scattered about.]

As the property which zinc possesses of causing the extrication of gas
(under the above circumstances) has a considerable influence on the
efficiency of a battery, it is well to understand thoroughly what
chemical action takes place which gives rise to this evolution of gas.


§ 18. All acids may be conveniently regarded as being built up of two
essential portions, viz.: firstly, a strongly electro-negative portion,
which may either be a single body, such as _chlorine_, _iodine_,
_bromine_, etc., or a compound radical, such as _cyanogen_; secondly,
the strongly electro-positive body _hydrogen_.

Representing, for brevity's sake, hydrogen by the letter H., and
chlorine, bromine, iodine, etc., respectively by Cl., Br., and I., the
constitution of the acids derived from these bodies may be conveniently
represented by:--

       H Cl             H Br           H I
       ----             ----           ---
  Hydrochloric[5]    Hydrobromic    Hydriodic
      Acid.             Acid.         Acid.


[Footnote 5: Spirits of salt.]

and the more complex acids, in which the electro-negative component is a
compound, such as sulphuric acid (built up of 1 atom of sulphur and 4
atoms of oxygen, united to 2 atoms of hydrogen) or nitric acid
(consisting of 1 nitrogen atom, 6 oxygen atoms, and 1 hydrogen atom),
may advantageously be retained in memory by the aid of the
abbreviations:--

    H_{2}SO_{4}             HNO_{6}
    -----------             -------
     Sulphuric    and       Nitric
      Acid.[6]              Acid.[7]


[Footnote 6: Oil of vitriol.]

[Footnote 7: Aquafortis.]

When zinc _does_ act on an acid, it displaces the hydrogen contained in
it, and takes its place; the acid losing at the same time its
characteristic sourness and corrosiveness, becoming, as chemists say,
_neutralized_. _One_ atom of zinc can replace _two_ atoms of hydrogen,
so that one atom of zinc can replace the hydrogen in two equivalents of
such acids as contain only one atom of hydrogen.

This power of displacement and replacement possessed by zinc is not
peculiar to this metal, but is possessed also by many other bodies, and
is of very common occurrence in chemistry; and may be roughly likened to
the substitution of a new brick for an old one in a building, or one
girder for another in an arch.

It will be well, therefore, to remember that in all batteries in which
acids are used to excite electricity by their behaviour along with zinc,
the following chemical action will also take place, according to which
acid is employed:--

  Hydrochloric Acid and Zinc, equal Zinc Chloride and Hydrogen Gas.

      2HCl           +   Zn    =      ZnCl_{2}       +     H_{2}

or:--

  Sulphuric Acid and Zinc, equal Zinc Sulphate and Hydrogen Gas.

      H_{2}SO_{4}   +   Zn    =       ZnSO_{4}      +     H_{2}

Or we may put this statement into a general form, covering all cases in
which zinc is acted on by a compound body containing hydrogen,
representing the other or electro-negative portion of the compound by
X:--

  Zn + H_{2}X = ZnX + H_{2}

the final result being in every case the corrosion and solution of the
zinc, and the extrication of the hydrogen gas displaced.


§ 19. We learn from the preceding statements that no electricity can be
manifested in a battery or cell (as such a combination of zinc acid and
metal is called) without consumption of zinc. On the contrary, we may
safely say that the more rapidly the _useful_ consumption of zinc takes
place, the greater will be the electrical effects produced. But here it
must be borne in mind that if the zinc is being consumed when we are
_not_ using the cell or battery, that consumption is sheer waste, quite
as much as if we were compelled to burn fuel in an engine whether the
latter were doing work or not. For this reason the use of commercial
zinc, in its ordinary condition, is not advisable in batteries in which
acids are employed, since the zinc is consumed in such, whether the
battery is called upon to do electrical work (by placing its plates in
connection through some conducting circuit) or not. This serious
objection to the employment of commercial zinc could be overcome by the
employment of chemically purified zinc, were it not that the price of
this latter is so elevated as practically to preclude its use for this
purpose. Fortunately, it is possible to confer, on the ordinary crude
zinc of commerce, the power of resisting the attacks of the acid (so
long as the plates are not metallically connected; or, in other words,
so long as the "circuit is broken"), by causing it to absorb
superficially a certain amount of mercury (quicksilver). The modes of
doing this, which is technically known as _amalgamating the zinc_, are
various, and, as it is an operation which every one who has the care of
batteries is frequently called upon to perform, the following working
details will be found useful:--


§ 20. To amalgamate zinc, it should first be washed with a strong
solution of common washing soda, to remove grease, then rinsed in
running water; the zinc plates, or rods, should then be dipped into a
vessel containing acidulated water (§ 17), and as soon as bubbles of
hydrogen gas begin to be evolved, transferred to a large flat dish
containing water. While here, a few drops of mercury are poured on each
plate, and caused to spread quickly over the surface of the zinc by
rubbing briskly with an old nail-brush or tooth-brush. Some operators
use a kind of mop, made of pieces of rag tied on the end of a stick, and
there is no objection to this; others recommend the use of the fingers
for rubbing in the mercury. This latter plan, especially if many plates
have to be done, is very objectionable: firstly, on the ground of
health, since the mercury is slowly but surely absorbed by the system,
giving rise to salivation, etc.; and, secondly, because any jewellery,
etc., worn by the wearer will be whitened and rendered brittle. When the
entire surface of the zinc becomes resplendent like a looking-glass, the
rubbing may cease, and the zinc plate be reared up on edge, to allow the
superfluous mercury to drain off. This should be collected for future
operations. It is important that the mercury used for this purpose
should be pure. Much commercial mercury contains lead and tin. These
metals can be removed by allowing the mercury to stand for some time in
a vessel containing dilute nitric acid, occasional agitation being
resorted to, in order to bring the acid into general contact with the
mercury. All waste mercury, drainings, brushings from old plates, etc.,
should be thus treated with nitric acid, and finally kept covered with
water. Sprague, in his admirable work on electricity, says:--"Whenever
the zinc shows a grey granular surface (or rather before this), brush
it well and re-amalgamate, remembering that a saving of mercury is no
economy, and a free use of it no waste; for it may all be recovered with
a little care. Keep a convenient sized jar, or vessel, solely for
washing zinc in, and brush into this the dirty grey powder which forms,
and is an amalgam of mercury with zinc, lead, tin, etc., and forms
roughnesses which reduce the protection of the amalgamation. Rolled
sheet zinc should always be used in preference to cast. This latter is
very hard to amalgamate, and has less electro-motive power[8]; but for
rods for use in porous jars, and particularly with saline solutions,
cast-zinc is very commonly used. In this case great care should be taken
to use good zinc cuttings, removing any parts with solder on them, and
using a little nitre as a flux, which will remove a portion of the
foreign metals."

[Footnote 8: Power to set up a current of electricity.]


§ 21. Another and very convenient mode of amalgamating zinc, specially
useful where solid rods or masses of zinc are to be used, consists in
weighing up the zinc and setting aside four parts of mercury (by weight)
for every hundred of the zinc thus weighed up. The zinc should then be
melted in a ladle, with a little tallow or resin over the top as a flux.
As soon as melted, the mercury should be added in and the mixture
stirred with a stick. It should then be poured into moulds of the
desired shape. This is, perhaps, the best mode of amalgamating cast
zincs.


§ 22. Some operators recommend the use of mercurial salts (such as
mercury nitrate, etc.) as advantageous for amalgamating; but, apart
from the fact that these salts are generally sold at a higher rate than
the mercury itself, the amalgamation resulting, unless a very
considerable time be allowed for the mercuric salts to act, is neither
so deep nor so satisfactory as in the case of mercury alone. It may here
be noted, that although the effect of mercury in protecting the zinc is
very marked in those batteries in which acids are used as the exciting
fluids, yet this action is not so observable in the cases in which
solutions of _salts_ are used as exciters; and in a few, such as the
Daniell cell and its congeners, the use of amalgamated zinc is
positively a disadvantage.


§ 23. If, having thus amalgamated the zinc plate of the little battery
described and figured at § 9, we repeat the experiment therein
illustrated, namely, of joining the wires proceeding from the two plates
over a suspended magnetic needle, and leave them so united, we shall
find that the magnetic needle, which was originally very much deflected
out of the line of the magnetic meridian (north and south), will very
quickly return near to its old and normal position; and this will be
found to take place long before the zinc has been all consumed, or the
acid all neutralised. Of course, this points to a rapid falling off in
the transmission of the electric disturbance along the united wires; for
had _that_ continued of the same intensity, the deflection of the needle
would evidently have remained the same likewise. What, then, can have
caused this rapid loss of power? On examining (without removing from
the fluid) the surface of the copper plate, we shall find that it is
literally covered with a coating of small bubbles of hydrogen gas, and,
if we agitate the liquid or the plates, many of them will rise to the
surface, while the magnetic needle will at the same time give a larger
deflection. If we entirely remove the plates from the acid fluid, and
brush over the surface of the copper plate with a feather or small
pledget of cotton wool fastened to a stick, we shall find, on again
immersing the plates in the acid, that the effect on the needle is
almost, if not quite, as great as at first; thus proving that the sudden
loss of electrical energy was greatly due to the adhesion of the free
hydrogen gas to the copper plate. This peculiar phenomenon, which is
generally spoken of as the _polarisation of the negative plate_, acts in
a twofold manner towards checking the electrical energy of the battery.
In the first place, the layer of hydrogen (being a bad conductor of
electricity) presents a great resistance to the transmission of
electrical energy from the zinc plate where it is set up to the copper
(or other) plate whence it is transmitted to the wires, or _electrodes_.
Again, the _copper_ or other receiving plate, in order that the electric
energy should be duly received and transmitted, should be more
electro-negative than the zinc plate; but the hydrogen gas which is
evolved, and which thus adheres to the negative plate, is actually very
highly electro-positive, and thus renders the copper plate incapable of
receiving or transmitting the electric disturbance. This state of things
may be roughly likened to that of two exactly equal and level tanks, Z
and C, connected by a straight piece of tubing. If Z be full and C have
an outlet, it is very evident that Z can and will discharge itself into
C until exhausted; but if C be allowed to fill up to the same level as
Z, then no farther flow can take place between the two.

It is, therefore, very evident that to ensure anything like constancy in
the working of a battery, at least until all the zinc be consumed or all
the acid exhausted, some device for removing the liberated hydrogen must
be put into practice. The following are some of the means that have been
adopted by practical men:--


§ 24. _Roughening the surface of the negative plate_, which renders the
escape of the hydrogen gas easier. This mode was adopted by Smee in the
battery which bears his name. It consists of a sheet of silver, placed
between two plates of zinc, standing in a cell containing dilute
sulphuric acid, as shown at Fig. 5.

[Illustration: Fig. 5.]

The silver sheet, before being placed in position, is _platinised_; that
is to say, its surface is covered (by electro-deposition) with a coating
of platinum, in the form of a fine black powder. This presents
innumerable points of escape for the hydrogen gas; and for this reason
this battery falls off much less rapidly than the plain zinc and smooth
copper form. A modification of Smee's battery which, owing to the large
negative surface presented, is very advantageous, is Walker's graphite
cell. In this we have a plate of zinc between two plates of gas-carbon
("scurf"), or graphite. The surface of this body is naturally much
rougher than metal sheets; and this roughness of surface is further
assisted by coating the surface with platinum, as in the case of the
Smee. The chief objection to the use of graphite is its porosity, which
causes it to suck up the acid fluid in which the plates stand, and this,
of course, corrodes the brass connections, or binding screws.

Other _mechanical_ means of removing the hydrogen have been suggested,
such as brushing the surface of the plate, keeping the liquid in a state
of agitation by boiling or siphoning; but the only really efficient
practical means with which we are at present acquainted are _chemical_
means. Thus, if we can have present at the negative plate some substance
which is greedy of hydrogen, and which shall absorb it or combine with
it, we shall evidently have solved the problem. This was first effected
by Professor Daniell; and the battery known by his name still retains
its position as one of the simplest and best of the "constant" forms of
battery. The term "constant," as applied to batteries, does not mean
that the battery is a constancy, and will run for ever, but simply that
so long as there is in the battery any fuel (zinc, acid, etc.), the
electrical output of that battery will be constant. The Daniell cell
consists essentially in a rod or plate of zinc immersed in dilute
sulphuric acid, and separated from the copper or collecting plate by a
porous earthen pot or cell. Around the porous cell, and in contact with
the copper plate, is placed a solution of sulphate of copper, which is
maintained saturate by keeping crystals of sulphate of copper (blue
stone, blue vitriol) in the solution. Sulphate of copper is a compound
built up of copper Cu, and of sulphur oxide SO_{4}. When the dilute
sulphuric acid acts on the zinc plate or rod (§ 18), sulphate of zinc is
formed, which dissolves in the water, and hydrogen is given off:--

  Zn + H_{2} SO_{4} = Zn SO_{4} + H_{2}.

  Zinc and sulphuric acid produce zinc sulphate and free hydrogen.

Now this free hydrogen, by a series of molecular interchanges, is
carried along until it passes through the porous cell, and finds itself
in contact with the solution of copper sulphate. Here, as the hydrogen
has a greater affinity for, or is more greedy of, the sulphur oxide,
SO_{4}, than the copper is, it turns the latter out, takes its place,
setting the copper free, and forming, with the sulphur oxide, sulphuric
acid. The liberated copper goes, and adheres to the copper plate, and,
far from detracting from its efficacy, as the liberated hydrogen would
have done, actually increases its efficiency, as it is deposited in a
roughened form, which presents a large surface for the collection of the
electricity. The interchange which takes place when the free hydrogen
meets the sulphate of copper (outside the porous cells) is shown in the
following equation:--

  H_{2} + Cu SO_{4} = H_{2} SO_{4} + Cu.

  Free hydrogen and copper sulphate produce sulphuric acid and free copper.

[Illustration: Fig. 6. DANIELL CELL.]


§ 25. The original form given to this, the Daniell cell, is shown at
Fig. 6, in which Z is the zinc rod standing in the porous pot P, in
which is placed the dilute sulphuric acid. A containing vessel, V, of
glazed earthenware, provided with a perforated shelf, S, on which are
placed the crystals of sulphate of copper, serves to hold the copper
sheet, C, and the solution of sulphate of copper. T and T' are the
terminals from which the electricity is led where desired.

In another form, the copper sheet itself takes the form and replaces the
containing vessel V; and since the copper is not corroded, but actually
increases in thickness during action, this is a decided advantage. A
modification, in which the porous cell is replaced by _sand_ or by
_sawdust_, is also constructed, and known as "Minotto's" cell: this,
owing to the greater thickness of the porous layer, offers more
resistance, and gives, consequently, less current. By taking advantage
of the greater specific gravity (_weight, bulk for bulk_) of the
solution of sulphate of copper over that of water or dilute sulphuric
acid, it is possible to construct a battery which shall act in a manner
precisely similar to a Daniell, without the employment of any porous
partition whatsoever. Fig. 7 illustrates the construction of one of
these, known as "Gravity Daniells."

[Illustration: Fig. 7. GRAVITY CELL.]

In this we have a plate, disc, or spiral of copper, C, connected by an
insulated copper wire to the terminal T'. Over this is placed a layer of
crystals of copper sulphate; the jar is then filled nearly to the top
with dilute sulphuric acid, or with a strong solution of sulphate of
zinc (which is more lasting in its effects, but not so energetic as the
dilute sulphuric acid), and on the surface of this, connected to the
other terminal, T, is allowed to rest a thick disc of zinc, Z. Speaking
of these cells, Professor Ayrton, in his invaluable "Practical
Electricity," says:--"All gravity cells have the disadvantage that they
cannot be moved about; otherwise the liquids mix, and the copper
sulphate solution, coming into contact with the zinc plate, deposits
copper on it. This impairs the action, by causing the zinc to act
electrically, like a copper one. Indeed, without any shaking, the
liquids mix by diffusion, even when a porous pot is employed; hence a
Daniell's cell is found to keep in better order if it be always allowed
to send a weak current when not in use, since the current uses up the
copper sulphate solution, instead of allowing it to diffuse." The use of
a solution of zinc sulphate to act on the zinc rod, or plate, is always
to be preferred in the Daniell cell, when long duration is of more
consequence than energetic action.


§ 26. There are many other bodies which can be used in batteries to
absorb the hydrogen set free. Of several of these we need only take a
passing notice, as the batteries furnished by their use are unfit for
electric bell work. Of these we may mention nitric acid, which readily
parts with a portion of the oxygen (§ 18) and reconverts the free
hydrogen into water. This acid is used as the "depolarizer"[9] in the
"Grove" and in the "Bunsen" cell. Another very energetic "depolariser"
is chromic acid, either in solution, in dilute sulphuric acid, or in the
form of potassic dichromate (bichromate of potash: bichrome). As one
form of chromic cell has found favour with some bell-fitters, we shall
study its peculiarities farther on.

[Footnote 9: Depolarizer is the technical name given to any body which,
by absorbing the free hydrogen, removes the false polarity of the
negative plate.]

Another class of bodies which readily part with their oxygen, and thus
act as depolarisers, are the oxides of lead and manganese. This latter
oxide forms the basis of one of the most useful cells for electric bell
work, namely: the one known as the "Leclanché." As the battery has been,
and will probably remain, long a favourite, the next paragraph will be
devoted to its consideration.


§ 27. The Leclanché cell, in its original form, consists in a rod or
block of gas carbon (retort scurf: graphite) standing in an upright
porous pot. Around this, so as to reach nearly to the top of the porous
cell, is tightly packed a mixture of little lumps of graphite and black
oxide of manganese (manganic dioxide: black wad), the porous cell itself
being placed in an outer containing vessel, which usually takes the form
of a square glass bottle. A zinc rod stands in one corner of the
bottle, and is prevented from coming into actual contact with the
porous cell by having an indiarubber ring slipped over its upper and
lower extremities. The glass containing vessel is then filled to about
two-thirds of its height with a solution of ammonium chloride (sal
ammoniac) in water, of the strength of about 2 oz. of the salt to each
pint of water. This soon permeates the porous cell and reaches the
mixture inside. The general appearance of the Leclanché cell is well
shown at Fig. 8.

[Illustration: Fig. 8.]

In order to ensure a large surface of contact for the terminal of the
carbon rod or plate, it is customary to cast a leaden cap on the top
thereof; and, as the porosity of the graphite, or carbon, is very apt
to allow the fluid in the battery to creep up to and corrode the
terminal, and thus oppose resistance to the passage of electricity, the
upper end of the carbon, before the lead cap is cast on, is soaked for
some time in melted paraffin wax, at a temperature of 110° Centigrade:
that is somewhat hotter than boiling water heat. This, if left on the
outside, would prevent the passage of electricity almost entirely; so
lateral holes are drilled into the carbon before the cap is finally cast
on. The action that takes place in the Leclanché cell may be summarised
as follows:--

When the zinc, Zn, is acted on by the ammonium chloride, 2NH_{4}Cl, the
zinc seizes the chlorine and forms with it zinc chloride, ZnCl_{2},
while the ammonium, 2NH_{4}, is liberated. But this ammonium, 2NH_{4},
does not escape. Being electro-positive, it is impelled towards the
negative plate, and in its passage thereto meets with another molecule
of ammonium chloride, from which it displaces the ammonium, in this
wise: 2NH_{4} + 2NH_{4}Cl = 2NH_{4}Cl + 2NH_{4}; in other words, this
electro-positive ammonium is able, by virtue of its electrical charge,
to displace the ammonium from the combined chloride. In so doing, it
sets the liberated ammonium in an electro-positive condition, as it was
itself, losing at the same time its electrical charge. This interchange
of molecules goes on (as we saw in the case of the Daniell's cell, § 24)
until the surface of the carbon is reached. Here, as there is no more
ammonium chloride to decompose, the ammonium 2NH_{4} immediately splits
up into ammonia 2NH_{3} and free hydrogen H_{2}. The ammonia escapes,
and may be detected by its smell; while the hydrogen H_{2}, finding
itself in contact with the oxide of manganese, 2MnO_{2}, seizes one atom
of its oxygen, O, becoming thereby converted into water H_{2}O; while
the manganese dioxide, 2MnO_{2}, by losing one atom of oxygen, is
reduced to the form of a lower oxide of manganese, known as manganese
sesquioxide, Mn_{2}O_{3}. Expressed in symbols, this action may be
formulated as below:--

In the zinc compartment--

  Zn + 2NH_{4}Cl  =  ZnCl_{2} + 2NH_{3} + H_{2}

In the peroxide of manganese compartment--

  H_{2} + 2MnO_{2}  =  Mn_{2}O_{3} + H_{2}O.

Ammonia gas therefore slowly escapes while this battery is in action,
and this corrodes all the brass work with which it comes into contact,
producing a bluish green verdigris. If there be not sufficient ammonium
chloride in solution, the water alone acts on the zinc: zinc oxide is
produced, which renders the solution milky. Should this be the case,
more sal ammoniac must be added. It is found that for every 50 grains of
zinc consumed in this battery, about 82 grains of sal ammoniac and 124
grains of manganese dioxide are needed to neutralize the hydrogen set
free. It is essential for the efficient working of this battery that
both the manganese dioxide and the carbon should be free from powder,
otherwise it will cake together, prevent the passage of the liquid, and
present a much smaller surface to the electricity, than if in a granular
form. For this reason, that manganese dioxide should be preferred which
is known as the "needle" form, and both this and the carbon should be
sifted to remove dust.


§ 28. In the admirable series of papers on electric bell fitting which
was published in the _English Mechanic_, Mr. F. C. Allsop, speaking of
the Leclanché cell, says:--"A severe and prolonged test, extending over
many years, has proved that for general electric bell work the Leclanché
has no equal; though, in large hotels, etc., where the work is likely to
be very heavy, it may, perhaps, be preferable to employ a form of the
Fuller bichromate battery. It is very important that the battery
employed should be a thoroughly reliable one and set up in a proper
manner, as a failure in the battery causes a breakdown in the
communication throughout the whole building, whilst the failure of a
push or wire only affects that portion of the building in which the push
or wire is fixed. A common fault is that of putting in (with a view to
economy) only just enough cells (when first set up) to do the necessary
work. This is false economy, as when the cells are but slightly
exhausted the battery power becomes insufficient; whereas, if another
cell or two had been added, the battery would have run a much longer
time without renewal, owing to the fact that each cell could have been
reduced to a lower state of exhaustion, yet still the battery would have
furnished the necessary power; and the writer has always found that the
extra expense of the surplus cells is fully repaid by the increased
length of time the battery runs without renewal."


§ 29. Another form of Leclanché, from which great things were expected
at its introduction, is the one known as the "Agglomerate block," from
the fact that, instead of simply placing the carbon and manganese
together loosely in a porous cell, solid blocks are formed by
compressing these materials, under a pressure of several tons, around a
central carbon core, to which the terminal is attached in the usual
manner. The following are some of the compositions used in the
manufacture of agglomerate blocks:--

No. 1.

  Manganese dioxide                    40 parts.
  Powdered gas carbon                  55  "
  Gum lac resin                         5  "

No. 2.

  Manganese dioxide (pyrolusite)       40 parts.
  Gas carbon (powdered)                52  "
  Gum lac resin                         5  "
  Potassium bisulphate                  3  "

These are to be thoroughly incorporated, forced into steel moulds
(containing the central carbon core) at a temperature of 100° C. (212°
Fahr.), under a pressure of 300 atmospheres, say 4,500 lbs. to the
square inch.

No. 3.

      _Barbier and Leclanché's Patent._

  Manganese dioxide                    49 parts.
  Graphite                             44  "
  Pitch ("brai gras")                   9  "
  Sulphur                             3/5  "
  Water                               2/5  "

The materials having been reduced to fine powder, and the proportion of
water stated having been added, are intimately mixed together by hand or
mechanically. The moist mixture is moulded at the ordinary temperature,
either by a simple compressing press, or by a press in which two pistons
moving towards each other compress the block on two opposite faces; or
the mixture may be compressed by drawing, as in the manufacture of
electric light carbon. After compression, the products are sufficiently
solid to be manipulated. They are then put in a stove, or oven, the
temperature of which is gradually raised to about 350° C. (about 662°
Fahr.); a temperature which is insufficient to decompose the
depolarising substance (manganese dioxide), but sufficient to drive out
first the volatile parts of the agglomerating material, and then to
transform its fixed parts in a body unattackable by the ammonia of the
cell. During the gradual heating, or baking, which lasts about two
hours, what remains of the water in the agglomerate is driven off; then
come the more volatile oils contained in the pitch, and finally the
sulphur. The sulphur is added to the mixture, not as an agglomerative,
but as a chemical re-agent (and this is a characteristic feature in the
invention), acting on what remains of the pitch, as it acts on all
carbo-hydrides at a high temperature, transforming it partially into
volatile sulphuretted compounds, which are expelled by the heat, and
partially into a fixed and unattackable body, somewhat similar to
vulcanite. The action of the sulphur on the pitch can very well be
likened to its action on caoutchouc (which is likewise a hydro-carbon)
during the process of vulcanisation.

These agglomerate blocks, however prepared, are placed in glass or
porcelain containing vessels, as shown in Fig. 9, with a rod of zinc,
separated from actual contact with the carbon by means of a couple of
crossed indiarubber bands, which serve at the same time to hold the zinc
rods upright. The exciting solution, as in the case of the ordinary
Leclanché consists in a solution of ammonium chloride.

[Illustration: Fig. 9.]

Among the various advantages claimed for the agglomerate form of
Leclanché over the ordinary type, may be mentioned the following:--

1st.--The depolarising power of the manganese oxide is used to the best
advantage, and that, owing to this, the electro-motive force of the
battery is kept at the same point.

2nd.--That, owing to the absence of the porous cell, there is less
internal resistance in the battery and therefore more available current.

3rd.--That the resistance of the battery remains pretty constant,
whatever work be put upon it.

4th.--That, owing to the fact that the liquid comes into contact with
both elements immediately, the battery is ready for use directly on
being charged.

5th.--That the renewal or recharging is exceedingly easy, since the
elements can be removed together, fresh solution added, or new
depolarising blocks substituted.

But when this battery came to be put to the test of practical work, it
was found the block form could not be credited with all these
advantages, and that their chief superiority over the old cell consisted
rather in their lower internal resistance than in anything else. Even
this is not an advantage in the case of bell work, except when several
bells are arranged _in parallel_, so that a large current is required.
The blocks certainly polarise more quickly than the old form, and it
does not appear that they depolarise any more rapidly. Probably the
enormous pressure to which the blocks are subjected, in the first two
processes, renders the composition almost impermeable to the passage of
the fluid, so that depolarisation cannot take place very rapidly.
Another and serious objection to these blocks is that, after a little
work, pieces break away from the blocks and settle on the zinc. This
sets up a "short circuit," and the zincs are consumed whether the
battery is in action or not.

The author has had no opportunity for making any practical tests with
the blocks prepared by process No. 3, but he is under the impression
that the blocks would be even more friable than those prepared under
greater pressure.


§ 30. A third form of Leclanché, and one which has given considerable
satisfaction, is the one known as "Judson's Patent." This consists, as
shown at Fig. 10, in a cylinder of corrugated carbon encased in an outer
coating of an insulating composition. Inside the cell are two or more
thin carbon sheets, cemented to the sides of the cell by Prout's elastic
glue, or some similar compound, so as to leave spaces, which are filled
in with granular carbon and manganese. The surface of the plates is
perforated, so as to allow ready access to the exciting fluid. The zinc
rod, which is affixed to the cover, stands in the centre of the cell,
touching it at no part. Owing to the very large surface presented by the
corrugations in the carbon, and by the perforated carbon plates, the
internal resistance of this form of battery is very low; hence the
current, if employed against a small outer resistance, is large. But
this, except in the case of bells arranged in parallel, is of no great
advantage.

[Illustration: Fig. 10.]


§ 31. The ordinary form of Leclanché is found in market in three sizes,
viz., No. 1, No. 2, and No. 3. Unfortunately, all makers do not use
these numbers in the same manner, so that while some call the smallest,
or _pint_ size, No. 1, others give this name to the largest, or
_three-pint_, size. No. 2 is always quart size, and this is the one
commonly employed. When several cells are employed to work a number of
bells, it is well, in order that they may not receive injury, that they
be enclosed in a wooden box. As it is necessary that the batteries
should be inspected from time to time, boxes are specially made with
doubled hinged top and side, so that when the catch is released these
fall flat; thus admitting of easy inspection or removal of any
individual cell. This form of battery box is shown at Fig. 11.

[Illustration: Fig. 11. BATTERY IN BOX.]


§ 32. There are certain ills to which the Leclanché cells are liable
that require notice here. The first is _creeping_. By creeping is meant
the gradual crystallisation of the sal ammonium up the inside and round
the outside of the glass containing jar. There are two modes of
preventing this. The first consists in filling in the neck with melted
pitch, two small funnel-like tubes being previously inserted to admit of
the addition of fresh sal ammoniac solution, and for the escape of gas.
This mode cannot be recommended, as it is almost impossible to remove
the pitch (in case it be required to renew the zinc, etc.) without
breaking the glass vessel. The best way to remove the pitch is to place
the cell in a large saucepan of cold water, and set it on a fire until
the water boils. The pitch is, by this treatment, so far softened that
the elements can be removed and the pitch scraped away with a knife.

[Illustration: Fig. 12.]

By far the better mode is to rub round the inside and outside of the
neck of the jar with tallow, or melted paraffin wax, to the depth of an
inch or thereabouts. This effectually prevents creeping and the
consequent loss of current. Messrs. Gent, of Leicester, have introduced
a very neat modification of the Leclanché cell, with a view to obviate
altogether the evils deriving from creeping. This cell is illustrated at
Fig. 12, and the following is the description supplied by the
patentees:--"All who have had experience of batteries in which a
solution of salts is used are aware of the difficulty experienced in
preventing it creeping over the outside of the jar, causing local loss,
and oftentimes emptying the jar of its solution. Many devices have been
tried to prevent this, but the only effectual one is our patent
insulated jar, in which a recess surrounds the top of the jar, this
recess being filled with a material to which the salts will not adhere,
thus keeping the outside of the jar perfectly clean. It is specially
adapted for use in hot climates, and is the only cell in which jars may
touch each other and yet retain their insulations. We confidently
recommend a trial of this cell. Its price is but little in excess of the
ordinary Leclanché." The battery should be set up in as cool a place as
possible, as heat is very conducive to creeping. It is also important
that the battery should be placed as near as convenient to the bell.

Sometimes the zincs are seen to become coated with a black substance, or
covered with crystals, rapidly wasting away at the same time, although
doing little or no work; a strong smell of ammonia being given off at
the same time. When this occurs, it points to an electrical leakage, or
short circuit, and this, of course, rapidly exhausts the battery. It is
of the utmost importance to the effective working of any battery that
not the slightest leakage or _local action_ should be allowed to take
place. However slight such loss be, it will eventually ruin the battery.
This leakage may be taking place in the battery, as a porous cell may be
broken, and carbon may be touching the zinc; or out of the battery,
along the conducting wires, by one touching the other, or through
partial conductivity of a damp wall, a metallic staple, etc., or by
creeping. If loss or local action has taken place, it is best, after
discovering and repairing the faults (see also _testing wires_), to
replace the old zincs by new ones, which are not costly.


§ 33. There is yet a modification of the Leclanché which is sometimes
used to ring the large bells in hotels, etc., known as the Leclanché
reversed, since the zinc is placed in the porous pot, this latter being
stood in the centre of the stoneware jar, the space between the two
being packed with broken carbon and manganese dioxide. By this means a
very much larger negative surface is obtained. In the Grenet cell, the
porous cell is replaced by a canvas bag, which is packed full of lumps
of graphite and carbon dioxide, a central rod of carbon being used as
the electrode. This may be used in out-of-the-way places where porous
cells are not readily obtainable, but I cannot recommend them for
durability.


§ 34. The only other type of battery which it will be needful to notice
in connection with bell work is one in which the depolariser is either
chromic acid or a compound of chromic acid with potash or lime. Chromic
acid consists of hydrogen united to the metal chromium and oxygen.
Potassic dichromate (bichromate of potash: bichrome) contains potassium,
chromium, and oxygen. If we represent potassium by K, chromium by Cr,
and oxygen by O, we can get a fair idea of its constitution by
expressing it as K_{2}Cr_{2}O_{7}, by which it is shown that one
molecule of this body contains two atoms of potassium united to two
atoms of chromium and seven atoms of oxygen. Bichromate of potash
readily parts with its oxygen; and it is upon this, and upon the
relatively large amount of oxygen it contains, that its efficiency as a
depolariser depends. Unfortunately, bichromate of potash is not very
soluble in water; one pint of water will not take up much more than
three ounces of this salt. Hence, though the solution of potassium
bichromate is an excellent depolariser as long as it contains any of the
salt, it soon becomes exhausted. When bichromate of potash is used in a
cell along with sulphuric acid and water, sulphate of potash and chromic
acid are formed, thus:--

  K_{2}Cr_{2}O_{7} + H_{2}SO_{4} + H_{2}O  =  K_{2}SO_{4} + 2H_{2}CrO_{4}
  ----------------   -----------   ------     -----------   -------------
    1 molecule of  & 1 molecule  &   1   give 1 molecule  & 2 molecules
      bichrome.          of       molecule        of             of
                      sulphuric       of        sulphate       chromic
                        acid.      water.      of potash.       acid.

From this we learn that before the potassium bichromate enters into
action in the battery, it is resolved into chromic acid. Chromic acid is
now prepared cheaply on a large scale, so that potassium bichromate may
always be advantageously replaced by chromic acid in these batteries;
the more so as chromic acid is extremely soluble in water. In the
presence of the hydrogen evolved during the action of the battery (§ 18)
chromic acid parts with a portion of its oxygen, forming water and
sesquioxide of chromium, Cr_{2}O_{3}, and this, finding itself in
contact with the sulphuric acid, always used to increase the
conductivity of the liquid, forms sulphate of chromium. The action of
the hydrogen upon the chromic acid is shown in the following equation:--

  2H_{2}CrO_{4}   +    3H_{2}     =      5H_{2}O    +   Cr_{2}O_{3}
  -------------        ------            -------        -----------
  2 molecules of    3 molecules        5 molecules      1 molecule
     chromic      &      of      give    of water.  &       of
       acid.          hydrogen.                           chromium
                                                        sesquioxide.

[Illustration: Fig. 13.]


§ 35. The "bottle" form of the bichromate or chromic acid battery (as
illustrated at Fig. 13) is much employed where powerful currents of
short duration are required. It consists of a globular bottle with a
rather long wide neck, in which are placed two long narrow graphite
plates, electrically connected to each other and to one of the binding
screws on the top. Between these two plates is a sliding rod, carrying
at its lower extremity the plate of zinc. This sliding rod can be
lowered and raised, or retained in any position, by means of a set
screw. The zinc is in metallic connection with the other binding screw.
This battery (which, owing to the facility with which the zinc can be
removed from the fluid, is extremely convenient and economical for short
experiments) may be charged with either of the following fluids:--

FIRST RECIPE.

_Bichromate Solution._

  Bichromate of potash (finely powdered)     3 oz.
  Boiling water                              1 pint.

Stir with a glass rod, allow to cool, then add, in a fine stream, with
constant stirring,

  Strong sulphuric acid (oil of vitriol)     3 fluid oz.

The mixture should be made in a glazed earthern vessel, and allowed to
cool before using.

SECOND RECIPE.

_Chromic Acid Solution._

  Chromic acid (chromic trioxide)      3 oz.
  Water                                1 pint.

Stir together till dissolved, then add gradually, with stirring,

  Sulphuric acid                       3 oz.

This also must not be used till cold.

In either case the bottle must not be more than three parts filled with
the exciting fluid, to allow plenty of room for the zinc to be drawn
right out of the liquid when not in use.


§ 36. The effects given by the above battery, though very powerful, are
too transient to be of any service in continuous bell work. The
following modification, known as the "Fuller" cell, is, however, useful
where powerful currents are required, and, when carefully set up, may be
made to do good service for five or six months at a stretch. The
"Fuller" cell consists in an outer glass or glazed earthern vessel, in
which stands a porous pot. In the porous pot is placed a large block of
amalgamated zinc, that is cast around a stout copper rod, which carries
the binding screw. This rod must be carefully protected from the action
of the fluid, by being cased in an indiarubber tube. The amalgamation of
the zinc must be kept up by putting a small quantity of mercury in the
porous cell. The porous cells must be paraffined to within about half an
inch of the bottom, to prevent too rapid diffusion of the liquids, and
the cells themselves should be chosen rather thick and close in texture,
as otherwise the zinc will be rapidly corroded. Water alone is used as
the exciting fluid in the porous cell along with the zinc. Speaking of
this form of cell, Mr. Perren-Maycock says:--"The base of the zinc is
more acted on (when bichromate crystals are used), because the porous
cells rest on the crystals; therefore let it be well paraffined, as also
the top edge. Instead of paraffining the pot in strips all round (as
many operators do) paraffin the pot all round, except at one strip about
half an inch wide, and let this face the carbon plate. If this be done,
the difference in internal resistance between the cell with paraffined
pot and the same cell with pot unparaffined will be little; but if the
portion that is unparaffined be turned away from the carbon, it will
make very nearly an additional 1 ohm resistance. It is necessary to have
an ounce or so of mercury in each porous cell, covering the foot of the
zinc; or the zincs may be cast short, but of large diameter, hollowed
out at the top to hold mercury, and suspended in the porous pot. The
zinc is less acted on then, for when the bichromate solution diffuses
into the porous pot, it obviously does so more at the bottom than at the
top."

[Illustration: Fig. 14.]

Fig. 14 illustrates the form usually given to the modification of the
Fuller cell as used for bell and signalling work.


§ 37. Before leaving the subject of batteries, there are certain points
in connection therewith that it is absolutely essential that the
practical man should understand, in order to be able to execute any work
satisfactorily. In the first place, it must be borne in mind that a cell
or battery, when at work, is continually setting up electric
undulations, somewhat in the same way that an organ pipe, when actuated
by a pressure of air, sets up a continuous sound wave. Whatever sets up
the electric disturbance, whether it be the action of sulphuric acid on
zinc, or caustic potash on iron, etc., is called _electromotive force_,
generally abbreviated E.M.F. Just in the same manner that the organ pipe
could give no sound if the pressure of air were alike inside and out, so
the cell, or battery, cannot possibly give _current_, or evidence of
electric flow, unless there is some means provided to allow the
_tension_, or increased atomic motion set up by the electromotive force,
to distribute itself along some line of conductor or conductors not
subjected to the same pressure or E.M.F. In other words, the "current"
of electricity will always tend to flow from that body which has the
highest tension, towards the body where the strain or tension is less.
In a cell in which zinc and carbon, zinc and copper, or zinc and silver
are the two elements, with an acid as an excitant, the zinc during the
action of the acid becomes of higher "potential" than the other
element, and consequently the undulations take place towards the
negative plate (be it carbon, copper, or silver). But by this very
action the negative plate immediately reaches a point of equal tension,
so that no current is possible. If, however, we now connect the two
plates together by means of any conductor, say a copper wire, then the
strain to which the carbon plate is subjected finds its exit along the
wire and the zinc plate, which is continually losing its strain under
the influence of the acid, being thus at a lower potential (electrical
level, strain) than the carbon, can and does actually take in and pass
on the electric vibrations. It is therefore evident that no true
"current" can pass unless the two elements of a battery are connected up
by a conductor. When this connection is made, the circuit is called a
"_closed circuit_." If, on the contrary, there is no electrical
connection between the negative and positive plates of a cell or
battery, the circuit is said to be open, or _broken_. It may be that the
circuit is closed by some means that is not desirable, that is to say,
along some line or at some time when and where the flow is not wanted;
as, for instance, the outside of a cell may be _wet_, and one of the
wires resting against it, when of course "leakage" will take place as
the circuit will be closed, though no useful work will be done. On the
other hand, we may actually take advantage of the practically unlimited
amount of the earth's surface, and of its cheapness as a conductor to
make it act as a portion of the conducting line. It is perfectly true
that the earth is a very poor conductor as compared with metals. Let us
say, for the sake of example, that damp earth conducts 100,000 times
worse than copper. It will be evident that if a copper wire 1/20 of an
inch in section could convey a given electric current, the same length
of earth having a section of 5,000 inches would carry the same current
equally well, and cost virtually nothing, beyond the cost of a metal
plate, or sack of coke, presenting a square surface of a little over 70
inches in the side at each end of the line. This mode of completing the
circuit is known as "the earth plate."


§ 38. The next point to be remembered in connection with batteries is,
that the electromotive force (E.M.F.) depends on the _nature_ of the
elements (zinc and silver, zinc and carbon, etc.) and the excitants used
in the cell, and has absolutely nothing whatever to do with their
_size_. This may be likened to difference of temperature in bodies.
Thus, whether we have a block of ice as large as an iceberg or an inch
square, the temperature will never exceed 32°F. as long as it remains
ice; and whether we cause a pint or a thousand gallons of water to boil
(under ordinary conditions), its temperature will not exceed 212°F. The
only means we have of increasing the E.M.F., or "tension," or
"potential," of any given battery, is by connecting up its constituent
cells in _series_; that is to say, connecting the carbon or copper plate
of the one cell to the zinc of the next, and so on. By this means we
increase the E.M.F. just in the same degree as we add on cells. The
accepted standard for the measure of electromotive force is called a
VOLT, and 1 volt is practically a trifle less than the E.M.F. set up by
a single Daniell's cell; the exact amount being 1·079 volt, or 1-1/12
volt very nearly. The E.M.F. of the Leclanché is very nearly 1·6 volt,
or nearly 1 volt and 2/3. Thus in Fig. 15, which illustrates 3 Leclanché
cells set up in series, we should get

  1·6 volt
  1·6 "
  1·6 "
  ---------
  4·8 volts

as the total electromotive force of the combination.

[Illustration: Fig. 15.]


§ 39. The _current_, or amplitude of the continuous vibrations kept up
in the circuit, depends upon two things: 1st, the electromotive force;
2nd, the resistance in the circuit. There is a certain amount of
resemblance between the flow of water under pressure and electricity in
this respect. Let us suppose we have a constant "head" of water at our
disposal, and allow it to flow through a tube presenting 1 inch
aperture. We get a certain definite flow of water, let us say 100
gallons of water per hour. More we do not get, owing to the resistance
opposed by the narrowness of the tube to a greater flow. If now we
double the capacity of the exit tube, leaving the pressure or "head" of
water the same, we shall double the flow of water. Or we may arrive at
the same result by doubling the "head" or pressure of water, which will
then cause a double quantity of water to flow out against the same
resistance in the tube, or conductor. Just in the same way, if we have a
given pressure of electric strain, or E.M.F., we can get a greater or
lesser flow or "current" by having less or more resistance in the
circuit. The standard of flowing current is called an AMPÈRE; and 1
ampère is that current which, in passing through a solution of sulphate
of copper, will deposit 18·35 grains of copper per hour. The unit of
resistance is known as an OHM. The resistance known as 1 ohm is very
nearly that of a column of mercury 1 square millimètre (1/25 of an inch)
in section, and 41-1/4 inches in height; or 1 foot of No. 41 gauge pure
copper wire, 33/10000 of an inch in diameter, at a temperature of 32°
Fahr., or 0° Centigrade.


§ 40. Professor Ohm, who made a special study of the relative effects of
the resistance inserted in the circuit, the electromotive force, and the
current produced, enunciated the following law, which, after him, has
been called "OHM'S LAW." It is that if we divide the number of
electromotive force units (volts) employed by the number of resistance
units (ohms) in the entire circuit, we get the number of current units
(ampères) flowing through the circuit. This, expressed as an equation is
shown below:

E/R = C or Electromotive force/Resistance = Current.

Or if we like to use the initials of volts, ampères, and ohms, instead
of the general terms, E, R, and C, we may write V/R = A, or Volts/Ohms =
Ampères.

From this it appears that 1 volt will send a current of 1 ampère through
a total resistance of 1 ohm, since 1 divided by 1 equals 1. So also 1
volt can send a current of 4 ampères through a resistance of 1/4 of an
ohm, since 1 divided by 1/4 is equal to 4. We can therefore always
double the current by halving the resistance; or we may obtain the same
result by doubling the E.M.F., allowing the resistance to remain the
same. In performing this with batteries we must bear in mind that the
metals, carbon, and liquids in a battery do themselves set up
resistance. This resistance is known as "_internal resistance_," and
must always be reckoned in these calculations. We can _halve_ the
internal resistance by _doubling_ the size of the negative plate, or
what amounts to the same thing by connecting two similar cells "_in
parallel_;" that is to say, with both their zincs together, to form a
positive plate of double size, and both carbons or coppers together to
form a single negative of twice the dimensions of that in one cell. Any
number of cells thus coupled together "_in parallel_" have their
resistances reduced just in proportion as their number is increased;
hence 8 cells, each having a resistance of 1 ohm if coupled together _in
parallel_ would have a joint resistance of 1/8 ohm only. The E.M.F.
would remain the same, since this does not depend on the size of the
plate (see § 38). The arrangement of cells in parallel is shown at Fig.
16, where three Leclanché cells are illustrated thus coupled. The
following little table gives an idea of the E.M.F. in volts, and the
internal resistance in ohms, of the cells mostly used in electric bell
work.

[Illustration: Fig. 16.]

TABLE SHOWING E.M.F. AND R. OF BATTERIES.

  ----------------+-------------------+-----------------+---------------
    Name of Cell. | Capacity of Cell. |  Electromotive  | Resistance in
                  |                   | force in Volts. |     Ohms.
  ----------------+-------------------+-----------------+---------------
  Daniell         |     2 quarts      |      1·079      |      1
     "    Gravity |     2 quarts      |      1·079      |     10
  Leclanché       |     1 pint        |      1·60       |      1·13
     "            |     2 pints       |      1·60       |      1·10
     "            |     3 pints       |      1·60       |      0·87
  Agglomerate     |     1 pint        |      1·55       |      0·70
     "            |     2 pints       |      1·55       |      0·60
     "            |     3 pints       |      1·55       |      0·50
  Fuller          |     1 quart       |      1·80       |      0·50
  ----------------+-------------------+-----------------+---------------

From this it is evident that if we joined up the two plates of a Fuller
cell with a short wire presenting no appreciable resistance, we should
get a current of (1·80 divided by 0·50) 3·6 ampères along the wire;
whereas if a gravity Daniell were employed the current flowing in the
same wire would only be a little over 1/10 of an ampère, since 1·079/10
= 0·1079. But every wire, no matter how short or how thick, presents
_some_ resistance; so we must always take into account both the internal
resistance (that of the battery itself) and the external resistance
(that of the wires, etc., leading to the bells or indicators) in
reckoning for any given current from any cell or cells.



CHAPTER III.

ON ELECTRIC BELLS AND OTHER SIGNALLING APPLIANCES.


§ 41. An electric bell is an arrangement of a cylindrical soft iron
core, or cores, surrounded by coils of insulated copper wire. On causing
a current of electricity to flow round these coils, the iron becomes,
_for the time being_, powerfully magnetic (see § 13). A piece of soft
iron (known as the _armature_), supported by a spring, faces the magnet
thus produced. This armature carries at its free extremity a rod with a
bob, clapper or hammer, which strikes a bell, or gong, when the
armature, under the influence of the pull of the magnet, is drawn
towards it. In connection with the armature and clapper is a device
whereby the flow of the current can be rapidly interrupted, so that on
the cessation of the current the iron may lose its magnetism, and allow
the spring to withdraw the clapper from against the bell. This device is
known as the "contact breaker" and varies somewhat in design, according
to whether the bell belongs to the _trembling_, the _single stroke_, or
the _continuous ringing_ class.


§ 42. In order that the electric bell-fitter may have an intelligent
conception of his work, he should _make_ a small electric bell himself.
By so doing, he will gain more practical knowledge of what are the
requisites of a good bell, and where defects may be expected in any he
may be called upon to purchase or examine, than he can obtain from pages
of written description. For this reason I reproduce here (with some
trifling additions and modifications) Mr. G. Edwinson's directions for
making an electric bell:--[10]

_How to make a bell._--The old method of doing this was to take a piece
of round iron, bend it into the form of a horse-shoe, anneal it (by
leaving it for several hours in a bright fire, and allowing it to cool
gradually as the fire goes out), wind on the wire, and fix it as a
magnet on a stout board of beech or mahogany; a bell was then screwed to
another part of the board, a piece of brass holding the hammer and
spring being fastened to another part. Many bells made upon this plan
are still offered for sale and exchange, but their performance is always
liable to variation and obstruction, from the following causes:--To
insure a steady, uniform vibratory stroke on the bell, its hammer must
be nicely adjusted to move within a strictly defined and limited space;
the least fractional departure from this adjustment results in an
unsatisfactory performance of the hammer, and often a total failure of
the magnet to move it. In bells constructed on the old plan, the wooden
base is liable to expansion and contraction, varying with the change of
weather and the humidity, temperature, etc., of the room in which the
bells are placed. Thus a damp, foggy night may cause the wood to swell
and place the hammer out of range of the bell, while a dry, hot day may
alter the adjustment in the opposite direction. Such failures as these,
from the above causes alone, have often brought electric bells into
disrepute. Best made bells are, therefore, now made with metallic
(practically inexpansible) bases, and it is this kind I recommend to my
readers.

[Footnote 10: "Amateur Work."]

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

_The Base_, to which all the other parts are fastened, is made of 3/4
in. mahogany or teak, 6 in. by 4 in., shaped as shown at Fig. 17, with a
smooth surface and French polished. To this is attached the metallic
base-plate, which may be cut out of sheet-iron, or sheet-brass (this
latter is better, as iron disturbs the action of the magnet somewhat),
and shaped as shown in Fig. 18; or it may be made of cast-iron, or cast
in brass; or a substitute for it may be made in wrought-iron, or brass,
as shown in Fig. 19. I present these various forms to suit the varied
handicrafts of my readers; for instance, a worker in sheet metal may
find it more convenient to manufacture his bell out of the parts
sketched in Figs. 17, 18, 20^A, 21, 23, 24^A, and 25; but, on the other
hand, a smith or engineer might prefer the improved form shown at Fig.
31, and select the parts shown at Figs. 20^A, 22, 19, choosing either to
forge the horse-shoe magnet, Fig. 20, or to turn up the two cores, as
shown at Fig. 21 (A), to screw into the metal base, Fig. 21 B, or to be
fastened by nuts, as shown at Fig. 19. The result will be the same in
the end, if good workmanship is employed, and the proper care taken in
fixing and adjusting the parts. A tin-plate worker may even cut his
base-plate out of stout block tin, and get as good results as if the
bell were made by an engineer. In some makes, the base-plate is cut or
stamped out of thick sheet-iron, in the form shown by the dotted lines
on Fig. 18, and when thus made, the part A is turned up at right angles
to form a bracket for the magnet cores, the opposite projection is cut
off, and a turned brass pillar is inserted at B to hold the contact
screw, or contact breaker (§ 41).

[Illustration: Fig. 19.]

[Illustration: Fig. 20.]

[Illustration: Fig. 20 A.]

The _Magnet_ may be formed as shown at Fig. 20, or at Fig. 20^A. Its
essential parts are: 1st. Two soft iron cores (in some forms a single
core is now employed); 2nd. An iron base, or yoke, to hold the cores
together; 3rd. Two bobbins wound with wire. The old form of magnet is
shown at Fig. 20. In this form the cores and yoke are made out of one
piece of metal. A length of round Swedish iron is bent round in the
shape of a horseshoe; this is rendered thoroughly soft by annealing, as
explained further on. It is absolutely essential that the iron be very
soft and well annealed, otherwise the iron cores retain a considerable
amount of magnetism when the current is not passing, which makes the
bell sluggish in action, and necessitates a higher battery power to make
it work (see § 14). Two bobbins of insulated wire are fitted on the
cores, and the magnet is held in its place by a transverse strip of
brass or iron secured by a wood screw passing between the two bobbins.
The size of the iron, the wire, the bobbins, and the method of winding
is the same as in the form next described, the only difference being
that the length of the iron core, before bending to the horse-shoe
form, must be such as to allow of the two straight portions of the legs
to be 2 in. in length, and stand 1-3/8 apart when bent. We may now
consider the construction of a magnet of the form shown at Fig. 20^A. To
make the cores of such a magnet, to ring a 2-1/2 in. bell, get two 2
inch lengths of 5/16 in. best Swedish round iron, straighten them,
smooth them in a lathe, and reduce 1/4 in. of one end of each to 4/16 of
an in., leaving a sharp shoulder, as shown at Fig. 21 A. Next, get a
2-in. length of angle iron, drill in it two holes 1-3/8 apart, of the
exact diameter of the turned ends of the cores, and rivet these securely
in their places; this may be done by fastening the cores or legs in a
vice whilst they are being rivetted. Two holes should be also bored in
the other flange to receive the two screws, which are to hold the magnet
to the base, as shown at Fig. 21 B. The magnet is now quite equal to the
horse-shoe form, and must be made quite soft by annealing. This is done
by heating it in a clear coal fire to a bright red heat, then burying it
in hot ashes, and allowing it to cool gradually for a period of from 12
to 24 hours; or perhaps a better guide to the process will be to say,
bury the iron in the hot ashes and leave it there until both it and they
are quite cold. The iron must be brought to a bright cherry red heat
before allowing it to cool, to soften it properly, and on no account
must the cooling be hurried, or the metal will be _hard_. Iron is
rendered hard by hammering, by being rapidly cooled, either in cold air
or water, and hard iron retains magnetism for a longer time than soft
iron. As we wish to have a magnet that will only act as such when a
current of electricity is passing around it, and shall return to the
state of a simple piece of unmagnetised iron when the current is broken,
we take the precaution of having it of soft iron. Many bells have failed
to act properly, because this precaution has been neglected, the
"residual" (or remaining) magnetism holding down the armature after
contact has been broken. When the magnet has been annealed, its legs
should be polished with a piece of emery cloth, and the ends filed up
level and smooth. If it is intended to fasten the cores into the
base-plate, this also should be annealed, unless it be made of brass, in
which case a thin strip of soft iron should connect the back ends of the
two legs before they are attached to the brass base (an iron yoke is
preferable, as it certainly is conducive to better effects to have a
massive iron yoke, than to have a mere strip as the connecting piece).
It will also be readily understood and conceded that the cores should be
cut longer when they are to be fastened by nuts, to allow a sufficient
length for screwing the ends to receive the nuts. The length and size of
the legs given above are suitable for a 2-1/2 in. bell only; for larger
bells the size increases 1/16 of an inch, and the length 1/4 of an inch,
for every 1/2 in. increase in the diameter of the bell.

[Illustration: Fig. 21.]

The _Bobbins_, on which the wire that serves to carry the magnetising
current is to be wound, next demand our attention. They may be turned
out of boxwood, ebony, or ebonite, or out of any hard wood strong enough
and dense enough to allow of being turned down thin in the body, a very
necessary requirement to bring the convolutions of wire as near the coil
as possible without touching it. Some amateurs use the turned ends of
cotton reels or spools, and glue them on to a tube of paper formed on
the cores themselves. If this tube be afterwards well covered with
melted paraffin wax, the plan answers admirably, but of course the
bobbins become fixtures on the magnets. There are some persons who are
clever enough to make firm bobbins out of brown paper (like rocket
cases), with reel ends, that can be slipped off and on the magnet cores.
To these I would say, "by all means at your command, do so if you can."
The size of the bobbins for a 2-1/2 in. bell should be: length 1-3/4
in., diameter of heads 3/4 of an in., the length increasing 1/4 of an
in. and the diameter 1/8 of an in. for every additional 1/2 in. in the
diameter of the bell. The holes throughout the bobbins should be of a
size to fit the iron cores exactly, and the cores should project 1/8 of
an inch above the end of the bobbins when these are fitted on. The wire
to be wound on the bobbins is sold by all dealers in electrical
apparatus. It is copper wire, covered with cotton or with silk, to
ensure insulation. Mention has already been made of what is meant by
insulation at § 3, but, in order to refresh the reader's memory, Mr. G.
Edwinson's words are quoted here. "To insulate, as understood by
electricians, means to protect from leakage of the electric current, by
interposing a bad conductor of electricity between two good conductors,
thus insulating[11] or detaching them from electric contact."

[Footnote 11: _Insula_ in Latin means an island, hence an electrified
body is said to be insulated when surrounded by non-conductors, as an
island by the sea.]

The following list will enable my readers to see at a glance the value
of the substances mentioned here as conductors or insulators, the best
conductors being arranged from the top downwards, and the bad conductors
or insulators opposed to them in similar order, viz., the worst
conductors or best insulators being at the top:--

    _Conductors._                 _Insulators._
  Silver.                      Paraffin Wax.
  Copper.                      Guttapercha.
  Iron.                        Indiarubber.
  Brass.                       Shellac.
  All Other Metals.            Varnishes.
  Metallic Solutions.          Sealing Wax.
  Metallic Salts.              Silk and Cotton.
  Wet Stone.                   Dry Clothing.
  Wet Wood.                    Dry Wood.
                               Oil, Dirt and Rust.

See also the more extended list given at § 5 for a more complete and
exact classification.

It will be seen, on reference to the above, that copper is a good
conductor, being excelled by silver alone in this respect; and that silk
and cotton are bad conductors. When, therefore, a copper wire is bound
round with silk or with cotton, even if two or more strands of such a
covered wire be superimposed, since these are electrically separated by
the non-conducting covering, no escape of electricity from one strand to
the other can take place, and the strands are said to be insulated. If
the copper wire had been coiled _naked_ round a bobbin, each convolution
touching its neighbour, the current would not have circled round the
whole length of the coils of wire, but would have leapt across from one
coil to the other, and thus the desired effect would not have been
obtained. A similar result, differing only in degree, would occur if a
badly insulating wire were used, say one in which the covering had been
worn in places, or had been badly wound, so as to expose patches of bare
copper wire. If the insulation of a wire be suspected, it should be
immersed in hot melted paraffin wax, and then hung up to drain and cool.
The size of wire to be used on a 2-1/2 in. bell should be No. 24 B. W.
G., the size falling two numbers for each 1/2 in. increase in the
diameter of the bell. In these wires the higher the number, the finer
the size, No. 6 being 1/5 and No. 40 being 1/200 of an inch in diameter.
Silk-covered wire has an advantage over cotton-covered wire, inasmuch as
the insulating material occupies less space, hence the convolutions of
wire lie closer together. This is important, as the current has less
effect on the iron if removed further from it, the decrease being as
the _square_ of the distance that the current is removed from the wire.
Magnets coiled with silk-covered wire admit also of better finish, but
for most purposes cotton-covered wire will give satisfaction, especially
if well paraffined. This wire must be wound on the bobbins, from end to
end regularly, with the coils side by side, as a reel of cotton is
wound. This may be done on a lathe, but a little practice will be
necessary before the inexperienced hand can guide the wire in a regular
manner. If, however, the spool of wire have a metal rod passed up its
centre, and this be held in the hand at a distance of a foot or more
from the bobbin on the lathe, the wire will almost guide itself on,
providing the guiding hand be allowed to follow its course. With a
little care, the wire for these little magnets may be wound entirely by
hand. Before commencing to wind on the bobbins, just measure off 8 in.
of the wire (not cutting it off) and coil this length around a pencil,
to form a small coil or helix. The pencil may then be withdrawn from the
helix thus formed, which serves to connect the wire with one of the
points of contact. This free end is to be fastened outside the bobbin by
a nick in the head; or the 1/8 in. length, before being formed into a
helix, may be pushed through a small hole made on the head of the
bobbin, so that 8 in. project _outside_ the bobbin, which projecting
piece may be coiled into a helix as above described. The wire should now
be wound exactly as a reel of cotton is wound, in close coils from end
to end, and then back again, until three layers of wire have been laid
on, so that the coiling finishes at the opposite end to that at which it
began. To prevent this uncoiling, it should be fastened by tying down
tightly with a turn or two of strong silk. The wire should now be cut
from the hank, leaving about 2 in. of free wire projecting at the
finishing end of each bobbin. In cases where many bobbins have to be
wound, either for bells, for relays, or for indicator coils, a device
similar to that illustrated at Fig. 21 A may be employed. This _electric
bobbin winder_ consists in a table which can be stood on a lathe or near
any other driving wheel. Two carriers, C C, somewhat similar to the back
centre and poppet head of a lathe, hollow inside, and furnished with a
spring and sliding piston spindle, stand one at each end of this table.
The sliding spindle of the one carries at its extremity a pulley, A, by
means of which motion can be transmitted from the band of the driving
wheel. The sliding spindles, B B, are fitted with recesses and screws, H
H H H, by means of which the temporary wooden cores, or the permanent
iron cores, of the bobbins can be held while the bobbins are being
wound. The bobbin is placed as shown at D; a flat piece of metal, E,
hinged at G, presses against the bobbin, owing to the spring F. The
centre figure shows details of the carrier, C, in section. At the bottom
is shown the spool of wire on a standard L. The wire passes from this
spot between the two indiarubber rollers, M M, on to the bobbin D.

[Illustration: Fig. 21 A.]

When the bobbins have been wound, they may be slipped over the magnet
cores. They should fit pretty tightly; if they do not, a roll of paper
may be put round the magnet cores, to ensure their not slipping when
the bell is at work. The helix ends of the bobbins should stand
uppermost, as shown at Fig. 22 A. A short length of the lower free ends
of wire (near the base or yoke) should now be bared of their covering,
cleaned with emery paper, twisted together tightly, as shown at Fig. 22
B, soldered together, and any excess of wire cut off with a sharp pair
of pliers. To prevent any chance electrical leakage between this bared
portion of the wire and the iron, it should be carefully coated with a
little melted guttapercha, or Prout's electric glue.

[Illustration: Fig. 22.]

Of course, if the operator has any skill at winding, he may wind both
bobbins with one continuous length of wire, thus avoiding joins, taking
care that the direction of the winding in the finished coils be as shown
at Fig. 22 B; that is to say, that the wire from the _under_ side of one
bobbin, should pass _over_ to the next in the same way as the curls of
the letter [rotated S].

[Illustration: Fig. 23.]

[Illustration: Fig. 24.]

[Illustration: Fig. 25.]

[Illustration: Fig. 26.]

[Illustration: Fig. 27.]

The part that next claims our consideration is the _armature_, with its
fittings. The armature is made out of 5/16 square bar iron, of the best
quality, soft, and well annealed, and filed up smooth and true. The
proportionate length is shown at Figs. 23 and 24; and the size of the
iron for other bells is regulated in the same ratio as that of the
cores. Two methods of making and attaching the springs and hammers are
shown. Fig. 24 shows the section of an armature fitted with back spring
and contact spring in one piece. This is cut out of hard sheet-brass, as
wide as the armature, filed or hammered down to the desired degree of
springiness, then filed up true on the edges. It may be attached to the
iron of the armature, either by soldering, by rivetting, or by means of
two small screws. Rivetting is, perhaps, the best mode, as it is not
liable to shake loose by the vibration of the hammer. The spring at its
shank end may be screwed or rivetted to the bracket. Mr. Edwinson
considers this the better form of contact spring. The other form is made
in two pieces, as shown at Fig. 23, where two strips of hard brass are
cut off, of the width of the armature, and the edges filed. A slot is
then cut in the back end of the armature to receive the two brass
strips, and these are soldered into it. The top strip is then bent back
over the armature to form the contact-spring, the other strip being
soldered or rivetted to a small bracket of angle brass. In either case a
short rod of stout hard brass wire is rivetted or screwed into the free
end of the armature, and to the end of this rod is screwed or soldered
the metal bead, or bob, which forms the hammer or "clapper" of the bell.
The next portion to be made is the contact pillar, or bracket, with its
screw, as shown at Fig. 25. This may either be a short stout pillar of
1/4 in. brass rod, about 1 in. high, tapped on one side to receive the
screw, which should be fitted with a back nut; or it may, as shown in
the figure, be made out of a stout piece of angle brass. The exact size
and length of the screw is immaterial; it must, however, be long enough
to reach (when put in its place behind the contact spring) the spring
itself, and still have a few threads behind the back nut to spare. The
screw should be nicely fitted to the pillar, and the lock nut should
clench it well, as when once the adjustment of the parts is found which
gives good ringing, it is advisable that no motion should take place,
lest the perfection of ringing be interfered with. Some makers use a
"set screw" at the side of the pillar wherewith to hold the contact
screw; others split the pillar and "spring" it against the contact
screw; but, all things considered, the back nut gives the greatest
satisfaction. When the bell is in action, a tiny spark is produced at
every make and break of contact between the contact spring and this
screw. This spark soon corrodes the end of the screw and the back of the
spring if brass alone is used, as this latter rusts under the influence
of the spark. To prevent this, a piece of platinum must be soldered or
rivetted to the spring, at the point where the screw touches, as shown
at Fig. 26, and also at the extremity of the contact screw itself. It is
better to rivet the platinum than to solder it, as the platinum is very
apt to absorb the solder, in which case it rusts quickly, and the
goodness of the contact is soon spoiled, when the bell ceases to ring.
To rivet the platinum piece on to the spring, as shown at Fig. 26, it is
only needful to procure a short length of No. 16 platinum wire, say 1/8
in., then, having drilled a corresponding hole at the desired spot in
the contact spring, put the platinum wire half way through the hole, and
give it one or two sharp blows on an anvil, with a smooth (pened)
hammer.

[Illustration: Fig. 28.]

[Illustration: Fig. 29.]

This will at once rivet it in its place, and spread it sufficiently to
make a good surface for contact. The screw must likewise be tipped with
platinum, by having a small hole bored in the centre of its extremity,
of the same diameter as the platinum wire, which must then be pushed in,
and rivetted by hammering the end, and burring the sides of the screw.
Whichever method be adopted, care must be taken that the platinum tip on
the screw and the speck on the contact spring are adjusted so as to
touch exactly in their centres. It will be hardly worth while for the
amateur to cast or even turn up his own bells (which are generally of
the class known as clock gongs), as these can now be procured so cheaply
already nickelled (see Fig. 28). The bell must be adjusted on its pillar
(see Fig. 29^A), which is itself screwed into a hole in the base-plate,
where it is held by a nut. The adjustment of the bell is effected by
placing it over the shoulder of the pillar, and then clenching it down
by screwing over it one or other of the nuts shown at Fig. 29. The bell
should clear the base, and should be at such a height as to be struck on
its edge by the hammer or clapper attached to the armature, Figs. 23 and
24. We still need, to complete our bell, two binding screws, which may
take either of the forms shown at Fig. 27; and an insulating washer, or
collar, made of ebonite or boxwood, soaked in melted paraffin, to
prevent the contact pillar (Fig. 25) making electrical contact with the
metal base. The best shape to be given to these washers is shown at Fig.
30. They consist in two thin circlets of wood or ebonite, that will just
not meet when dropped, one on the one side, and one on the other of the
hole through which the shank of the contact pillar passes when set up on
the base-plate. If a wooden base be used below the metal base-plate,
then only one washer, or collar, need be used--that is, the one
_above_--since the screw of the pillar will pass into the wood, and this
is not a conductor. If the metal base alone be used, both washers must
be employed, and a small nut (not so large as the washer) used to
tighten up and hold the pillar firm and immovable in its place opposite
the contact spring.

[Illustration: Fig. 30.]

Having now all the parts at hand, we can proceed to fit them together,
which is done as follows:--The bell pillar, with its bell attached, is
fastened by its shank into the hole shown near B, Fig. 17, where it is
screwed up tight by the square nut shown at Fig. 29 _c_. In the same
manner, we must fasten the contact pillar, or bracket, shown at Fig. 24
A. Whichever form be used, we must take great care that it be insulated
from metallic contact with the metal base-plate by washers, as shown at
Fig. 30 (similar washers must be used for the two binding screws if the
_whole_ base-plate be made in metal). This being done, the metal frame,
Fig. 18, is put in position on the wooden base, as shown at Fig. 17, and
screwed down thereto by the screws indicated at _s s s_. The magnet may
then be screwed down to the metal frame as shown. The small bracket of
angle brass marked B, in Figs. 23 and 24, is next screwed into its
place; that is, in such a position that the armature stands squarely
facing the poles of the electro-magnet, but not quite touching them (say
1/16 of an inch for a 2-1/2 in. bell). In setting up this and the
contact pillar, the greatest care must be taken that the platinum tip of
the contact screw, Fig. 25, should touch lightly the centre of the
platinum speck at the back of the spring, Figs. 23 and 24, shown full
size at Fig. 26.

The free ends of the helically coiled electro-magnet wires should now be
inserted into short lengths of small indiarubber tubing (same as used
for feeding bottles), the extremities being drawn through and 1 in. of
the copper wire bared of its covering for the purpose of making good
metallic contact with the connections. One of these ends is to be
soldered, or otherwise metallically connected, to the angle brass
carrying the armature, spring and clapper, the other being similarly
connected with the left-hand binding-screw, shown at Fig. 17. Another
short length of wire (also enclosed in rubber tubing) must be arranged
to connect the contact screw pillar Fig. 17, with the right-hand
binding-screw. When this has been done, we may proceed to test the
working of the bell by connecting up the binding screws with the wires
proceeding from a freshly-charged Leclanché cell. If all have been
properly done, and the connections duly made, the armature should begin
to vibrate at once, causing the "bob," or hammer, to strike the bell
rapidly; that is, provided the platinum tipped screw touches the
platinum speck on the contact spring. Should this not be the case, the
screw must be turned until the platinum tip touches the platinum speck.
The armature will now begin to vibrate. It may be that the clapper runs
too near the bell, so that it gives a harsh, thuddy buzz instead of a
clear, ringing sound; or, possibly, the clapper is "set" too far from
the bell to strike it. In either case a little bending of the brass wire
carrying the clapper (either from or towards the bell, as the case may
dictate) will remedy the defect. It is also possible that the armature
itself may have been set too near, or too far from the electro-magnet.
In the latter case, the clapper will not vibrate strongly enough, in the
former the vibration will be too short, and the clapper may even stick
to the poles of the electros, especially if these have not been
carefully annealed. A little bending of the spring, to or from the
magnets, will remedy these deficiencies, unless the distance be very
much too great, in which case the bending of the spring would take the
platinum tip out of the centre of the platinum speck.

[Illustration: Fig. 31.]


§ 43. Having thus constructed an efficient electric bell we may proceed
to study its action and notice some of the defects to which it may be
subject. In the first place, if we connect up the bell with the battery
as shown in Fig. 17, viz., the left-hand binding-screw with the wire
proceeding from the carbon of the Leclanché, and the right-hand screw
with the wire from the zinc, then, if the platinum tipped screw touches
the platinum speck, at the back of the contact spring, a current of
electricity flows from the left-hand binding-screw all round the coils
of the electro-magnets, passes along the contact spring and platinum
speck, thence to the platinum tipped screw along the short length of
wire to the right-hand binding-screw, whence it returns to the zinc
element of the battery, thus completing the circuit. The current, in
thus passing around the electro-magnet cores, converts them, _pro tem._,
into a powerful magnet (see § 13); consequently, the armature, with its
contact spring and hammer, is pulled towards the electro-magnets and at
the same time gives a blow to the bell. Now, if instead of having the
platinum speck attached to a flexible spring, it had been attached
bodily to the rigid iron armature, directly the electro-magnets felt the
influence of the current, the platinum speck would have also been pulled
out of contact with the platinum screw, therefore the electro-magnet
cores would have _immediately_ lost their magnetism (see § 13, last five
lines). This would have been disadvantageous, for two reasons: 1st,
because the _stroke_ of the hammer would have been very short, and
consequently the ring of the bell very weak; and, 2nd, because, as even
the softest iron requires some appreciable time for the electric current
to flow round it to magnetise it to its full capacity, it would need a
much greater battery power to produce a given stroke, if the contact
were so very short. The use of an elastic contact spring is, therefore,
just to lengthen the time of contact. But the electro-magnets, even when
the flexible spring is used, do actually pull the platinum speck out of
contact with the platinum screw. When this takes place, the circuit is
broken, and no more current can flow round the electro-magnets, the
spring reasserts its power, and the contact is again made between the
contact screw and contact spring, to be again rapidly broken, each break
and make contact being accompanied by a correspondingly rapid vibration
of the armature, with its attendant clapper, which thus sets up that
characteristic rapid ringing which has earned for these bells the name
of trembling, chattering, or vibrating bells.


§ 44. From a careful consideration of the last two sections it will be
evident that the possible defects of electric bells may be classed under
four heads: viz., 1st, Bad contacts; 2nd, Bad adjustment of the parts;
3rd, Defective insulation; 4th, Warpage or shrinkage of base. We will
consider these in the above order. Firstly, then, as to bad contacts.
Many operators are content with simply turning the terminal wires round
the base of the binding-screws. Unless the binding-screws are firmly
held down on to the wires by means of a back nut, a great loss is sure
to occur at these points, as the wires may have been put on with sweaty
hands, when a film of oxide soon forms, which greatly lowers the
conductivity of the junction. Again, at the junction points of the wires
with the contact angle brass and contact pillar, some workmen solder the
junctions, using "killed spirits" as a flux. A soldered contact is
certainly the best, electrically speaking, but "killed spirits," or
chloride of zinc, should never be used as a flux in any apparatus or at
any point that cannot be washed in abundance of water, as chloride of
zinc is very _deliquescent_ (runs to water), rottens the wire, and
spoils the insulation of the adjacent parts. If solder be used at any
parts, let _resin_ be used as a flux. Even if any excess of resin remain
on the work, it does no harm and does not destroy the insulation of any
of the other portions. Another point where bad contact may arise is at
the platinum contacts. Platinum is a metal which does not rust easily,
even under the influence of the electric spark given at the point of
contact. Therefore, it is preferred to every other metal (except,
perhaps, iridium) for contact breakers. Platinum is an expensive metal,
the retail price being about 30s. an ounce, and as it is nearly twice as
heavy as lead (Lead 11. Platinum 21·5) very little goes to an ounce. For
cheap bells, therefore, there is a great temptation to use some other
white metal, such as silver, german silver, platinoid, etc.

The tip of the platinum screw may be tested for its being veritably
platinum in the following mode: Touch the tip with the stopper of a
bottle containing aquafortis, so as to leave a tiny drop on the extreme
point of the suspected platinum. If it boils up green, or turns black,
it is _not_ platinum; if it remains unaltered, it may be silver or
platinum. After it has stood on the tip for a minute, draw it along a
piece of white paper, so as to produce a streak of the acid. Expose the
paper for a few minutes to sunlight. If the streak turns violet or pinky
violet, the metal is _silver_; if the paper simply shows a slightly
yellowish streak, the metal is platinum. The tip of the platinum screw
must be carefully dried and cleaned after this trial before being
replaced.

Secondly, as to bad adjustment. It is evident that the magnets and the
armature must stand at a certain distance apart to give the best effects
with a given battery power. The distance varies from 1/24 in. in the
very smallest, to 1/8 in. in large bells. Sometimes (but only in very
badly made instruments) the armature adheres to the poles of the
electro-magnet. This is due to _residual_ _magnetism_ (see § 14), and
points to hard or unannealed iron in the cores or armature. As a
make-shift, this defect may be partially remedied by pasting a thin
piece of paper over that surface of the armature which faces the poles
of the electro-magnets. Another bad adjustment is when the platinum
screw does not touch fairly on the centre of the platinum speck, but
touches the spring or the solder. Rust is then sure to form, which
destroys the goodness of the contact. To adjust the contact spring at
the right distance from the platinum screw, hold the hammer against the
bell or gong. The armature should now _just not touch_ the poles of the
electro-magnet. Now screw up the platinum screw until it _clears_ the
contact spring by about the thickness of a sheet of brown paper (say
1/50 of an inch). Let the hammer go, and notice whether the contact
spring makes good contact with the platinum screw. This may be tried by
the Leclanché cell as well, so as to make sure of the character of the
_ringing_. When this has been satisfactorily adjusted the back-nut or
set screw may be tightened, to insure that the vibration of the hammer
shall not alter the adjustment. It sometimes happens that the spring
that bears the armature is itself either too strong (or set back too
far) or too weak. In the former case, the electro-magnet cannot pull the
armature with sufficient force to give a good blow; in the latter, the
spring cannot return the armature, with its attendant contact spring,
back to its place against the platinum screw. To ascertain which of
these two defects obtains, it is only necessary, while the bell is in
action, to press the spring lightly with a bit of wire, first _towards_
and then _away_ from the electro-magnets. If the ringing is improved in
the first case, the spring is too strong; if improvement takes place in
the latter case, the spring is too weak. The third source of inefficient
action, defective insulation, is not likely to occur in a newly-made
bell, except by gross carelessness. Still, it may be well to point out
where electrical leakage is likely to occur, and how its presence may be
ascertained, localized, and remedied. If the wire used to wind the
electro-magnet be old, badly covered, or bared in several places in
winding, it probably will allow the current to "short circuit," instead
of traversing the whole length of the coils. If this be the case, the
magnet will be very weak: the magnet of a 2-1/2-in. bell should be able
to sustain easily a 1 lb. weight attached by a piece of string to a
smooth piece of 1/2-in. square iron placed across its poles, when
energized by a single pint Leclanché cell. If it will not do this, the
insulation may be suspected. If the wire has been wound on the bare
cores (without bobbins), as is sometimes done, bared places in the wire
may be touching the iron. This may be ascertained by connecting one pole
of a bottle bichromate, or other powerful battery, with one of the wires
of the electro-magnet coils, and drawing the other pole of the battery
across the clean iron faces of the electro-magnet poles. If there is any
leakage, sparks will appear on making and breaking contact. Nothing but
unwinding and rewinding with a well covered wire can remedy these
defects. The other points where the insulation may be defective are
between the binding screws and the base, if this be all of metal; or
between the contact spring block and the base, and the contact pillar.
It is also probable (if the connecting wires have not been covered with
indiarubber tubing, as recommended) that leakage may be taking place
between these wires and some portion of the metal work of the base or
frame. This must be carefully examined, and if any point of contact be
observed, a little piece of Prout's elastic glue, previously heated,
must be inserted at the suspected places. With regard to the binding
screws, if they stand on the wooden base, their insulation (unless the
base be very damp indeed) will be sufficiently good; but if the base is
entirely metallic, then ebonite or boxwood washers must be used to
insulate them from contact with the base-plate. With regard to the
contact spring block and the platinum screw pillar, it is _permissible_
that one or the other should not be insulated from the base or frame;
but one or the other _must_ be insulated by means of ebonite or other
insulating washers. Personally, I prefer to insulate both; but in many
really good bells only the platinum screw pillar is thus insulated. Any
such leakage can be immediately detected by holding one pole of a
powerful battery against the suspected binding-screw, or block, or
pillar, and while in this position, drawing the other pole across some
bare iron portion of the frame or metal base. Sparks will appear if
there is any leakage.

The fourth defect--that is, warpage or shrinkage of the base--can only
occur in badly-made bells, in which the entire base is of wood. A
cursory examination will show whether the board is warped or swollen, or
whether it has shrunk. Warping or swelling will throw the electro-magnet
too far from the armature, or "set" the pillar out of place; shrinkage,
on the contrary, will bring the parts too close together and jamb the
magnets, the armature, and the contact pillar into an unworkable
position.


§ 45. Before quitting the subject of the defects of bells, it may not be
out of place to mention that no bell that is set to do real work should
be fitted up without a cover or case. The dust which is sure to
accumulate, not to speak of damp and fumes, etc., will certainly
militate against good contacts and good action if this important point
be neglected. The cover or case generally takes the form of a shallow
box, as shown at Fig. 32, and may be made from 1/4-in. teak, mahogany,
or walnut, dovetailed together and well polished. It is fastened to the
base in the same manner as the sides of a Dutch clock, by means of
studs, hooks and eyes. At the bottom of the box is cut a slot, of
sufficient width and length to admit the play of the hammer shank.

[Illustration: Fig. 32.]

In the annexed table is given a general idea of the proportion which
should be observed in the construction of bells of different sizes. It
must be noted that if the bells are to be used at long distances from
the battery, rather more of a finer gauge of wire must be employed to
wind the magnets than that herein recommended, unless, indeed, _relays_
be used in conjunction with the bells.


§ 46.--

TABLE

Showing proportions to be observed in the different parts of electric
bells.

  ---------+---------+----------+--------+---------+----------
  Diameter |Length of|Diameter  |Length  |Diameter | B. W. G.
     of    | Magnet  |of Magnet |  of    |of Bobbin| of Wire
    Bell.  | Cores.  | Cores.   |Bobbin. |  Head.  |on Bobbin.
  ---------+---------+----------+--------+---------+----------
    2-1/2" |  2"     |   5/16"  | 1-3/4" |    3/4" |    24
    3      |  2-1/4  |   3/8    | 2      |    7/8  |    24
    3-1/2  |  2-1/2  |   7/16   | 2-1/4  |  1      |    22
    4      |  2-3/4  |   1/2    | 2-1/2  |  1-1/8  |    22
    4-1/2  |  3      |   9/16   | 2-3/4  |  1-1/4  |    20
    5      |  3-1/4  |   5/8    | 3      |  1-3/8  |    18
    5-1/2  |  3-1/2  |  11/16   | 3-1/4  |  1-1/2  |    16
    6      |  3-3/4  |   3/4    | 3-1/2  |  1-5/8  |    16
    6-1/2  |  4      |  13/16   | 3-3/4  |  1-3/4  |    16
    7      |  4-1/4  |   7/8    | 4      |  1-7/8  |    16
    7-1/2  |  4-1/2  |  15/16   | 4-1/4  |  2      |    14
    8      |  4-3/4  | 1        | 4-1/2  |  2-1/8  |    14
    8-1/2  |  5      | 1-1/16   | 4-3/4  |  2-1/4  |    14
    9      |  5-1/4  | 1-1/8    | 5      |  2-3/8  |    14
    9-1/2  |  5-1/2  | 1-3/16   | 5-1/4  |  2-1/2  |    14
   10      |  5-3/4  | 1-1/4    | 5-1/2  |  2-5/8  |    14
   10-1/2  |  6      | 1-5/16   | 5-3/4  |  2-3/4  |    12
   11      |  6-1/4  | 1-3/8    | 6      |  2-7/8  |    12
   11-1/2  |  6-1/2  | 1-7/16   | 6-1/4  |  3      |    10
   12      |  6-3/4  | 1-1/2    | 6-1/2  |  3-1/8  |    10
  ---------+---------+----------+--------+---------+----------

[Illustration: Fig. 33 A.]

[Illustration: Fig. 33 B.]

[Illustration: Fig. 34.]


§ 47. We can now glance at several modifications in the shape and mode
of action of electric bells and their congeners. Taking Figs. 33 A and B
as our typical forms of trembling bell, the first notable modification
is one by means of which the bell is made to give a single stroke only,
for each contact with the battery. This form, which is known as the
"single stroke bell," lends itself to those cases in which it may be
required to transmit preconcerted signals; as also where it is desired
to place many bells in one circuit. Fig. 34 illustrates the construction
of the single stroke bell. It differs from the trembling bell in the
mode in which the electro-magnet is connected up to the binding screws.
In the trembling bell, Fig. 33, the circuit is completed through the
platinum screw pillar, to the binding screw marked Z, hence the circuit
is rapidly made and broken as long as by any means contact is made with
the battery, and the binding screws L and Z. But in the single stroke
bell, Fig. 34, the wires from the electro-magnet are connected directly
to the two binding screws L and Z, so that when contact is made with the
battery, the armature is drawn to the poles of the electro-magnet, and
kept there so long as the battery current passes. By this means, only
one stroke or blow is given to the bell for each contact of the battery.
Of course, directly the connection with the battery is broken, the
spring which carries the armature and clapper flies back ready to be
again attracted, should connection again be made with the battery. To
regulate the distance of the armature from the poles of the
electro-magnets, a set screw Q takes the place of the platinum screw in
the ordinary form, while to prevent the hammer remaining in contact with
the bell (which would produce a dull thud and stop the clear ring of the
bell), a stop (_g_) is set near the end of the armature, or two studs
are fixed on the tips of the poles of the electro-magnets. The mode of
adjusting this kind of bell, so as to obtain the best effect, differs a
little from that employed in the case of the trembling bell. The
armature must be pressed towards the poles of the electro-magnets, until
it rests against the stop or studs. A piece of wood or cork may be
placed between the armature and the set screw Q, to retain the armature
in this position, while the rod carrying the hammer or clapper is being
bent (if required) until the hammer just clears the bell. If it touches
the bell, a thud instead of a ring is the result; if it is set off too
far, the ring will be too weak. The armature can now be released, by
removing the wood or cork, and the set screw Q driven forwards or
backwards until the best effect is produced when tested with the
battery. The tension of the armature spring must be carefully looked to
in these single stroke bells. If it is too strong, the blow will be
weak; if too weak, the hammer trembles, so that a clear single stroke is
not obtainable, as the spring _chatters_.


§ 48. _The continuous ringing bell_ is the modification which next
demands our attention. In this, the ringing action, when once started by
the push,[12] or other contact maker, having been touched, continues
either until the battery is exhausted, or until it is stopped by the
person in charge. The great use of this arrangement is self-evident in
cases of burglar alarms, watchman's alarms, etc., as the continuous
ringing gives notice that the "call" has not received attention. The
continuous ringing bell differs but little from the ordinary trembling
bell. The chief difference lies in the addition of an automatic device
whereby contact is kept up with the battery, even after the "push"
contact has ceased. As it is desirable for the person in charge to be
able to stop the ringing at will, without proceeding to the place where
the "push" stands, so it is not usual to make the continuous ringing
arrangement dependent on the "push," though, of course, this could be
done, by causing it to engage in a catch, which would keep up the
contact, when once made. Continuous ringing bells may be conveniently
divided into two classes; viz., 1st, those in which a device is attached
to the framework of the bell; which device, when once upset by the first
stroke of the bell, places the bell in direct communication with the
battery independent of the "push" or usual contact; and 2ndly, those in
which a separate device is used, for the same purpose. This latter
arrangement admits of the use of an ordinary trembling bell.

[Footnote 12: A "push," of which several forms will hereafter be
described and figured, consists essentially in a spring carrying a stud,
standing directly over, but not touching, another stud, fixed to a base.
The lower stud is connected to one terminal of battery, the spring is
connected to the bell. When the spring is pressed down, the two studs
come into contact, the current flows, and the bell rings.]

[Illustration: Fig. 35.]

[Illustration: Fig. 36.]

Fig. 35 illustrates the action of bells of the first class. In the first
place it will be noticed that there are three binding screws instead of
two, as in the ordinary pattern, one marked C connected as usual with
the carbon element of the battery; another marked L, which connects with
line wire, and a third, Z, connected by means of a branch wire (shunt
wire), proceeding from the zinc of the battery. It will be seen, that
if the battery current is by means of the push caused to flow through
the coils of the electro-magnets, the armature is attracted as usual by
them, and in moving towards them, releases and lets fall the lever
contact, which, resting on the contact screw, completes the circuit
between Z and C, so that the bell is in direct communication with its
battery, independently of the push. Hence the bell continues ringing,
until the lever is replaced. This can be done, either by pulling a
check string (like a bell-pull) attached to an eye in the lever, or by
means of a press-button and counter-spring; as shown in Fig. 36, A and
B.

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

[Illustration: Fig. 39.]

In continuous ringing bells of the second class, a detent similar to
that shown at Fig. 35 D is used, but this, instead of being actuated by
the electro-magnet belonging to the bell itself, is controlled by a
separate and entirely independent electro-magnet, which, as it may be
wound with many coils of fine wire, and have a specially light spring
for the armature, can be made very sensitive. This second
electro-magnet, which serves only to make contact with a battery, is
known as a _Relay_, and is extensively employed in many cases where it
is desired to put one or more batteries into, or out of circuit, from a
distance. The relay may be looked upon as an automatic hand, which can
be made to repeat at a distant point contacts made or broken by hand at
a nearer one. Fig. 37 shows this arrangement, attached to the same base
board as the bell itself. On contact being made with the push, the
current enters at C, circulates round the cores of the relay, thus
converting it into a magnet. The armature _a_ is thereby pulled to the
magnet, and in so doing releases the detent lever, which falls on the
contact screw, thus at one and the same time breaking the circuit
through the relay, and making the circuit through the bell magnets B B´,
back to the battery by Z. A second modification of this mode of causing
an ordinary bell to ring continuously is shown at Fig. 38, the peculiar
form of relay used therewith being illustrated at Fig. 39. Here, the
relay is placed on a separate base board of its own, and could, if
necessary, be thrown out of circuit altogether, by means of a
_switch_,[13] so that the bell can be used as an ordinary bell or
continuous action at will. It will be noticed that the relay has in this
sketch only one core. But the delicacy of the action is not impaired
thereby, as the armature, by means of the steel spring _s_, is made to
form part and parcel of the magnet, so that it becomes magnetised as
well as the core, and is attracted with more force than it would be, if
it were magnetically insulated. The battery current enters by the wires
C and W, passes round the coils of the electro-magnet, and returns by Z.
In so doing it energises the electro-magnet E, which immediately
attracts its armature A. The forward movement of the armature A,
releases the pivoted arm L, to which is attached a platinum-tipped
contact prong P. This, it will be noticed, is in metallic connection
with the pillar P', and with the base, and, therefore, through the wire
W, with the battery. When the arm L falls, the contact prong completes
the circuit to the bell, through the insulated pillar X. The relay is
thus thrown out of the circuit at the same time that the bell is thrown
in. A device similar to those illustrated at Fig. 36 can be employed to
reset the arm L.

[Footnote 13: Described at § 61.]

[Illustration: Fig. 40.]

A rather more complicated arrangement for continuous bell ringing is
shown at Fig. 40. It is known as Callow's, and is peculiarly adapted to
ringing several bells from one attachment, etc. Owing to the relay in
this form being wound with two sets of wires, it takes a little more
battery power; but this disadvantage is compensated by its many good
points. The following description, taken from F. C. Allsop's papers in
the _English Mechanic_, will render the working of Callow's attachment
perfectly clear. "When the button of the push P is pressed, the current
in the main circuit flows from the positive pole C of the battery D
through the relay coil _a_, and thence by the wire _d_ and push P, to
the zinc of the battery. This attracts the armature A of the relay R,
closing the local bell circuit, the current flowing from C of the
battery to armature A of the relay R, through contact post _p_, terminal
L of the bell, through bell to terminal Z, and thence by the wire _g_ to
the zinc of the battery. Part of the current also flows along the wire
from the bell terminal L through the relay coil _b_ and switch W, to
terminal Z of the bell, thus keeping the armature of the relay down,
after the main circuit (through the push) has been broken; the bell
continuing to ring until the shunt circuit is broken by moving the arm
of the switch W over to the opposite (or non-contact) side. The bell can
also be stopped by short circuiting the relay, which can be effected by
an ordinary push. It will be seen that more than one bell can be rung
from the same attachment, and the bell can, by moving the arm of the
switch W, be made continuous ringing or not, at will. If the arm of the
switch is moved over to the opposite side to which it is shown in the
figure, the shunt circuit of the bell through the relay is broken, and
the bell will ring only so long as the button of the push is kept in.
This continuous arrangement is very convenient for front doors, etc.,
where trouble is experienced in securing immediate attention to the
summons. Instead of being taken to the switch, as in Fig. 40, the two
wires are taken to a contact piece fixed on the side of the door frame,
and so arranged that when the door is opened, it either short circuits
or breaks the shunt circuit: thus when the push is pressed, the bell
rings until the door is opened, the continual ringing of the bell
insuring prompt attention."

Mr. H. Thorpe, of 59, Theobald's Road, London, has devised a very
ingenious arrangement for the continuous ringing of one or more bells
for a stated period of time. This is shown at Fig. 40 A. It is set in
action by pulling the ring outside the bottom of the core. The bell or
bells then start ringing, as contact is established and kept up. The
novelty lies in the fact that the duration of the contact, and
consequently of the ringing, can be accurately timed from 5 seconds to
30 seconds, by merely inserting a pin at different holes in the rod, as
shown. After the bells have rung the required time the instrument
automatically resets itself.

[Illustration: Fig. 40 A.]


§ 49. The modifications we are now about to consider, differ from the
ordinary bell, either in the shape or material of the bell itself, the
relative disposition of the parts, or some structural detail; but not
upon the introduction of any new principle. The most striking is
certainly the Jensen bell, which is shown in section at Fig. 41.

[Illustration: Fig. 41.]

According to Mr. Jensen's system of electric bells, the bell may take
any desired form, that of the ordinary church bell being preferred, and
the electro-magnetic apparatus is placed entirely inside the bell
itself. To attain this end the electro-magnetic apparatus must be
compact in form. A single electro-magnet has pole pieces at each end
opposite to which an armature is suspended from a pivot and balanced by
the hammer of the bell. At the back of the armature there may be a make
and break arrangement, whereby a continuous succession of strokes is
effected, or this may be omitted, in which case a single stroke is
given when the contact with the battery is made, or both may be effected
by separate wires, make contact with one wire, and a single stroke is
struck; make it with the other and the current passes through the make
and break and a succession of strokes is heard. When the contact-breaker
is used, it is so arranged that a slight rub is caused at every stroke,
so keeping the contact clean. The flexible break, with the ingenious
wiping contact, is a great improvement over the ordinary screw, which
often becomes disarranged.

The form of the magnet is such that a considerable degree of magnetic
force is caused by a comparatively small battery power. The
electro-magnetic apparatus being within the bell the latter forms a very
effective and handsome shield for the former. Not only can the bell
shield the electro-magnet from wet but the whole of the conducting wires
as well.

The bell may be screwed to a tube through which passes the conducting
wire, which makes contact with an insulated metallic piece in the centre
of the top of the bell. Both the wire and the contact piece are as
completely shielded from the weather as if within the bell itself.

[Illustration: Fig. 42.]

The great point of departure is the discarding of the unsightly magnet
box, and the hemispherical bell (_see_ Fig. 32), and substituting a bell
of the Church type (see Fig. 42), and placing inside it an
electro-magnet specially arranged. The inventors use a single solenoidal
magnet of a peculiar construction, by which the armature is attracted by
both poles simultaneously. By this means less than half the usual
quantity of wire is required, thus reducing the external resistance of
the circuit one half. Moreover the armature, besides being magnetised by
induction, as acted on in the ordinary method of making electric bells,
is by Messrs. Jensen's plan directly polarised by being in actual
magnetic contact by the connection of the gimbal (which is one piece
with the armature) with the core iron of their magnet. It is thus
induced to perform the largest amount of work with the smallest
electro-motive force. Instead of the armature and clapper being in a
straight line attached to a rigid spring, which necessitates a
considerable attractive power to primarily give it momentum, in the
Jensen Bell the armature and hammer are in the form of an inverted [U],
and being perfectly balanced from the point of suspension, the lines of
force from a comparatively small magnetic field suffice to set this
improved form of armature into instant regular vibration. By using a
flexible break and make arrangement instead of the usual armature spring
and set screw (at best of most uncertain action), it is found that a
much better result is attained, and by this device the armature can be
set much nearer the poles of the magnet with sufficient traverse of the
hammer. This is in strict accordance with the law of inverse squares,
which holds that the force exerted between two magnetic poles is
inversely proportionate to the square of the distance between them, or,
in other words, that magnets increase proportionately in their power of
attraction as they decrease in the square of the distance. It will now
be seen why these bells require so little battery power to ring them:
firstly, the armature and hammer are so perfectly balanced as to offer
but little resistance; secondly, the external resistance to the current
is reduced; and thirdly, the best possible use is made of the
electro-magnetic force at disposal.


§ 50. The next modification which demands attention is the so-called
"Circular bell." This differs from the ordinary form only in having the
action entirely covered by the dome. Except, perhaps, in point of
appearance, this presents no advantages to that. The bells known as
"Mining bells" resemble somewhat in outward appearance the circular
bell; but in these mining bells the action is all enclosed in strong,
square teak cases, to protect the movement, as far as possible, from the
effects of the damp. All the parts are, for the same reason, made very
large and strong; the armature is pivoted instead of being supported on
a spring, the hammer shank being long, and furnished with a heavy bob.
The domes or bells are from 6 inches to 12 inches in diameter, and are
generally fitted with _single stroke_ movement, so as to enable them to
be used for signalling. The hammer shank, with its bob, and the dome,
which stands in the centre of the case, are the only parts left
uncovered, as may be seen on reference to Figs. 43 A and B, where the
exterior and interior of such a bell are shown.

[Illustration: Fig. 43 A.]

[Illustration: Fig. 43 B.]

[Illustration: Fig 44.]


§ 51. In the "Electric Trumpet," introduced by Messrs. Binswanger, of
the General Electric Company, we have a very novel and effective
arrangement of the parts of an electric bell and telephone together.
This instrument, along with its battery, line and push, is illustrated
at Fig. 44, where A is a hollow brass cylinder, in which is placed an
ordinary electro-magnet similar to Figs. 20 or 20 A. At the front end,
near B, is affixed by its edges a thin disc of sheet iron, precisely as
in the Bell telephone,[14] and over against it, at B, is an insulated
contact screw, as in the ordinary trembling bell. On the disc of sheet
iron, at the spot where the screw touches, is soldered a speck of
platinum. The wires from the electro-magnet are connected, one to the
upper binding screw, the other to the brass case of the instrument
itself, which is in metallic communication with the sheet iron disc.
The return wire from the contact screw is shown attached to the
insulated piece, and is fastened to another binding screw (not visible)
on the base board. When contact is made with the battery, through the
press or push, the magnet becomes energised, and pulls the iron disc or
diaphragm towards it, causing it to buckle inwards. In doing this,
contact is broken with the screw B; consequently the diaphragm again
straightens out, as the magnet no longer pulls it. Again contact is
made; when of course the same round of performances is continuously
repeated. As the plate or diaphragm vibrates many hundreds of times per
second, it sets up a distinctly musical and loud sound wave, not unlike
the note of a cornet-a-piston, or a loud harmonium reed. With a number
of these "trumpets," each diaphragm being duly tuned to its proper
pitch, it would be possible to construct a novel musical instrument,
working solely by electricity. The "pushes" need only take the form of
pianoforte keys to render the instrument within the grasp of any
pianoforte or organ player.

[Footnote 14: See "Electrical Instrument Making for Amateurs." Whittaker
& Co. Second edition.]


§ 52. Sometimes the gong or "dome" of the ordinary bell is replaced by a
coil spring, as in the American clocks; sometimes quaint forms are given
to the parts covering the "movement," so as to imitate the head of an
owl, etc. But bells with these changes in outward form will not present
any difficulty, either in fixing or in management, to those who have
mastered the structural and working details given in this chapter.



CHAPTER IV.

ON CONTACTS, PUSHES, SWITCHES, KEYS, ALARMS, AND RELAYS.


§ 53. All the appliances which have hitherto been described, would be
utterly useless for the purposes intended, had we not at hand some means
of easily, certainly and rapidly completing and breaking the circuit
between the bell or bells, on the one hand, and the battery on the
other. This necessary piece of apparatus, which is simply a contact
maker, receives different names, dependent on its application. When it
is intended to be actuated directly by hand, it is known as a "push," a
"pressel," or "pull," according to the mode in which the contact is
made. At Fig. 45, A, B, C, D, and E, show the outward forms of various
"pushes," in wood and china, as sent out by the leading makers. (The
ones figured are from Messrs. Binswanger & Co.) At F is a sectional view
of one of these pushes, and G shows the interior when the cover has been
removed. From these two latter illustrations it will be easily
understood that the "push" consists essentially in two pieces of metal
one or both of which are springs, and one of which is connected with one
of the wires from the battery, while the other is attached to the wire
proceeding to the bell. When the button is pressed the upper spring
comes into contact with the lower metal spring or plate. The circuit is
now complete; hence the bell rings. But as soon as the finger is removed
from the stud or button of the "push," the spring returns to its old
place, contact being thereby broken when the bell ceases to ring, unless
it be fitted with a continuous ringing arrangement (see § 48). In
fastening the leading wires to these pushes, care must be taken that
the ends of the wires be scraped, and sand papered quite clean and
bright, bent into a loop which must be inserted under the head of the
screw that holds the wire to the spring pieces; the screws being then
tightened up carefully to ensure a good grip and contact with the wires.

[Illustration: Fig. 45.]

[Illustration: Fig. 46.]


§ 54. A "pressel" (Fig. 46) is simply a push which instead of being made
a fixture by being fastened in the wall or door, is attached to a
metallic wired line, so that it is generally made to resemble somewhat
in outward appearance the knob or tassel of the bell-pull of the last
generation, the interior arrangement is precisely similar to that of the
push; that is to say, the pressel consists in a pear-shaped or
acorn-shaped hollow wooden box, with a projecting knob or button below.
This button is attached to a spring, the tension of which keeps the knob
protruding from the end of the box, and at the same time prevents
contacts with the second spring at the bottom of the box. Two insulated
wires, one from the battery, the other from the bell, are connected to
separate screws at the top of the pressel. One of these screws connects
with the lower spring, the other with the upper.

[Illustration: Fig. 47.]


§ 55. The "pull" (Fig. 47), as its name implies, makes contact and rings
the bell on being pulled. The knob has a rather long shank bar, around
which is coiled a pretty stiff spring. At the farther extremity is an
ebonite or boxwood collar ending in a rather wider metal ring. The wires
from the bell and battery are connected respectively to two flat
springs, _a a'_, by the screws _b b'_. When the knob is pulled, the
metal collar touches both springs, and the circuit is completed. Closely
allied to the "pull" is a form of bedroom contact, which combines
pear-push or pressel and pull in one device. This will be readily
understood on reference to Fig. 48. Another form of bedroom pull, with
ordinary rope and tassel, consists in a box containing a jointed metal
lever, standing over a stud, from which it is kept out of contact by a
counter spring. To the projecting end of the lever is attached the bell
rope. When this is pulled the lever touches the stud, contact is made,
and the bell rings. This is clearly shown in Fig. 49 A. In all these
contacts, except the door pull (Fig. 47) where the friction of the
action of pulling keeps the surfaces bright, the points of contact
should be tipped with platinum. Another form of contact to be let in the
floor of the dining-room, within easy reach of the foot of the carver,
or other persons at the head of the table, is shown at Fig. 49 B.

[Illustration: Fig. 48.]

[Illustration: Fig. 49 A.]

[Illustration: Fig. 49 B.]

Mr. Mackenzie has introduced a very ingenious contrivance whereby the
ringer may know whether the bell at the distant end has rung. This is
effected by inclosing in the push a device similar to that shown at Fig.
43 A. That is to say, an electro-magnet wound with wire, and surmounted
by a thin iron disc, is placed in circuit with the line wires. The
ringing of the bell rapidly magnetises and demagnetises the
electro-magnet, and causes a humming sound, which clearly indicates
whether the bell is ringing or not. As this device can be made very
small, compact, and not liable to derangement, it is of easy
application.


§ 56. The next form of contact to which our attention must be directed,
is that known as the _burglar alarm_, with its variant of door-contacts,
sash-contacts, till-contacts, etc.

The "burglar's pest" (as the contrivance we illustrate is called) is one
of the most useful applications of electricity for the protection of
property against thieves. It consists usually, first, of a brass plate
(Fig. 50), upon which a platinum contact piece is fixed, and second, of
a spring made of hardened brass or steel insulated from the plate; or of
a cylindrical box with a spiral spring inside (see Fig. 51). It is so
arranged that as long as the stud is kept pressed in, the platinum
points of contact are kept apart; this is the position when fixed in the
rebate of a closed door or window; but as soon as opened, the stud
passes outward through the hole, and the points of contact come together
and complete the circuit of the wires in connection with the bell. The
bell is best to be a continuous ringing one. It may be fixed in the
master's bedroom, or outside the premises in the street.

[Illustration: Fig. 50.]

[Illustration: Fig. 51.]

Legge's Window Blind contact is an arrangement by which the blind is
secured at the bottom by attaching it to a hook or button. A slight
pressure against the blind (caused by anyone trying to enter after
having broken a window) sets the electric bell in motion unknown to the
intruder.

[Illustration: Fig. 52.]

[Illustration: Fig. 53.]

A form of floor contact, which may be placed under a light mat or
carpet, illustrated at Fig. 52, serves to give notice if anyone be
waiting at the door, or stepping into places which are desired to be
kept private. All these arrangements, to be serviceable, should be
connected with continuous ringing bells (see § 48). Wherever it is
likely that these arrangements may stand a long time without being
called into play, it is better to employ some form of contact in which a
_rubbing_ action (which tends to clean the surfaces and then make a good
contact) is brought into play, rather than a merely _dotting_ action.
For this reason, spring contacts in which the springs connected with the
wires are kept apart by an insulating wedge (shown at Fig. 53) as long
as the door or window are kept closed, are preferred. In the case of
windows, strips of brass let into the frame on each side of the sash,
are thrown into contact by the springs _a_ and _a'_ in the sash itself,
as shown at Fig. 54. For shop doors and others, where a short contact
only is required, and this only when the door is opened, a contact such
as shown at Fig. 55 is well adapted. It consists, as will be seen, in a
peculiarly shaped pivoted trigger _a_, which is lifted forwards when the
door is opened, so that it makes contact with the spring _b_. Owing to
the curved shape of the arm of the trigger, the contact is not repeated
when the door is closed.

[Illustration: Fig. 54.]

[Illustration: Fig. 55.]


§ 57. In all forms of burglar or thief alarms, the ordinary system of
having the circuit broken, until contact is made by the intruder
involuntarily making contact at some point, presents one great
disadvantage; and that is, that if "_notre ami l'ennemi_," viz., the
thief or burglar, be anything of an electrician (and alas! to what base
uses may not even science be perverted) he will begin by cutting all
suspicious-looking wires before he attempts to set about any serious
work. This disadvantage may be entirely overcome by the adoption of a
simple modification, known as the "closed circuit system" of bell
ringing. For this the bells, etc., are continuously in contact with the
batteries, but owing to the peculiar connections, do not ring unless the
circuit is broken. To render the working of such a system clear to my
readers, I quote the description given in the _English Mechanic_, by one
of our leading electricians:--

Writing on the subject of Closed Circuit Bell-ringing, Mr. Perren
Maycock says:--"This is principally adopted for alarm purposes. Its
superiority over the open circuit system lies in the fact that notice is
given on opening (breaking) the circuit, which is the reverse to the
usual practice. In the ordinary method it becomes necessary to have a
contact maker, differing in form for various purposes and situations,
which, along with the leading wires, must be artfully concealed. All
this entails great expense; besides which one can never be sure that the
contacts and wires are in proper order without actually trying each one.
On the other hand, with the "closed circuit" system, one has merely to
place the wire in any convenient position, it being better _seen_ than
_hidden_. The very fact that alarm is given on breaking the contact
renders the method applicable in circumstances and under conditions
which would render the "open" method difficult and expensive, if not
impossible. One can always be certain that everything is in order. The
modern burglar, electrically educated as regards common practise in
such matters, would naturally make a point of cutting all wires that
fall in his path. From these and other obvious considerations, it is
evident how simple and yet how perfect a means of protection such a
system provides. I will now proceed to explain the manner of
application. The bell used differs from the ordinary, only in the
arrangement of its external connections.

[Illustration: Fig. 56 A.]

Fig. 56 A represents a single-alarm circuit. When contact is broken
externally, there is a closed circuit in which are the battery and bell
magnet coils. Consequently the armature is drawn away from the contact
stud, close up to the electro-magnet, and is held so. When a break
occurs, the armature flies back, completes the local circuit, and rings
so long as the external circuit remains broken. There is a switch for
use when the alarm is not required.

[Illustration: Fig. 56 B.]

[Illustration: Fig. 57.]

Fig. 56 B represents a case in which notice is given at two places. By
insulating a key as shown, reply signalling can be carried on between
the points at which the bells are placed. A special gravity Daniell
modification (§ 25) is used for this class of work (Fig. 57): a narrow
lead cylinder, about 2" in diameter, watertight except at the bottom,
where it opens out into an inverted cone, the surface of which is
pierced with holes. This stands immersed in dilute sulphuric acid. A
saturated solution of copper sulphate is next carefully introduced, so
as to displace the acid upwards. Crystals of sulphate of copper are
introduced into the open end at the top of cylinder, to fill the
perforated portion at the bottom. From the wooden cover of cell a thick
flat ring of amalgamated zinc hangs suspended in the dilute acid. Care
should be taken not to introduce the zinc till the two solutions have
become well separated. During action this becomes coppered, while in
contact with the sulphate of copper, but it is not attacked by the acid.
It is, however, preferable to _paint_ that portion of the lead, which is
surrounded by the acid. The height of the cell is about 14.''

It will be readily understood that if this latter system be employed,
special contacts, which break contact when the pressure is removed, must
be employed for the door or window contacts. A simple form is shown at
Fig. 58.

[Illustration: Fig. 58.]

Contacts similar to Figs. 50, 53 and 54, may be fitted on tills or
drawers.


§ 58. Another useful application of "contact" is for the notification of
any rise or fall of temperature beyond certain fixed limits. The devices
used for this purpose are known as "fire alarms," "frost alarms," and
"thermometer alarms." The thermometer alarm is at once the most
effective and trustworthy of the forms known, as, besides its delicacy,
it has the advantage of being able to give notice of low, as well as of
abnormally high temperature. The form usually given to the electric
alarm thermometer, is well shown at Fig. 59. It consists in an ordinary
thermometer with a wire projecting into the tube to a certain point, say
100 degrees. The mercury in the bulb being also connected with another
wire. When the temperature is within the usual climatic range, the
mercury does not reach the upper wire. If by reason of fire or any other
abnormal heat, the temperature rises beyond that to which the instrument
is set, the mercury rises and touches the upper wire, contact is thus
established, and the bell rings.

[Illustration: Fig. 59.]

By giving the thermometer the shape of a letter [U], it is possible to
notify also a fall below a certain degree, as well as a rise beyond a
certain fixed point. These thermometers are specially used by nurserymen
and others, to warn them of the too great lowering of temperature, or
_vice versâ_, in the houses under their charge.

Other forms of fire alarms are shown at Fig. 60 and 61. If a strip be
built up of two thin layers of dissimilar metals riveted together, as
the two metals do not expand at the same rate, the strip will bend to
the _right_ if heated, and to the _left_ if cooled. In the instrument
shown at Fig. 60, the application of heat causes the flexible strip
carrying the contact screw, to bend over till it touches the lower stop,
when, of course, the bell rings. If two stops are employed instead of
the lower one only, the bell will ring when a low temperature is
reached, which causes the strip to bend in the opposite direction.

[Illustration: Fig. 60.]

[Illustration: Fig. 61.]

At Fig. 61 is illustrated a novel form, in which the expansion of air
causes contact to be made. It consists in an air chamber hermetically
closed by a corrugated metal plate I, similar to that used in the
aneroid barometers. When the temperature rises to a certain point, the
expansion of the air in the chamber brings the metallic plate into
contact with the screw, as shown below. This closes the circuit and
rings the bell in the usual manner. In all these fire or thermometer
alarms, the exact degree of heat at which the bell shall ring, can be
pretty accurately adjusted by means of the contact screws.


§ 59. Closely allied to these forms of contacts are the devices whereby
an ordinary clock or watch can be made to arouse the over-drowsy sleeper
by the ringing of an electric bell, which in this case should be of the
continuous type. All these depend in their action upon some arrangement
whereby when the hour hand of the clock or watch arrives at a certain
given point in its travel, it makes contact between the battery and
bell. In general the contact piece is attached bodily to the clock, but
in the very ingenious arrangement illustrated at Fig. 62 (devised by
Messrs. Binswanger) the contacts are attached to an outer case, and as
the case of the watch itself forms one point of contact, any watch that
will slip in the case, may be set to ring the bell.

[Illustration: Fig. 62.]

[Illustration: Fig. 63.]

Messrs. Gent, of Leicester, have also perfected an electric watchman's
clock, which records the number of places the watchman in charge has
visited or missed on his rounds. This we illustrate at Fig. 63. We quote
Messrs. Gent's own words, in the following description:--

"It consists of an eight-day clock, to which is attached a disc or table
revolving upon a vertical axis and driven by the mechanism of the clock.
The disc is covered with a sheet of paper, attached to it by a binding
screw so that it can be removed when used and a clean sheet substituted
for it. Each sheet of paper is divided longitudinally into hours and, if
necessary, parts of hours, and crosswise into as many divisions as there
are places to be visited by the watchman--any number from one to twenty.
Each division has a corresponding marker, which indicates, by the
impression it makes upon the paper, the time the watchman visits the
place connected with that marker. Wires are carried from the terminals
of the clock, one to the battery, and one to each press-button fixed at
the points intended to be visited by the watchman; another wire is
carried from each press-button to the other end of the battery. The
action is very simple: when the button is pressed in the current passes
through a coil carrying an armature and contact breaker with a point at
the end of a long arm; a hammer-like motion is given to the pointer, and
a distinct perforation made in the card. It is usual to have the
press-button in a box locked up, of which the watchman only has the key.

"The clock may be in the office or bedroom of the manager or head of
the establishment, who can thus, from time to time, satisfy himself of
the watchman's vigilance. The record should be examined in the morning,
and replaced by a clean sheet of card.

"This clock received the special mention of Her Majesty's Commissioners
in Lunacy, and has been adopted by some of the largest asylums in the
country.

"We have recently made an important improvement by adding a relay for
every marker, thus enabling a local battery of greater power to be used
for actuating the markers. This has made no alteration in the appearance
of the clock, as the relays are contained within the cornice at the top
of the clock case."


§ 60. By means of a float, it is possible to give notice of the height
of water in a tank, a reservoir, or even of the state of the tide. In
these cases all that is needed is a float with an arm, having a suitable
contact attached, so that when the water rises to the level of the float
and lifts it, it causes the contact piece to complete the circuit
through a set screw. Or the float may be attached to an arm having a
certain play in both directions, _i.e._, up and down, within which no
contact is made, as the arm has a contact piece on either side, which
can touch either an upper or a lower contact screw, according to whether
the tide is low or high, or whether the lock or tank is nearly empty or
too full.

[Illustration: Fig. 64.]


§ 61. Sometimes it is convenient to be able to ring an ordinary
trembling bell continuously, as when a master wishes to wake a member of
his family or a servant; or again, to cut a given bell or bells out of
circuit altogether. The arrangements by which this can be effected, are
known as "switches." Of switches there are two kinds, namely,
_plugswitches_ or _interruptors_, and _lever switches_. The former
consists essentially in two stout plates of brass affixed to a base
board of any insulating material. These brass plates are set parallel to
each other, a short distance apart, and the centre of the facing edge is
hollowed out to take a brass taper plug. A binding or other screw is
fixed to each brass plate, to connect up to the leading wires. When the
plug is in its socket, the circuit between the two plates (and
consequently between the battery and bell, etc.) is complete; when the
plug is out, the contact is broken. This form of switch is subject to
work out of order, owing to the fact that the taper plug gradually
widens the hole, so that the contact becomes uncertain or defective
altogether. By far the better form of switch is the lever switch, as
shown at Fig. 64. This consists in a movable metal lever or arm, which
is held by a strong spring in contact with the upper binding screw. It
can be made to slide over to the right or left of the centre, at its
lower or free end, as far as the binding screws or studs shown, which
act at once as stops and point of connection to wires. When the arm or
lever is in the centre no contact is made but if it be pushed over to
the right, it slides on a brass strip let into and lying flush with the
base. Contact is thus made between the upper binding screw and the
left-hand screw. If there is another brass strip on the left-hand side
(as shown in the figure), contact may be made with another bell, etc.,
by sliding the arm to the left; or again, if no metal strip be placed on
the left side the contact may be broken by pushing the arm towards the
left-hand stud.


§ 62. A _key_ is another form of contact, by means of which a long or
short completion of circuit can be made by simply tapping on the knob.
It is particularly useful when it is desired to transmit signals, either
by ringing or otherwise. It consists, as may be seen at Fig. 65, of a
lever or arm of brass, pivoted at its centre, furnished with a spring
which keeps the portion under the knob out of contact with the stud in
the front of the base-board. As both the stud and the lever are
connected to binding screws communicating with the battery and bell,
etc., it is evident that on depressing the key the circuit with the bell
will be completed for a longer or shorter period, varying with the
duration of the depression. Hence, either by using preconcerted signals
of short and long rings to signify certain common words, such as a long
ring for _No_, and a short one for _Yes_, or by an adaptation of the
ordinary Morse code, intelligible conversation can be kept up between
house and stable, etc., etc., by means of a key and a bell. As Mr.
Edwinson has given much time to the elucidation of this system of bell
signalling, I cannot do better than quote his instructions, as given in
_Amateur Work_:--

"For this purpose preconcerted signals have been agreed upon or invented
as required, and these have been found to be irksome and difficult to
remember, because constructed without any reference to a definite plan.
We may, however, reduce bell signals to a definite system, and use this
system or code as a means to carry on conversation at a distance as
intelligently as it can be done by a pair of telegraph instruments. In
fact, the Morse telegraph code can be easily adopted for use with
electric bells of the vibrating or trembling type, and its alphabet, as
appended below, easily learnt. The letters of the alphabet are
represented by long strokes and short strokes on the bell, as here
shown.--

  A ·-
  B -···
  C -·-·
  D -··
  E ·
  F ··-·
  G --·
  H ····
  I ··
  J ·---
  K -·-
  L ·-··
  M --
  N -·
  O ---
  P ·--·
  Q --···
  R ·-·
  S ···
  T -
  U ··-
  V ···-
  W ·--
  X -··-
  Y -·--
  Z --··
  Ch ----
  Ä (æ) ·-·-
  Ö ([oe]) ---·
  Ü (ue) ··--
  1 ·----
  2 ··---
  3 ···--
  4 ····-
  5 ·····
  6 -····
  7 --···
  8 ---··
  9 ----·
  0 -----

"It will be noticed that the strokes to represent a letter do not in any
case exceed four, and that all the figures are represented by five
strokes of varying length to each figure. Stops, and other marks of
punctuation, are represented by six strokes, which are in their
combination representations of two or three letters respectively, as
shown below:--

  Comma           (,)   by  A A A  or  ·-·-·-
  Full stop       (.)   "   I I I  "   ······
  Interrogation   (?)   "   U D    "   ··--··
  Hyphen          (-)   "   B A    "   -····-
  Apostrophe      (')   "   W G    "   ·----·
  Inverted commas (")   "   A F    "   ·-··-·
  Parenthesis      ()   "   K K    "   -·--·-
  Semi-colon      (;)   "   K Ch   "   ·-----
  Surprise        (!)   "   N Ch   "   -·----
  Colon           (:)   "   I Ch   "   ··----

"In sending signals to indicate stops, no regard must be had to the
letters which they represent; these are only given as aids to memory,
and are not to be represented separately on the bell. Bell signals must
be given with a certain amount of regularity as to time; indeed, to
carry on a conversation in this way it is necessary to be as punctilious
in time as when playing a piece of music on a piano, if the signals are
to be understood. The dots of the signal should therefore be represented
in time by _one_, and the dashes by _two_, whilst the spaces between
words and figures where a stop does not intervene should be represented
by a pause equal to that taken by a person counting _three_, the space
between a word and a stop being of the same duration. To make this more
clear I give an example. The mistress signals to her coachman:--

   G  | E | T | | T |  H   | E |
  --· | · | - | | - | ···· | ·
  221 | 1 | 2 |3| 2 | 1111 | 1 | 3

   C   |  A |   R |   R |   I |  A |  G  | E |
  -·-· | ·- | ·-· | ·-· | ··  | ·- | --· | · |
  2121 | 12 | 121 | 121 | 11  | 12 | 221 | 1 | 3

   R  | E |  A |  D  |   Y
  ·-· | · | ·- | -·· | -·--
  121 | 1 | 12 | 211 | 2122

"The coachman replies:--

   R  | E |  A |  D  |   Y
  ·-· | · | ·- | -·· | -·--
  121 | 1 | 12 | 211 | 2122

"When the mistress is ready she signals:--

    B  |  R  | I  | N  |  G  |  |  T |  H   | E  |
  -··· | ·-· | ·· | -· | --· |  |  - | ···· | ·  |
  2111 | 121 | 11 | 21 | 221 | 3|  2 | 1111 | 1  | 3

   C   |  A |   R |   R |   I |  A |  G  | E
  -·-· | ·- | ·-· | ·-· | ··  | ·- | --· | ·
  2121 | 12 | 121 | 121 | 11  | 12 | 221 | 1

"And the coachman replies with a single long ring to signify that he
understands. It will be found convenient to have an answering signal
from the receiving end of the line to each word separately. This must be
sent in the pause after each word, and consists of the short signal E ·
when the word is understood, or the double short signal I ·· when the
word is not understood. A negative reply to a question may be given by
the signal for N -·, and an affirmative by the signal for Æ ·-·-; other
abbreviations may be devised and used where desired. The code having
been committed to memory, it will be quite easy to transpose the words
and send messages in cypher when we wish to make a confidential
communication; or the bells may be muffled under a thick cloak, and
thus, whilst the measured beats are heard by the person for whom the
signal is intended, others outside the room will not be annoyed by
them."

[Illustration: Fig. 65.]


§ 63. At § 48, we noticed that a device known as a _Relay_ is a
convenient, if not an essential mode of working continuous ringing
bells. Here we will direct our attention to its structural arrangement,
and to its adaptations. Let us suppose that we had to ring a bell at a
considerable distance, so far indeed that a single battery would not
energise the electro-magnets of an ordinary bell, sufficiently to
produce a distinct ring. It is evident that if we could signal, ever so
feebly, to an attendant at the other end of the line to make contact
with another battery at the distant end of the line to _his_ bell, by
means, say, of a key similar to that shown at Fig. 65, we should get a
clear ring, since this second battery, being close to the bell, would
send plenty of current to energise the bell's magnets. But this would
require a person constantly in attendance. Now the _relay_ does this
automatically; it _relays_ another battery in the circuit. The manner in
which it effects this will be rendered clear, on examination of Fig. 66.
Here we have an armature A attached to a light spring, which can play
between an insulated stop C, and a contact screw B. The play of this
armature can be regulated to a nicety by turning the screws B or C.
These two screws are both borne by a double bent arm (of metal) affixed
to the pillar D. This pillar is separated from the rest of the frame by
an insulating collar or washer of ebonite, so that no current can pass
from E to D, unless the armature be pulled down so as to make contact
with the contact screw B. Just under the armature, stands the
electro-magnet G, which when energised can and does pull down the
armature A. It will be readily understood that if we connect the wires
from the electro-magnet G, to the wires proceeding from the battery and
push (or other form of contact) at the distant station, the
electro-magnet, being wound with a large quantity of fine wire, will
become sufficiently magnetized to pull the armature down through the
small space intervening between C and B; so that if the screws D and E
are connected respectively to the free terminals of a battery and bell
coupled together at the nearer station, this second battery will be
thrown into circuit with the bell, and cause it to ring as well and as
exactly as if the most skilful and most trustworthy assistant were in
communication with the distant signaller. Every tap, every release of
the contact, (be it push, key, or switch) made at the distant end, will
be faithfully reproduced at the nearer end, by the motion of the
armature A. For this reason we may use a comparatively weak battery to
work the relay, which in its turn brings a more powerful and _local_
battery into play, for doing whatever work is required. In cases where a
number of calls are required to be made simultaneously from one centre,
as in the case of calling assistance from several fire engine stations
at once, a relay is fixed at each station, each connected with its own
local battery and bell. The current from the sending station passes
direct through all the relays, connecting all the local batteries and
bells at the same time. This is perhaps the best way of ringing any
number of bells from one push or contact, at a distant point. Ordinary
trembling bells, unless fitted with an appropriate contrivance, cannot
well be rung if connected up in _series_. This is owing to the fact that
the clappers of the bells do not all break or make contact at the same
time, so that intermittent ringing and interruptions take place. With
single stroke bells, this is not the case, as the pulling down of the
armature does not break the contact.

[Illustration: Fig. 66.]

[Illustration: Fig. 67.]

[Illustration: Fig. 68.]


§ 64. We now have to consider those contrivances by means of which it is
possible for an attendant to know when a single bell is actuated by a
number of pushes in different rooms, etc., from whence the signal
emanates. These contrivances are known as _indicators_. Indicators may
be conveniently divided into 3 classes, viz.:--1st, indicators with
_mechanical_ replacements; 2nd, those with electrical replacements; and
3rdly, those which are self replacing. Of the former class we may
mention two typical forms, namely, the ordinary "fall back" indicator,
and the drop indicator. All indicators depend in their action on the
sudden magnetisation of an electro-magnet by the same current that works
the electric bell at the time the call is sent. To understand the way in
which this may be effected, let the reader turn to the illustration of
the Relay (Fig. 66), and let him suppose that the pillar D, with its
accompanying rectangle B C, were removed, leaving only the
electro-magnet G, with its frame and armature A. If this armature holds
up a light tablet or card, on which is marked the number of the room, it
is evident that any downward motion of the armature, such as would occur
if the electro-magnet were energised by a current passing around it,
would let the tablet fall, so as to become visible through a hole cut in
the frame containing this contrivance. It is also equally evident that
the card or tablet would require replacing by hand, after having once
fallen, to render it capable of again notifying a call. Fig. 67 shows
the working parts of one of these "drop" indicators, as sent out by
Messrs. Binswanger. In another modification, known as Thorpe's
"Semaphore Indicator," we have a most ingenious application of the same
principle in a very compact form. In this (Fig. 68), the electro-magnet
is placed directly behind a disc-shaped iron armature, on which is
painted or marked the number of the room etc. (in this case 4); this
armature is attached by a springy shank to the drop bar, shown to the
left of the electro-magnet. In front of the armature is a light metal
disc, also pivoted on the drop bar. This engages in a catch above, when
pushed up so as to cover the number. When pushed up, the spring of the
armature retains it in its place so that the number is hidden. When the
current passes around the electro-magnet, the armature is pulled toward
it, and thus frees the covering disc, which therefore falls, and
displays the number. The ordinary form of "fall back" indicator (a
misnomer, by the way, since the indicator falls forwards) is well
illustrated at Fig 69. Here we have an ordinary electro-magnet A, with
its wires _w_ _w'_ standing over an armature B attached to a spring C,
which bears on its lower extremity, a toothed projection which serves to
hold up the short arm of the bent lever D, which supports the number
plate E. When the electro-magnet A is energised by the current, it pulls
up the armature B, which releases the detent D from the tooth C; the
number plate therefore falls forwards, as shown by the dotted lines,
and shows itself at the aperture E´, which is in front of the indicator
frame. To replace the number out of sight, the attendant pushes back the
plate E, till it again engages the bent lever D in the tooth C. This
replacement of the number plate, which the attendant in charge is
obliged to perform, gives rise to confusion, if through carelessness it
is not effected at once, as two or more numbers may be left showing at
one time. For this reason, indicators which require no extraneous
assistance to replace them, are preferred by many. Indicators with
electrical replacements meet in part the necessities of the case. This
form of indicator consists usually of a permanent bar magnet pivoted
near its centre, so that it can hang vertically between the two poles of
an electro-magnet placed at its lower extremity. The upper extremity
carries the number plate, which shows through the aperture in the frame.
This bar magnet is made a trifle heavier at the upper end, so that it
must rest against either the one or other pole of the electro-magnet
below. If the _north_ pole of the bar magnet rests against the _right_
hand pole of the electro-magnet when the number does not show, we can
cause the bar magnet to cross over to the other pole, and display the
number by sending a current through the electro-magnet in such a
direction as to make its right hand pole a north pole, and its left hand
a south pole. This is because the two north poles will repel each other,
while the south will attract the north. On being once tilted over, the
bar magnet cannot return to its former position, until the person who
used the bell sends a current in the opposite direction (which he can do
by means of a reversing switch), when the poles of the electro-magnet
being reversed, the bar magnet will be pulled back into its original
position. Indicators of this class, owing to the fact that their
replacement depends on the _polarity_ of the bar magnet, are also known
as "polarised indicators."

[Illustration: Fig. 69.]


§ 65. For general efficiency and trustworthiness, the _pendulum
indicator_; as shown at Fig. 70, is unsurpassed. It consists of an
electro-magnet with prolongation at the free end on which is delicately
pivoted a soft iron armature. From the centre of this armature hangs,
pendulum fashion, a light brass rod carrying a vane of fluted silver
glass, or a card with a number on it, as may be found most convenient.
This vane or card hangs just before the aperture in the indicator frame.
Stops are usually placed on each side of the pendulum rod to limit the
swing. When the electro-magnet is magnetised by the passage of the
current, the armature is pulled suddenly on one side, and then the
pendulum swings backwards and forwards in front of the aperture for some
minutes before it comes to rest. When fitted with silver fluted glass,
the motion of the vane is clearly visible even in badly lighted places.
As the pendulum, after performing several oscillations, comes to rest by
itself in front of the aperture, this indicator requires no setting.
Messrs. Binswanger fit these indicators with double core magnets, and
have a patented adjustment for regulating the duration of the swings of
the pendulum, which may be made to swing for two or three minutes when
the circuit is completed by pressing the push; it then returns to its
normal position, thus saving the servant the trouble of replacing the
"drop."

[Illustration: Fig. 70.]

Messrs. Gent, of Leicester, have also patented a device in connection
with this form of indicator, which we give in the patentee's own
words:--"The objection so frequently urged against the use of Electric
Bells, that the servants cannot be depended upon to perform the
operation of replacing the signals, cannot any longer apply, for the
pendulum signals require no attention whatever. It consists of an
electro-magnet having forks standing up in which [V] openings are made. An
armature of soft iron, with a piece of thin steel projecting at each end
lies suspended at the bottom of the [V] opening, a brass stem carrying the
signal card is screwed into the armature, the action being, that when a
current is allowed to pass through the electro-magnet the armature with
the pendulum is drawn towards it and held there until the current ceases
to pass, when it instantly looses its hold of the armature, which swings
away and continues to oscillate for two or three minutes, so that if the
servant happens to be out of the way, it may be seen on her return which
pendulum has been set in motion. The Pendulum Indicator we have recently
patented is entirely self-contained. The magnet has its projecting poles
riveted into the brass base which carries the flag. The flag is
constructed as Fig. 70, but swings in closed bearings, which prevents
its jerking out of its place, and enables us to send it out in position
ready for use. It will be seen this _patented_ improvement makes all
screws and plates as formerly used for securing the parts unnecessary.
It will be seen at once that this is simplicity itself, and has nothing
about it which may by any possibility be put out of order, either by
warping or shrinking of the case or carelessness of attendants."

[Illustration: Fig. 71.]

There is only one point that needs further notice with regard to these
pendulum indicators, and that is, that since the rapid break and make
contact of the ringing bell interferes somewhat with the proper action
of the indicator magnet, it is always advisable to work the indicator by
means of a relay (fixed in the same frame) and a _local_ battery. This
is shown in Fig. 71, where a second pair of wires attached to C and C,
to the extreme right of the indicator frame, are brought from the same
battery to work the indicator and contained relay. It is not advisable,
however, with the pendulum indicator, to use the same battery for the
indicator; the relay should throw a local battery into the indicator
circuit. In Fig. 71 six pushes are shown to the left of the indicator
frame. These, of course, are supposed to be in as many different rooms.

[Illustration: Fig. 72.]

We close this chapter with an engraving of a very compact and neat form
of drop indicator devised by Messrs. Gent, and called by them a
"Tripolar Indicator." It consists, as the name implies, of a single
magnet, having one end of the iron core as one pole, the other end
extending on each side like a [V], forming, as it were, three poles.
Though but one bobbin is used, the effect is very powerful. There are no
springs or other complications, so that the arrangement is adapted for
ship use, as are also those represented at Figs. 67 and 68. Pendulum and
fall-back indicators, as well as polarised indicators, owing to the
delicacy of the adjustments, are unfitted for use on board ship, or in
the cabs of lifts, where the sudden jolts and jerks are sure to move the
indicators, and falsify the indications. The tripolar indicator is
illustrated at Fig. 72.



CHAPTER V.

ON WIRING, CONNECTING UP, AND LOCALISING FAULTS.


§ 66. However good may be the bells, indicators, batteries, etc., used
in an electric bell installation, if the _wiring_ be in any wise faulty,
the system will surely be continually breaking down, and giving rise to
dissatisfaction. It is therefore of the highest importance that the
workman, if he value his good name, should pay the greatest attention to
ensure that this part of his work be well and thoroughly done. This is
all the more necessary, since while the bells, batteries, relays,
pushes, etc., are easily got at for examination and repair, the wires,
when once laid, are not so easily examined, and it entails a great deal
of trouble to pull up floor boards, to remove skirtings etc., in order
to be able to overhaul and replace defective wires or joints. The first
consideration of course, is the kind and size of wire fitted to carry
the current for indoor and outdoor work. Now this must evidently depend
on three points. 1st, The amount of current (in ampères) required to
ring the bell. 2nd, The battery power it is intended to employ. 3rd, The
distance to which the lines are to be carried. From practical
experience I have found that it is just possible to ring a 2-1/2" bell
with 1/2 an ampère of current. Let us consider what this would allow us
to use, in the way of batteries and wire, to ring such a bell. The
electro-motive force of a single Leclanchè cell is, as we have seen at §
38, about 1·6 volt, and the internal resistance of the quart size, about
1·1 ohm. No. 20 gauge copper wire has a resistance of about 1·2 ohm to
the pound, and in a pound (of the cotton covered wire) there are about
60 yards. Supposing we were to use 60 yards of this wire, we should have
a wire resistance of 1·2 ohm, an internal resistance of 1·1 ohm, and a
bell resistance of about 0·1 of an ohm, altogether about 2·4 ohms. Since
the E.M.F. of the cell is 1·6 volt, we must divide this by the total
resistance to get the amount of current passing. That is to say:--

  Ohms. Volts. Ampères.
  2·4)  1·60   (0·66,

or about 2/3 of an ampère; just a little over what is absolutely
necessary to ring the bell. Now this would allow nothing for the
deterioration in the battery, and the increased resistance in the
pushes, joints, etc. We may safely say, therefore, that no copper wire,
of less diameter than No. 18 gauge (48/1000 of an inch diameter) should
be used in wiring up house bells, except only in very short circuits of
two or three yards, with one single bell in circuit; and as the
difference in price between No. 18 and No. 20 is very trifling, I
should strongly recommend the bell-fitter to adhere to No. 18, as his
smallest standard size. It would also be well to so proportion the size
and arrangement of the batteries and wires, that, at the time of setting
up, a current of at least one ampère should flow through the entire
circuit. This will allow margin for the weakening of the battery, which
takes place after it has been for some months in use. As a guide as to
what resistance a given length of copper wire introduces into any
circuit in which it may be employed, I subjoin the following table of
the Birmingham wire gauge, diameter in 1,000ths of an inch, yards per
lb., and resistance in ohms per lb. or 100 yards, of the wires which the
fitter is likely to be called upon to employ:--

  ------------------------------------------------------------
            Table of Resistance and lengths per lbs.
          & 100 yards of cotton covered copper wires.
  ------------------------------------------------------------
  Birmingham  | Diameter in |   Yards  |   Ohms.  | Ohms. per
  Wire Gauge. |  1000th of  |  per lb. |  per lb. | 100 yards.
              |   an inch.  |          |
  ------------+-------------+----------+----------+-----------
  No. 12      |       100   |      9   |  0·0342  |  0·0038
      14      |        80   |     15   |  0·0850  |  0·0094
      16      |        62   |     24   |  0·2239  |  0·0249
      18      |        48   |     41   |  0·6900  |  0·0766
      20      |        41   |     59   |  1·2100  |  0·1333
      22      |        32   |    109   |  3·1000  |  0·3444
  ------------------------------------------------------------


§ 67. Whatever gauge wire be selected, it must be carefully insulated,
to avoid all chance contact with nails, staples, metal pipes or other
wires. The best insulation for wires employed indoors is gutta-percha,
surrounded with a coating of cotton wound over it, except only in cases
when the atmosphere is excessively dry. In these, as the gutta-percha
is apt to crack, india-rubber as the inner coating is preferable. If No.
18 wire be used, the thickness of the entire insulating coating should
be thick enough to bring it up to No. 10 gauge, say a little over 1/10th
inch in diameter. There is one point that will be found very important
in practice, and that is to have the cotton covering on the wires
_leading_ to the bells of a different colour from that on the _return_
wires; in other words, the wires starting from the zinc poles of the
battery to the bells, indicators, relays, etc., should be of a different
colour from that leading from the carbon poles to the bells, etc.
Attention to this apparently trifling matter, will save an infinite
amount of trouble in connecting up, repairing, or adding on fresh branch
circuits. For outdoor work, wire of the same gauge (No. 18) may
generally be used, but it must be covered to the thickness of 1/10" with
pure gutta-percha, and over this must be wound tape served with
Stockholm tar. Wires of this description, either with or without the
tarred tape covering, may be obtained from all the leading electricians'
sundriesmen. Many firms use copper wire _tinned_ previous to being
insulated. This tinning serves two good purposes, 1st, the copper wire
does not verdigris so easily; 2ndly, it is more easily soldered. On the
other hand, a tinned wire is always a little harder, and presents a
little higher resistance. Whenever wires are to be joined together, the
ends to be joined must be carefully divested of their covering for a
length of about three inches, the copper carefully cleaned by scraping
and sand-papering, twisted tightly and evenly together, as shown in
Fig. 73 A, and soldered with ordinary soft solder (without spirits), and
a little resin or composite candle as a flux. A heavy plumber's
soldering iron, or even a tinman's bit, is not well adapted for this
purpose, and the blowpipe is even worse, as the great heat melts and
spoils the gutta-percha covering. The best form of bit, is one made out
of a stout piece of round copper wire 1/4" thick with a nick filed in
its upper surface for the wire to lie in (see Fig. 73 B). This may be
fastened into a wooden handle, and when required heated over the flame
of a spirit lamp. When the soldering has been neatly effected, the waste
ends _a_ and _b_ of the wire should be cut off flush. The wire must then
be carefully covered with warm Prout's elastic or softened gutta-percha,
heated and kneaded round the wire with the fingers (moistened so as not
to stick) until the joint is of the same size as the rest of the covered
wire. As a further precaution, the joints should be wrapped with a layer
of tarred tape. Let me strongly dissuade the fitter from ever being
contented with a simply twisted joint. Although this may and does act
while the surfaces are still clean, yet the copper soon oxidises, and a
poor non-conducting joint is the final result.

"That'll do" will not do for electric bell-fitting.

[Illustration: Fig. 73.]


§ 68. Whenever possible, the wiring of a house, etc., for bell work,
should be done as soon as the walls are up and the roof is on. The
shortest and straightest convenient route from bell to battery, etc.,
should always be chosen where practicable to facilitate drawing the wire
through and to avoid the loss of current which the resistance of long
lengths of wire inevitably entails. The wires should be run in light
zinc tubes nailed to the wall.

In joining up several lengths of tubing, the end of one piece of tube
should be opened out _considerably_ of a trumpet shape for the other
piece to slip in; and the end of this latter should also be _slightly_
opened out, so as not to catch in the covering of any wire drawn through
it. The greatest care must be exercised in drawing the wires through the
tubes or otherwise, that the covering be not abraded, or else leakage at
this point may take place. In cases where tubes already exist, as in
replacing old crank bells by the electric bells, the new wires can be
drawn through the tubes, by tying the ends of the new wire to the old
wire, and carefully pulling this out, when it brings the new wire with
it. Or if the tubes are already empty, some straight stout wire may be
run through the tubes, to which the new wires may be attached, and then
drawn through, using, of course, every possible precaution to avoid the
abrasion of the insulating covering of the wire, which would surely
entail leakage and loss of current. All the old fittings, cranks,
levers, etc., must be removed, and the holes left, carefully filled with
dowels or plaster. In those cases where it is quite impossible to lay
the wires in zinc or wooden tubes (as in putting up wires in furnished
rooms already papered, etc.), the wires may be run along the walls, and
suspended by staples driven in the least noticeable places; but in no
case should the two wires (go and return) lie under the same staple, for
fear of a short circuit. It must be borne in mind that each complete
circuit will require at least two wires, viz., the one leading from the
battery to the bell, and the other back from the bell to the battery;
and these until connection is made between them by means of the
"contact" (pull, push, or key) must be perfectly insulated from each
other. In these cases, as far as possible, the wires should be laid in
slots cut in the joists under the floor boards, or, better still, as
tending to weaken the joists less, small holes may be bored in the
joists and the wires passed through them; or again, the wires may be led
along the skirting board, along the side of the doorpost, etc., and when
the sight of the wires is objectionable, covered with a light ornamental
wood casing. When the wires have been laid and the position of the
"pushes," etc., decided upon, the _blocks_ to which these are to be
fastened must be bedded in the plaster. These blocks may be either
square or circular pieces of elm, about 3 inches across, and 1 inch
thick, bevelled off smaller above, so as to be easily and firmly set in
the plaster. They may be fastened to the brickwork by two or three
brads, at such a height to lie level with the finished plaster. There
must of course be a hole in the centre of the block, through which the
wires can pass to the push. When the block has been fixed in place, the
zinc tube, if it does not come quite up to the block, should have its
orifice stopped with a little paper, to prevent any plaster, etc.,
getting into the tube. A little care in setting the block will avoid the
necessity of this makeshift. A long nail or screw driven into the block
will serve to mark its place, and save time in hunting for it after the
plastering has been done. When the blocks have been put in their places,
and the plastering, papering, etc., done, the wires are drawn through
the bottom hole of the push (after the lid or cover has been taken off),
Fig. 74, and a very small piece of the covering of the wire having been
removed from each wire, and brightened by sand papering, one piece is
passed round the shank of the screw connected with the lower spring,
shown to the _right_ in Fig. 74, and the other round the shank of the
screw connected to the upper spring, shown to the _left_ in the Fig. The
screws must be loosened to enable the operator to pass the wire under
their heads. The screws must then be tightened up to clench the wire
quite firmly. In doing this, we must guard against three things.
Firstly, in pulling the wire through the block, not to pull so tightly
as to cut the covering against the edge of the zinc tube. Secondly, not
to uncover too much of the wire, so as to make contact between the wires
themselves either at the back of the push, or at any other part of the
push itself. Thirdly, to secure good contact under the screws, by having
the ends of the wires quite clean, and tightly screwed down.

[Illustration: Fig. 74.]


§ 69. In all cases where the wires have to be taken out of doors, such
as is necessitated by communication from house to outhouses, stables,
greenhouses, etc., over head lines (No. 18 gauge, gutta-percha tape and
tar covering) should be used. Where overhead lines are not admissible,
either as being eyesores, or otherwise, the wires may be laid in square
wooden casings of this section [box open up], the open part of
which must be covered by a strip of wood laid over it. The wood must
have been previously creosoted, in the same manner as railway sleepers.
This mode admits of easy examination. Iron pipes must, however, be used
if the lines have to pass under roads, etc., where there is any heavy
traffic. And it must be borne in mind that however carefully the iron
pipes, etc., be cemented at the joints, to make them watertight, there
will always be more electrical leakage in underground lines than in
overhead ones. In certain rare cases it may be needful to use _iron_
wires for this purpose instead of copper; in this case, as iron is six
or seven times a worse conductor than copper, a much heavier wire must
be employed to get the same effect. In other words, where iron wire is
used, its section must be not less than seven times that of the copper
wire which it replaces.


§ 70. It is always preferable, where great distance (and, consequently,
greater expense) do not preclude it, to use wire for the leading as well
as for the returning circuit. Still, where for any reason this is not
practicable, it is perfectly admissible and possible to make a good
return circuit through the _earth_, that is to make the damp soil carry
the return current (see § 37). As recommended at the section just
quoted, this earth circuit must have at each extremity a mass of some
good conductor plunged into the moist ground. In _towns_, where there
are plenty of water mains and gas mains, this is a matter of no
difficulty, the only point being to ensure _good_ contact with these
masses of metal. In other places a hole must be dug into the ground
until the point of constant moisture is reached; in this must be placed
a sheet of lead or copper, not less than five square feet surface, to
which the _earth_ wires are soldered, the hole then filled in with
ordinary coke, well rammed down to within about six inches of the
surface, and then covered up with soil well trodden down. In making
contact with water or gas pipes, care must be taken to see that these
are _main_ pipes, so that they _do_ lead to earth, and not to a cistern
or meter only, as, if there are any white or red lead joints the circuit
will be defective. To secure a good contact with an iron pipe, bare it,
file its surface clean, rub it over with a bit of blue stone (sulphate
of copper) dipped in water; wipe it quite dry, bind it tightly and
evenly round with some bare copper wire (also well cleaned), No. 16
gauge. Bring the two ends of the wire together, and twist them up
tightly for a length of three or four inches. Now heat a large soldering
bit, put some resin on the copper wire, and solder the wire, binding
firmly down to the iron pipe. Do likewise to the projecting twist of
wire, and to this twist solder the end of the _return_ wire. On no
account should the two opposite _earth_ wires be soldered to water mains
and gas mains at the same time, since it has been found that the
different conditions in which these pipes find themselves is sufficient
to set up a current which might seriously interfere with the working of
the battery proper. Sometimes there is no means of getting a good
_earth_ except through the gas main: in this case we must be careful to
get to the street side of the meter, for the red lead joints will
prevent good conductivity being obtained. In out of the way country
places, if it is possible to get at the metal pipe leading to the well
of a pump, a very good "earth" can be obtained by soldering the wires to
that pipe, in the same manner as directed in the case of the water main.
The operator should in no case be contented with a merely twisted joint,
for the mere contact of the two metals (copper and iron) sets up in the
moist earth or air a little electric circuit of its own, and this
speedily rusts through and destroys the wires. The following
suggestions, by Messrs. Gent, on the subject of wiring, are so good,
that we feel that we shall be doing real service to the reader to quote
them here in full:--

"1st.--The description of wire to be used. It is of the utmost
importance that all wires used for electric bell purposes be of pure
copper and thoroughly well insulated. The materials mostly employed for
insulating purposes are indiarubber, gutta-percha, or cotton saturated
with paraffin. For ordinary indoor work, in dry places, and for
connecting doors and windows with burglar alarms, or for signalling in
case of fire, indiarubber and cotton covered wires answer well; but for
connecting long distances, part or all underground, or along walls, or
in damp cellars or buildings, gutta-percha covered wire is required, but
it should be fixed where it will not be exposed to heat or the sun, or
in very dry places, as the covering so exposed will perish, crack, and
in time fall off. This may be, to some extent, prevented by its being
covered with cotton; but we recommend for warm or exposed positions a
specially-prepared wire, in which rubber and compound form the
insulating materials, the outside being braided or taped.

"For ordinary house work, we refer to lay a wire of No. 18 or 20 copper,
covered to No. 14 or 11 with gutta-percha, and an outer covering of
cotton, which we called the 'battery' wire, this being the wire which
conveys the current from the battery to every push, etc., no matter how
many or in what position. The reason for selecting this kind is, that
with the gutta-percha wires the joints may be more perfectly covered and
made secure against damp. This is of the utmost importance in the case
of '_battery wires_,' as the current is always present and ready to
take advantage of any defect in the insulation to escape to an adjoining
wire, or to '_earth_,' and so cause a continuous waste of current. The
wires leading from the pushes to the signalling apparatus or bell we
call the 'line' wires. In these, and the rest of the house wires, the
perfect covering of the joints is important. For _line wires_ we usually
prefer No. 18 or 20 copper, covered with indiarubber, and an outer
coating of cotton, well varnished. In joining the '_battery wires_,' the
place where the junction is to be made must be carefully uncovered for
the distance of about an inch; the ends of the wire to be joined, well
cleaned, and tightly twisted together; with the flame of a spirit lamp
or candle the joint must be then heated sufficiently to melt fine solder
in strips when held upon it, having first put a little powdered resin on
the joint as a flux; the solder should be seen to run well and adhere
firmly to the copper wire. A piece of gutta-percha should then be taken
and placed upon the joint while warm, and with the aid of the spirit
lamp and wet fingers, moulded round until a firm and perfect covering
has been formed. _On no account use spirits_ in soldering. With the
_line wire_, it is best, as far as possible, to convey it all the way
from the push to the signal box or bell in _one continuous_ length. Of
course, when two or more pushes are required to the same wire, a
junction is unavoidable. The same process of joining and covering, as
given for the battery wire, applies to the line wire. Where many wires
are to be brought down to one position, a large tube may be buried in
the wall, or a wood casing fixed flush with the plaster, with a
removable front. The latter plan is easiest for fixing and for making
alterations and additions. For stapling the wires, in no case should the
wires be left naked. When they pass along a damp wall, it is best to fix
a board and _loosely_ staple them. _In no case allow more than one wire
to lie under the same staple_, and do not let the staples touch one
another. In many cases, electric bells have been an incessant annoyance
and complete failure, through driving the staples _tight up to the
wires_, and several wires to the same staple,--this must not be done on
any account. A number of wires may be twisted into a cable, and run
through a short piece of gutta-percha tube, and fastened with ordinary
gas hooks where it is an advantage to do so. In running the wires, avoid
hot water pipes, and do not take them along the same way as plumber
pipes. Underground wires must be laid between pieces of wood, or in a
gas or drain pipe, and not exposed in the bare earth without protection,
as sharp pieces of stone are apt to penetrate the covering and cause a
loss; in fact, in this, as in every part of fixing wires, the best wire
and the best protection is by far the cheapest in the end. The copper
wire in this case should not be less than No. 16 B.W.G., covered with
gutta-percha, to No. 9 or 10 B.W.G., and preferably an outer covering of
tape or braid well tarred. Outside wire, when run along walls and
exposed to the weather, should be covered with rubber and compound, and
varnished or tarred on an outer covering of tape or braid. Hooks or
staples must be well galvanised to prevent rusting, and fixed loosely.
If the wire is contained within an iron pipe, a lighter insulation may
be used: _but the pipe must be watertight_. In a new building, wires
must be contained within zinc or copper bell tubes. A 3/8 inch tube will
hold two wires comfortably. The tubes should be fixed to terminate in
the same positions in the rooms as ordinary crank bell levers,--that is,
about three feet from the floor. At the side of the fireplace a block of
wood should be fixed in the wall before any plaster is put on, and the
end of the tube should terminate in the centre of the same. A large nail
or screw may be put in to mark the place, so that the end of the tube
may be found easily when the plastering is finished. Bend the tube
slightly forward at the end, and insert a short peg of wood to prevent
dirt getting into the tube. Do the same at the side of, or over the bed
in bedroom. If the tubes are kept clean, the wires may be easily drawn
up or down as the case may require. The best way is to get a length of
ordinary copper bell wire, No. 16, sufficient to pass through the tube,
and having stretched it, pass it through and out at the other end. Here
have your coils of insulated wire, viz., one battery wire, which is
branched off to every push, and one line wire, which has to go direct to
the indicator or bells, and having removed a short portion of the
insulation from the end of each, they are tied to the bare copper wire
and drawn through. This is repeated wherever a push is to be fixed
throughout the building. In making connection with binding screws or
metal of any kind, it is of the utmost importance that everything should
be _perfectly clean_. _Joints_ in wire, whether tinned or untinned,
_must be soldered and covered_. We cannot impress this too earnestly on
fixers. Never bury wires in plaster unprotected, and in houses in course
of erection, the _tubes_ only should be fixed until the plastering is
finished, the wires to be run in at the same time that the other work is
completed."

[Illustration: Fig. 75.]


§ 71. The wires having been laid by any of the methods indicated in the
preceding five sections, the fixer is now in a position to _connect up_.
No two houses or offices will admit of this being done in _exactly_ the
same way; but in the following sections most of the possible cases are
described and illustrated, and the intelligent fixer will find no
difficulty, when he has once grasped the principle, in making those
trifling modifications which the particular requirements may render
necessary. The first and simplest form, which engages our attention, is
that of a _single bell, battery, and push_, connected by wire only. This
is illustrated at Fig. 75. Here we see that the bell is connected by
means of one of the wires to the zinc pole of the battery, the push or
other contact being connected to the carbon pole of the same battery. A
second wire unites the other screw of the push or contact with the
second binding screw of the bell. There is no complete circuit until the
push is pressed, when the current circulates from the carbon or positive
pole of the battery, through the contact springs of the push, along the
wire to the bell, and then back again through the under wire to the zinc
or negative pole of the battery.[15] It must be clearly understood that
the exact position of battery, bell, and push is quite immaterial. What
is essential is, that the relative connections between battery, bell,
and push be maintained unaltered. Fig. 76 shows the next simplest case,
viz., that in which a single bell and push are worked by a single cell
through an "earth" return (see § 70). Here the current is made to pass
from the carbon pole of the battery to the push, thence along the line
wire to the bell. After passing through the bell, it goes to the
right-hand earth-plate E, passing through the soil till it reaches the
left-hand earth-plate E, thence back to the zinc pole of the battery. It
is of no consequence to the working of the bell whether the battery be
placed between the push and the left-hand earth-plate, or between the
bell and the right-hand earth-plate; indeed, some operators prefer to
keep the battery as near to the bell as possible. At Fig. 77 is shown
the mode by which a single battery and single bell can be made to ring
from two (or more) pushes situated in different rooms. Here it is
evident that, whichever of the two pushes be pressed, the current finds
its way to the bell by the upper wire, and back home again through the
lower wire; and, even if both pushes are down at once, the bell rings
just the same, for both pushes lead from the same pole of the battery
(the carbon) to the same wire (the line wire).

[Footnote 15: It must be borne in mind that the negative element is that
to which the positive pole is attached, and _vice versâ_ (see ss. 8 and
9).]

[Illustration: Fig. 76.]

[Illustration: Fig. 77.]

In Fig. 78, we have a slight modification of the same arrangement, a
front-door _pull_ contact being inserted in the circuit; and here, in
view of the probably increased resistance of longer distance, _two_
cells are supposed to be employed instead of _one_, and these are
coupled up in series (§ 40), in order to overcome this increased
resistance.

[Illustration: Fig. 78.]

The next case which may occur is where it is desired to ring two or more
bells from one push. There are two manners of doing this. The first mode
is to make the current divide itself between the two bells, which are
then said to be "_in parallel_." This mode is well illustrated both at
Figs. 79 and 80. As in these cases the current has to divide itself
among the bells, larger cells must be used, to provide for the larger
demand; or several cells may be coupled up in parallel (§ 40). At Fig.
79 is shown the arrangement for two adjoining rooms; at Fig. 80, that to
be adopted when the rooms are at some distance apart. If, as shown at
Fig. 81, a switch similar to that figured in the cut Fig. 64 be inserted
at the point where the line wires converge to meet the push, it is
possible for the person using the push to ring both bells at once, or to
ring either the right-hand or the left-hand bell at will, according to
whether he turns the arm of the switch-lever on to the right-hand or
left-hand contact plate.

[Illustration: Fig. 79.]

[Illustration: Fig. 80.]

[Illustration: Fig. 81.]

The second mode of ringing two or more bells from one push is that of
connecting one bell to the other, the right-hand binding screw of the
one to the left-hand binding screw of the next, and so on, and then
connecting up the whole series of bells to the push and battery, as if
they were a single bell. This mode of disposing the bells is called the
_series_ arrangement. As we have already noticed at § 63, owing to the
difference in the times at which the different contact springs of the
various bells make contact, this mode is not very satisfactory. If the
bells are single stroke bells, they work very well in series; but, to
get trembling bells to work in series, it is best to adopt the form of
bell recommended by Mr. F. C. Allsop. He says: "Perhaps the best plan is
to use the form of bell shown at Fig. 82, which, as will be seen from
the figure, governs its vibrations, not by breaking the circuit, but by
shunting its coils. On the current flowing round the electro-magnet, the
armature is attracted, and the spring makes contact with the lower
screw. There now exists a path of practically no resistance from end to
end. The current is therefore diverted from the magnet coils, and passes
by the armature and lower screw to the next bell, the armature falling
back against the top screw, and repeating the previous operation so long
as the circuit is closed. Thus, no matter how many bells there be in the
series, the circuit is never broken. This form of bell, however, does
not ring so energetically as the ordinary form, with a corresponding
amount of battery power."

[Illustration: Fig. 82.]

[Illustration: Fig. 83.]

Fig. 83 illustrates the mode in which a bell, at a long distance, must
be coupled up to work with a local battery and relay. The relay is not
shown separately, but is supposed to be enclosed in the bell case. Here,
on pressing the push at the external left-hand corner, the battery
current passes into the relay at the distant station, and out at the
right-hand earth-plate E returning to the left-hand earth-plate E. In
doing this, it throws in circuit (just as long as the push is held down)
the right-hand local battery, so that the bell rings by the current sent
by the local battery, the more delicate relay working by the current
sent from the distant battery.

[Illustration: Fig. 84.]

[Illustration: Fig. 85.]

At Fig. 84, we have illustrated the mode of connecting up a continuous
ringing bell, with a wire return. Of course, if the distance is great,
or a roadway, etc., intervene, an overhead line and an earth plate may
replace the lines shown therein, or both lines may be buried. It is
possible, by using a Morse key (Fig. 65) constructed so as to make
contact in one direction when _not_ pressed down, and in the other
_when_ pressed down, to signal from either end of a circuit, using only
one line wire and one return. The mode of connecting up for this purpose
is shown at Fig. 85. At each end we have a battery and bell, with a
double contact Morse key as shown, the Morse key at each end being
connected through the intervention of the line wire through the central
stud. The batteries and bells at each station are connected to earth
plates, as shown. Suppose now we depress the Morse key at the right-hand
station. Since by so doing, we lift the back end of the lever, we throw
our own bell out of circuit, but make contact between our battery and
the line wire. Therefore the current traverses the line wire, enters in
the left-hand Morse key, and, since this is not depressed, can, and
does, pass into the bell, which therefore rings, then descends to the
left-hand earth-plate, returning along the ground to the battery from
whence it started at the right-hand E. If, on the contrary, the
_left_-hand Morse key be depressed, while the right-hand key is not
being manipulated, the current traverses in the opposite direction, and
the right-hand bell rings. Instead of Morse keys, _double contact_
pushes (that is, pushes making contact in one direction when _not_
pressed, and in the opposite _when_ pressed) may advantageously be
employed. This latter arrangement is shown at Fig. 86.

[Illustration: Fig. 86.]

It is also possible, as shown at Fig. 87, to send signals from two
stations, using but one battery (which, if the distance is great, should
be of a proportionate number of cells), two bells, and two ordinary
pushes. Three wires, besides the earth-plate or return wire, are
required in this case. The whole of the wires, except the _return_, must
be carefully insulated. Suppose in this case we press the right-hand
button. The current flows from the battery along the lower wire through
this right-hand push and returns to the distant bell along the top wire,
down the left-hand dotted wire back to the battery, since it cannot
enter by the left-hand press, which, not being pushed, makes no contact.
The left-hand bell therefore rings. If, on the other hand, the left-hand
push be pressed, the current from the carbon of the battery passes
through the left-hand push, traverses the central line wire, passes into
the bell, rings it, and descends to the right-hand earth plate E,
traverses the earth circuit till it reaches the left-hand earth plate E,
whence it returns to the zinc pole of the battery by the lower dotted
line.

[Illustration: Fig. 87.]

Fig. 88 shows how the same result (signalling in both directions) may be
attained, using only two wires, with earth return, and two Morse keys.
The direction of the current is shown by the arrows. Both wires must be
insulated and either carried overhead or underground, buried in tubes.
Fig. 89 shows the proper mode of connecting the entire system of bells,
pushes, etc., running through a building. The dotted lines are the wires
starting from the two poles of the battery (which should consist of more
cells in proportion as there is more work to do), the plain lines being
the wires between the pushes and the bell and signalling box. In this
illustration a door-pull is shown to the extreme left. Pendulum
indicators are usually connected up as shown in this figure, except that
the bell is generally enclosed in the indicator case. The wire,
therefore, has to be carried from the left-hand screw of the indicator
case direct to the upper dotted line, which is the wire returning to the
zinc pole of the battery. N.B.--When the wires from the press-buttons
are connected with the binding-screw, of the top of or inside of the
indicator case, the insulating material of the wires, at the point where
connection is to be made, must be removed, and the wires _carefully
cleaned_ and _tightly clamped down_.

[Illustration: Fig. 88.]

[Illustration: Fig. 89.]

When it is desired to connect separate bells to ring in other parts of
the building, the quickest way is to take a branch wire out of the
nearest _battery wire_ (the wire coming from the carbon pole), and carry
it to the push or pull, from thence to the bell, and from the bell back
to the zinc of the battery.


§ 72. We should advise the fixer always to draw out a little sketch of
the arrangement he intends to adopt in carrying out any plan, as any
means of saving useless lengths of wire, etc., will then easily be seen.
In doing this, instead of making full sketches of batteries, he may use
the conventional signs [battery] for each cell of the battery, the thick
stroke meaning the carbon, the thin one the zinc. Pushes may be
represented by (·), earth-plates by [E] and pulls, switches, &c., as
shown in the annexed cut, Fig. 90, which illustrates a mode of
connecting up a lodge with a house, continuous bells being used, in such
a way that the lodge bell can be made to ring from the lodge pull, the
house bell ringing or not, according to the way the switch (shown at top
left-hand corner) is set. As it is set in the engraving, only the lodge
bell rings.

[Illustration: Fig. 90.]


§ 73. There are still two cases of electric bell and signal fitting, to
which attention must be directed. The first is in the case of _ships_.
Here all the connections can be made exactly as in a house, the only
exception to be made being that the indicators must not be of the
_pendulum_, or other easily displaced type; but either of the form shown
at Fig. 67 or 68, in which the electro-magnet has to lift a latch to
release the fall or drop, against a pretty stiff spring. Besides being
thus firmly locking, so as not to be affected by the ship's motion, all
the wood work should be soaked in melted paraffin wax, the iron work
japanned, and the brass work well lacquered, to protect all parts from
damp. The second case requiring notice is that of _lifts_. Every
well-appointed lift should be fitted with electric bells and
indicators. In the cab of the lift itself should be placed an electric
bell, with as many double contact pushes and indicators as there are
floors to be communicated with. At the top and at the bottom of the left
shaft, as near to the landing side as possible, must be set two stout
wooden blocks (oak, elm, or other non-perishable wood). From top to
bottom of the shaft must then be stretched, in the same manner as a
pianoforte is strung, on stout metal pins, with threading holes and
square heads, as many No. 12 or 14 bare copper wires as there are floors
or landings, and two more for the battery and return wire respectively.
Care must be taken that these wires are strung perfectly parallel, and
that they are stretched quite taut, but not strained, otherwise they
will surely break. To the top of the cab, and in connection in the usual
manner by wires with the bell and indicator (which, as in the case of
ships, must be of the locking type, lest the jolts of the cab disturb
their action) must be attached a number of spoonbill springs, which
press against the naked wires running down the shaft. The shape of these
springs (which should be of brass) at the part where they press against
the bare wires, is similar to that of the spoon break of a bicycle. Some
operators use rollers at the end of the spring instead of spoonbills,
but these latter _rub_ the wires and keep up good contact, while the
rollers slip over the wires and do not keep them clean. By means of
these springs, the current from the batteries, which are best placed
either at the top of the lift itself, or in one of the adjacent rooms
(never at the bottom of the shaft, owing to the damp which always reigns
there), can be taken off and directed where it is desired, precisely as
if the batteries were in the cab itself. It is usual (though not
obligatory) to use the two wires _furthest_ from the landing as the go
and return battery wires, and from these, through the other wires, all
desired communication with the landings can be effected. To obtain this
end, it will be necessary to furnish every landing with a double contact
push and bell, and each bell and push must be connected up to the shaft
wires in the following mode:--

A wire must be led from the _lower_ contact spring of the double contact
push, to the _main battery carbon wire_ in the shaft. A second wire is
led from the _upper contact stop_ of the double contact push to the
bell, and thence to the _main battery zinc wire_ on the shaft. Lastly, a
third wire is taken from the _upper contact spring_ of the push and
connected to that particular wire in the shaft which by means of the
spoonbill springs connects the particular push and indicator in the cab,
destined to correspond with it. It will be seen that with the exception
of using the rubbing spoonbill springs and return wires in the shaft,
this arrangement is similar to that illustrated at Fig. 87.

[Illustration: Fig. 91.]

A glance at Fig. 91 will render the whole system of wiring and
connecting up with lifts and landing, perfectly clear. In connecting the
branch lines to the main bare copper wires in the shaft, in order that
the spoonbill springs should not interfere with them, they (the ends of
the branch wires) must be bent at right angles, like a letter [L], and the
upright portion soldered neatly to the _back_ of the shaft wire. Any
solder which may flow over to the _front_ of the wire must be carefully
scraped off to prevent any bumps affecting the smooth working of the
contact springs. It will be evident on examination of Fig. 91, that if
any of the pushes on the landings be pressed, the circuit is completed
between the battery at the top, through the two battery wires to the
bell and one of the indicators to the cab, and, on the other hand, that
if a push be pressed in the cab, a corresponding bell on the landing
will be rung, precisely as in Fig. 87.

Some fitters employ a many-stranded cable to convey the current to and
from the battery to the cab and landing, instead of the system of
stretched wires herein recommended; but this practice cannot be
advocated, as the continual bending and unbending of this cable,
repeated so frequently every day, soon breaks the leading wires
contained in the cable.


§ 74. In many cases where a "call" bell alone is required, the battery
may be entirely dispensed with, and a small dynamo (§ 15) employed
instead. The entire apparatus is then known as the "magneto-bell," and
consists essentially of two parts, viz., the generator, Fig. 92, and the
bell, Fig. 93. The _generator_ or _inductor_ consists of an armature,
which by means of a projecting handle and train of wheels can be
revolved rapidly between the poles of a powerful magnet; the whole being
enclosed in a box. The current produced by the revolution of the
armature is led to the two binding screws at the top of the box. By
means of two wires, or one wire and an earth circuit, the current is led
to the receiver or bell case, Fig. 93. Here, there are usually two
bells, placed very near one another, and the armature attached to the
bell clapper is so arranged between the poles of the double-bell
magnets, that it strikes alternately the one and the other, so that a
clear ringing is kept up as long as the handle is being turned at the
generator.

[Illustration: Fig. 92.]

[Illustration: Fig. 93.]

[Illustration: Fig. 94.]

If a _combined_ generator and bell be fitted at each end of a line, it
becomes possible to communicate both ways; one terminal of each
instrument must be connected to the line, and the other terminal on each
to earth. A combined generator and bell is shown at Fig. 94. These
instruments are always ready for use, require no battery or
press-buttons. The generator, Fig. 92, will ring seven bells
simultaneously, if required, so powerful is the current set up; and by
using a switch any number of bells, placed in different positions, can
be rung, by carrying a separate wire from the switch to the bell.

[Illustration: Fig. 95.]


§ 75. Our work would not be complete unless we pointed out the means
necessary to detect faults in our work. In order to localise faults, two
things are requisite: first, a means of knowing whether the battery
itself is working properly, that is to say, giving the due _amount_ of
current of the right _pressure_, or E.M.F.; secondly, a means of
detecting whether there is leakage, or loss of current, or break of
circuit in our lines. Fortunately, the means of ascertaining these data
can be all combined in one instrument, known as a linesman's
galvanometer or detector, of which we give an illustration at Fig. 95.
It will be remembered (§ 10) that if a current be passed over or under a
poised magnetic needle, parallel to it, the needle is immediately
deflected out of the parallel line, and swings round to the right or
left of the current, according to the _direction_ of the current;
likewise that the needle is deflected farther from the original position
as the current becomes stronger. The deflections, however, are not
proportionate to the strength of the current, being fairly so up to
about 25 to 30 degrees of arc out of the original position, but being
very much less than proportionate to the current strength as the needle
gets farther from the line of current; so that a current of infinite
strength would be required to send the needle up to 90°. On this
principle the detector is constructed. It consists of a lozenge-shaped
magnetic needle, suspended vertically on a light spindle, carrying at
one end a pointer, which indicates on a card, or metal dial, the
deflection of the needle. Behind the dial is arranged a flat upright
coil of wire (or two coils in many cases) parallel to the needle, along
which the current to be tested can be sent. The needle lies between the
front and back of the flat coil. The whole is enclosed in a neat wooden
box, with glazed front to show the dial, and binding screws to connect
up to the enclosed coil or coils. If the coil surrounding the needle be
of a few turns of coarse wire, since it opposes little resistance to the
passage of the current, it will serve to detect the presence of large
_quantities_ of electricity (many ampères) at a low pressure; this is
called a _quantity_ coil. If, on the other hand, the coil be one of fine
wire, in many convolutions, as it requires more _pressure_, or E.M.F.,
or "intensity" to force the current through the fine high-resistance
wire, the instrument becomes one fitted to measure the voltage or
_pressure_ of the current, and the coil is known as the "intensity." If
both coils are inserted in the case, so that either can be used at will,
the instrument is capable of measuring either the quantity of
electricity passing, or the pressure at which it is sent, and is then
known as a quantity and intensity detector. No two galvanometers give
exactly the same deflection for the same amount of current, or the same
pressure; the fitter will therefore do well to run out a little table
(which he will soon learn by heart) of the deflection _his_ instrument
gives with 1, 2, 3, 4, 5 and 6 Leclanché's _coupled in parallel_, when
connected with the quantity coil. He will find the smaller sizes give
less current than the larger ones. In testing the deflections given by
the intensity coil, he must remember to couple his cells _in series_, as
he will get no increase in _tension_ or _pressure_ by coupling up in
parallel. In either case the cells should be new, and freshly set up,
say, within 24 hours. As some of my readers may like to try their skill
at constructing such a detector, I transcribe the directions given in
"Amateur work" by Mr. Edwinson:--


§ 76. "Such an instrument, suitable for detecting the currents in an
electric bell circuit, may be made up at the cost of a few shillings for
material, and by the exercise of a little constructive ability. We shall
need, first of all, a magnetised needle; this can be made out of a piece
of watch spring. Procure a piece of watch spring two inches long, soften
it by heating it to redness, and allowing it to cool gradually in a bed
of hot ashes; then file it up to the form of a long lozenge, drill a
small hole in the centre to receive the spindle or pivot, see that the
needle is quite straight, then harden it by heating it again to a bright
red and plunging it at once into cold water. It now has to be
magnetised. To do this, rub it on a permanent horse-shoe, or other
magnet, until it will attract an ordinary sewing needle strongly, or
wrap it up in several turns of insulated line wire, and send many jerky
charges of electricity from a strong battery through the wire. When it
has been well magnetised, mount it on a spindle of fine hard wire, and
secure it by a drop of solder. We will next turn our attention to the
case, bobbin, or chamber in which the needle has to work. This may be
made out of cardboard entirely, or the end pieces may be made of ivory
or ebonite, or it may be made out of thin sheet brass; for our purpose
we will choose cardboard. Procure a piece of stout cardboard 4-3/4
inches long by 2 inches wide, double it to the form of a Tãndstickor
match-box, and pierce it in exactly opposite sides, and in the centre of
those sides with holes for the needle spindle. Now cut another piece of
stout, stiff cardboard 2-3/4 inches long by 3/4 inch wide, and cut a
slit with a sharp knife to exactly fit the ends of the case or body
already prepared. The spindle holes must now be bushed with short
lengths of hard brass or glass bugles, or tubing, made to allow the
spindle free movement, and these secured in position by a little melted
shellac, sealing-wax, or glue. The needle must now be placed in the
case, the long end of the spindle first, then the short end in its
bearing; then, whilst the case with the needle enclosed is held between
the finger and thumb of the left hand, we secure the joint with a little
glue or with melted sealing-wax. The end-pieces are now to be put on,
glued, or sealed in position, and set aside to get firm, whilst we turn
our attention to other parts. The case, 5 inches by 4 inches by 2 inches
in depth, may be improvised out of an old cigar-box, but is best made of
thin mahogany or teak, nicely polished on the outside, and fitted with a
cover sliding in a groove, or hinged to form the back of the instrument.
The binding screws should be of the pattern known as the telegraph
pattern, fitted with nuts, shown at Fig. 27. A small brass handle to be
fitted to the top of the instrument, will also be handy. A circular
piece of smooth cardboard 3-1/4 inches in diameter, with a graduated
arc, marked as shown in Fig. 95, will serve the purpose of a dial, and a
piece of thin brass, bent to the form of [box open down], will be
required as a needle guard. The face of the dial may be a circular piece
of glass, held in a brass ogee, or a hole the size of the dial may be
cut in a piece of thin wood; this, glazed on the inside with a square of
glass, may be made to form the front of the instrument over the dial. An
indicating needle will also be required for an outside needle; this is
usually made of watch spring, and nicely blued; but it may be made of
brass or any other metal, one made of aluminium being probably the best
on account of its lightness. It must be pierced with a hole exactly in
the centre, so as to balance it as the beam of scales should be
balanced, and should one end be heavier than the other it must be filed
until they are equal.

We will now turn our attention to the coil.

Procure sixpennyworth of No. 36 silk-covered copper wire and wind three
layers of it very evenly on the coil case or bobbin, being careful in
passing the needle spindle not to pinch it or throw it out of truth.
When this has been wound on, it will be found that one end of the wire
points to the left and the other end to the right. These are destined to
be connected to the under side of the binding screws shown on the top of
Fig. 95. We therefore secure them to their respective sides with a touch
of sealing wax, and leave enough wire free at the ends to reach the
binding screws--say, about 6 inches. It is handy to have an additional
coil for testing strong currents, and as this may be combined in one
instrument at a trifle additional cost, we will get some line wire (No.
22) and wind six or eight turns of it around the coil outside the other
wire; one end of this wire will be attached to an additional binding
screw placed between the others, and the other end to left binding screw
shown. The coil thus prepared may now be mounted in position. Pierce the
board dial and the wood at its back with a hole large enough for the
needle spindle to pass through from the back to the centre of the dial.
See that the thick end of the inside needle hangs downwards, then place
the coil in the position it is intended to occupy, and note how far the
needle spindle protrudes on the face of the dial. If this is too long,
nip off the end and file it up taper and smooth until it will work
freely in a hole in the needle guard, with all parts in their proper
places. This being satisfactory, secure the coil in its place by sealing
wax, or, better still, by two thin straps of brass, held by screws at
each end, placed across the coil. Now clean the free ends of the coil
wires, insert them under the nuts of the binding screws, fix the
indicating needle on the end of the spindle outside, and see that it
hangs in a vertical position with the inside needle when the instrument
is standing on a level surface. Secure it in this position, screw on the
needle guard, fasten on the glass face, and the instrument will be
complete.


§ 77. Provided thus with an efficient detector, the fitter may proceed
to test his work. In cases of _new installations_, take the wire off
the carbon binding screw of the battery and attach it to one screw of
the galvanometer (on the intensity coil side), next attach a piece of
wire from the other binding screw of the galvanometer (the central one)
so as to place the galvanometer in circuit. _There should be no movement
of the needle_, and in proportion to the deflection of the needle, so
will the loss or waste be. If loss is going on, every means must be used
to remedy it. It is of the utmost importance to the effective working of
the battery and bells that not the _slightest leakage_ or _local action_
should be allowed to remain. However slight such loss may be, it will
eventually ruin the battery. Let damp places be sought out, and the
wires removed from near them. Bad or injured coverings must also be
looked for, such as may have been caused by roughly drawing the wires
across angular walls, treading on them, or driving staples too tightly
over them. Two or more staples may be touching, or two or more wires
carelessly allowed to lie under one staple. The wire may have been bared
in some places in passing over the sharp edges of the zinc tube. The
backs of the pushes should be examined to see if too much wire has been
bared, and is touching another wire at the back of the push-case itself.
Or the same thing may be taking place at the junction with the relays or
at the indicator cases. Should the defect not be at any of these places,
the indicator should next be examined, and wire by wire detached (not
cut) until the particular wire in which the loss is going on has been
found. This wire should then be traced until the defect has been
discovered. In testing underground wires for a loss or break, it will be
necessary first to uncouple the _distant_ end, then to disconnect the
other end from the instruments, and attach the wire going underground to
the screw of the galvanometer. A piece of wire must then be taken from
the other screw of the detector to the carbon end of the battery, and a
second wire from the zinc end of the battery to the earth plate or other
connection. Proceeding to that part of the wire where the injury is
suspected, the wire is taken up, and a temporary earth connection having
been made (water main, gas pipe, etc.), and by means of a sharp knife
connected with this latter, the covering of the suspected wire
penetrated through to the wire, so as to make a good connection between
this suspected wire and the temporary earth plates. If, when this is
done, the needle is deflected fully, the injury is farther away from the
testing end, and other trials must be made farther on, until the spot is
discovered. Wherever the covering of the wire has been pierced for
testing, it must be carefully recovered, finished off with Prout's
elastic glue, or gutta-percha, and made quite sound. The connections
with the earth plates very frequently give trouble, the wires corrode or
become detached from the iron pipes etc., and then the circuit is
broken.


§ 78. When the fitter is called to localise defects which may have
occurred in an installation which has been put up some time, before
proceeding to work let him ask questions as to what kind of defect there
is, and when and where it evinces itself. If all the bells have broken
down, and will not ring, either the battery or the main go and return
wires are at fault. Let him proceed to the battery, examine the binding
screws and connected wires for corrosion. If they are all right, let the
batteries themselves be tested to see if they are giving the right
amount of current. This should be done with the quantity coil of the
detector. Should the battery be faulty, it will be well to renew the
zincs and recharge the battery, if the porous cell be still in good
condition; if not, new cells should be substituted for the old ones.
Should the battery be all right, and still none of the bells ring, a
break or bad contact, or short circuit in the main wires near the
battery may be the cause of the mischief. If some bell rings
continuously, there must be a short circuit in the push or pushes
somewhere; the upper spring of one of the pushes may have got bent, or
have otherwise caught in the lower spring. _Pulls_ are very subject to
this defect. By violent manipulations on the part of mischievous butcher
or baker boys, the return spring may be broken, or so far weakened as
not to return the pull into the "off" position. If, the batteries being
in good order, any bell rings feebly, there is either leakage along its
line, or else bad contact in the push or in the connections of the wires
to and from the push. There should be platinum contacts at the ends of
the push springs; if there are not, the springs may have worked dirty at
the points of contact, hence the poor current and poor ringing. It is
seldom that the bells themselves, unless, indeed, of the lowest
quality, give any serious trouble. Still the set screw may have shaken
loose (which must then be adjusted and tightened up), or the platinum
speck has got solder on its face and therefore got oxidised. This may be
scraped carefully with a penknife until bright. Or, purposely or
inadvertently, no platinum is on the speck at all, only the solder. A
piece of platinum foil should be soldered on the spot, if this is so. Or
again (and this only in very bad bells), the electro-magnets being of
hard iron, may have retained a certain amount of _permanent magnetism_,
and pull the armature into permanent contact with itself. This can be
remedied by sticking a thin piece of paper (stamp paper will do) over
the poles of the magnet, between them and the armature. In no case
should the fitter _cut_ or _draw up_ out of tubes, etc., any wire or
wires, without having first ascertained that the fault is in that wire;
for, however carefully joints are made, it is rare that the jointed
places are so thoroughly insulated as they were before the cutting and
subsequent joining were undertaken. To avoid as much as possible cutting
uselessly, let every binding screw be examined and tightened up, and
every length of wire, which it is possible to get at, be tested for
continuity before any "slashing" at the wires, or furious onslaughts on
the indicator be consummated.

In conclusion, I beg to record my thanks for the very generous
assistance which I have received in the compilation of the foregoing
pages from the electrical firms of Messrs. Blakey Emmot, Binswanger,
Gent, Judson, Jensen, and Thorpe.



ADDENDUM.

THE GASSNER BATTERY.


Since the compilation of the foregoing pages, a _dry battery_, known by
the above name, has found great favour with electric-bell fitters. Its
peculiarity consists in the zinc element forming the outside cell. In
this is placed the carbon, which is separated from the zinc by a thick
paste or jelly made of gypsum and oxide of zinc. The cell can be placed
in any position, works as well on its side as upright, is not subject to
creeping, has an E.M.F. of about 1·5 volt, with an internal resistance
of only 0·25 ohm in the round form, and 0·6 in the flat form. The
Gassner dry battery polarizes much less quickly than the ordinary
Leclanché. The only defects at present noticeable, are the flimsy
connections, and the fact that the outer cases being _metal_ must be
carefully guarded from touching one another. This can be effected by
enclosing in a partitioned _wooden box_.



INDEX.


  A.

  Acid, Chromic, 33, 46

  ---- Hydrobromic, 20

  ---- Hydrochloric, 20

  ---- Hydriodic, 20

  ---- Nitric, 20

  ---- Sulphuric, 20

  Action in Bichromate, 47

  ---- Dotting, 116

  ---- of electric bell, 81

  ---- Leclanché, 35

  ---- Relay, 134

  ---- Rubbing, 116

  ---- of zinc on acids, 21

  Agglomerate block, 38

  ---- Cell, 38

  ---- Compo, 38

  Alarms, Burglar, 113

  ---- Fire, 123

  ---- Frost, 121

  ---- Thermometer, 122

  ---- Thief, 113

  ---- Watch, 124

  Amber, 1

  Ampère, 55

  Ampère's law, 11

  Annealing iron, 13

  Arrangement of bells for lifts, 171

  ---- Ships, 170

  Attraction, 3


  B.

  Batteries, 18

  Battery agglomerate, 39

  Battery, Bichromate, 48

  ---- Bunsen, 33

  ---- Chromic acid, 46

  ---- Daniell's, 29

  ---- Gassner (addendum), 186

  ---- Gent's, 44

  ---- Gravity, 31

  ---- Modified, 120

  ---- Grenet, 46

  ---- Grove, 33

  ---- Judson's, 41

  ---- Leclanché, 33

  ---- Reversed, 46

  ---- Minotto, 31

  ---- Smee's, 27

  ---- Walker's, 27

  Bell action, case for, 88

  Blocks, wooden, 150

  Bobbins, electric bell, 67

  Box for batteries, 43

  Brushes, dynamo, 17


  C.

  Cable, many stranded, 174

  Case for bell action, 88

  Cells in parallel, 57

  ---- series, 53

  Charging fluid, recipes, 48

  ---- Fuller, 49

  Circuits, closed, 52, 118

  ---- Of bells complete in house, 168

  ---- For signalling, 167

  ---- In both directions, 168

  Circuits of bells with Morse key, 165
    In parallel, 161
    Series, 162
    With relay, 164
    Single bell and wire, 159
    Earth, 160
    Two pushes, 161
    Push and pull, 161
    Open, 52

  Closed circuit system, 118

  Code for signalling, 130

  Coil spring, 108

  Conductors, 3

  Connecting up, 144, 159

  Contacts, burglar alarm, 113
    Door, 116
    Drawer, 121
    Floor, 113
    For closed circuits, 121
    Mackenzie's humming, 113
    Shop door, 116
    Till, 121
    Watch alarm, 124
    Window sash, 116

  Corrugated carbons, 41

  Creeping in cells, 43
    To remedy, 44

  Callow's attachment, 99

  Current, 54
    To ring bell, 145


  D.

  Daniell's cell, 29
    Action in, 29

  Deflection of needle, 9, 11

  Detector or galvanometer, to make, 178

  Detent lever, 94

  Door contact, 116

  Dotting action, 116

  Drawing out plans, 169

  Dynamo, 15
    Armature, 16
    Brushes, 17
    Commutator, 17

  Dynamo, Cumulative effects, 17
    Field magnets, 16


  E.

  Earth, 52
    Plate, 53
    Return, 153

  Electric bell, action of, 81
    Armature, 74
    Base, 61
    Bobbins, 67
    Contact screw, 75
    Continuous, 92
    Circular bell, 106
    Gong, 77
    How to make, 60
    In lifts, 171
    Ships, 170
    Jensen's, 101
    Joining E. M. wire, 73
    Magnets, 63
    Magneto, 174
    Mining, 106
    Paraffining, 69
    Platinum tip, 76
    Putting together, 78
    Single stroke, 91
    Spring, 74
    Thorpe's, 100
    Trembling, 81, 90
    Winding wire on, 71
    Wire for, 69
    Trumpet, 107

  Electricity, sources of, 2

  Electrodes, 26

  Electro-motive force, 51

  Electron, 1

  E.M.F., 51

  Excitation, 6


  F.

  Faults to detect, 182

  Fire alarms, 123

  Floor contacts, 113

  Frost alarms, 121

  Fuller charging, 49


  G.

  Galvanometer, 176

  Gas evolved, 18

  Gassner battery (addendum), 186

  Generator (magneto), 174

  Gent's battery, 44

  Glue, Prout's elastic, 148

  Graphite, 27

  Gravity battery, 31
    Daniell battery, 31
    Modified, 120

  Grenet battery, 46

  Grove battery, 33

  Gutta-percha, 148


  I.

  Indicator, 135
    Automatic, 138
    Drop, 136
    Electric replacement, 136
    Gent's, 140
    Tripolar, 143
    Mechanical  replacement, 136
    Mode of coupling up, 142
    Pendulum, 139
    Polarised, 139
    Self replacing, 136
    Semaphore, 136

  Inductor, 174

  Insulation, 68

  Insulators, 4

  Internal resistance, 56

  Interior of push, 151

  Iron, importance of soft, 65
    Yoke, 66


  J.

  Jensen's bell, 101

  Joining wires to push, 151

  Judson's cell, 41


  K.

  Key, Morse, 129


  L.

  Leakage, 52

  Leclanché cell, 33
    reversed, 46

  Legge's contact, 115

  Lever switches, 128

  Lifts, bells for, 171

  Localising faults, 144, 175

  Lodge bell, 169


  M.

  Magnetic field, 14

  Magneto bells, 175
    Electric machines, 14, 15

  Magnets, 13

  Magnets producing electricity, 14

  Magnetisation of iron, 12
    Steel, 13

  Manganese oxide, 33

  Minotto cell, 31

  Modified gravity battery, 120

  Morse key, 129

  Musical instrument, novel, 108


  N.

  Negative electricity, 7

  Non-conductors, 3

  Novel musical instrument, 108


  O.

  Ohm, 55

  Ohm's law, 55

  Open circuit, 52

  Overhead lines, 152


  P.

  Paraffin, 69, 170

  Percha, gutta, 148

  Plans, drawing out, 169

  Platinum, riveting, 76

  Platinum, use of, 76

  Plug switches, 128

  Polarisation, 26

  Positive electricity, 7

  Proportions of bell parts, table of, 89

  Pressels, 111

  Prout's elastic glue, 148

  Pulls, 111

  Push, 92, 151, 109
    Interior of, 151
    Joining wires to, 151


  R.

  Relay, 96, 133
    Action of, 134

  Repulsion, 3

  Resinous electricity, 7

  Resistance of wire, table of, 146

  Return current, 153

  Riveting platinum, 76

  Rubbing action, 116


  S.

  Ships, bells for, 170

  Shop door contact, 116

  Signalling by bells, 130
    Code, 130

  Silver platinised, 27

  Single cell, 9

  Sizes of Leclanché's, 42

  Smee's cell, 27

  Spring coil, 108

  Standard size of wires, 146

  Switches, lever, 128
    Plug, 128


  T.

  Table of batteries, E.M.F. and R., 58
    Conductors and insulators, 4, 68
    Metals in acid, 8

  Table of Proportions of bell parts, 89
    Wire resistance, etc., 146

  Testing new work, 182
    Old, 183

  Thermometer alarms, 122

  Thorpe's Ball, 100


  U.

  Use of platinum, 76


  V.

  Vitreous electricity, 7

  Volt, 53


  W.

  Walker's cell, 27

  Watchman's clock, 124

  Water level indicator, 127

  Washer, insulating, 77

  Window sash contact, 116

  Wiping contact, 102

  Wire covering, 147
    In iron pipes, 152
    In wooden boxes, 152
    Iron, 152
    Joining, 148
    To push, 151
    Laying in tubes, 149
    Leading, 147, 150
    Overhead, 152
    Resistance, table of, 146
    Return, 147, 150
    Soldering iron, 148
    Tinned, 147
    Underground, 152

  Wiring, general instructions, 155
    Up, 144


  Z.

  Zinc, amalgamated, 22
    Blacking, 45
    Consumption, 21
    Commercial, 19
    Pure, 19


WILLIAM RIDER AND SON, PRINTERS, LONDON.

       *       *       *       *       *

  _Small crown 8vo, cloth._         _With many Illustrations._


             WHITTAKER'S LIBRARY OF ARTS, SCIENCES,
                  MANUFACTURES AND INDUSTRIES.

    MANAGEMENT OF ACCUMULATORS AND PRIVATE ELECTRIC LIGHT INSTALLATIONS.

    A Practical Handbook by Sir DAVID SALOMONS, Bart., M. A.

    4th Edition, Revised and Enlarged, with 32 Illustrations. Cloth 3s.

    "To say that this book is the best of its kind would be a poor
    compliment, as it is practically the only work on accumulators that
    has been written."--_Electrical Review._

    ELECTRICAL INSTRUMENT-MAKING FOR AMATEURS. A Practical Handbook. By
    S. R. BOTTONE, Author of "The Dynamo," &c. With 60 Illustrations.
    Second Edition. Cloth 3s.

    ELECTRIC BELLS. By S. R. BOTTONE. With numerous Illustrations.


IN PREPARATION.

    THE PROTECTION OF BUILDINGS FROM LIGHTNING. A Treatise on the Theory
    of Lightning Conductors from a Modern Point of View. Being the
    substance of two lectures delivered before the Society of Arts in
    March, 1888. By OLIVER J. LODGE, LL.D., D.Sc, F.R.S., Professor of
    Physics in University College, Liverpool.

    Published with various amplifications and additions, with the
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    ELECTRICAL INFLUENCE MACHINES: Containing a full account of their
    historical development, their modern Forms, and their Practical
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    ELECTRICAL ENGINEERING IN OUR WORKSHOPS. A Practical Handbook. By
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                                                        [_Ready Shortly_

       *       *       *       *       *


Transcriber's Note

Page 12: changed "guage" to "gauge" (... cotton-covered copper wire,
say No. 20 gauge ...)

Page 35: changed "change" to "charge" (... losing at the same time its
electrical charge ...)

Page 55: changed "guage" to "gauge" (... 1 foot of No. 41 gauge pure
copper wire ...)

Page 64: changed "exaet" to "exact" (... of the exact diameter of the
turned ends of the cores ...)

Page 73: moved comma "Rivetting, is" to "Rivetting is," (Rivetting, is
perhaps, the best mode ...)

Page 81: added hyphen (... along the short length of wire to the
right-hand binding-screw ...)

Page 83: changed "head" to "heads" (... the possible defects of
electric bells may be classed under four heads: ...)

Page 92: changed "its" to "it" (... until it rests against the stop or
studs.)

Page 102: changed "contract-breaker" to "contact-breaker" (When the
contact-breaker is used, ...)

Page 103: changed "instead" to "Instead" (Instead of the armature and
clapper ...)

Page 132: in the Morse code for "BRING THE", the code for "H" has been
corrected from two dots to four dots.

Page 136: changed "eletro-magnet" to "electro-magnet" (... if the
electro-magnet were energised ...)

Page 137: changed "idicator" to "indicator" (since the indicator falls
forwards)

Page 146: changed "arrangment" to "arrangement" (the size and
arrangement of the batteries and wires)

Page 146: added comma "nails," (... chance contact with nails,
staples, metal pipes or other wires ...)

Page 179: changed "carboard" to "cardboard" (... for our purpose we
will choose cardboard.)

Page 179: changed "Tanstickor" to "Tãndstickor" (... double it to the
form of a Tãndstickor match-box, ...)

Page 185: suspected typo (unchanged) "Emmot" should perhaps be
"Emmott" (... the electrical firms of Messrs. Blakey Emmot, ...)

Page 186: changed "Leclanchè" to "Leclanché" (... polarizes much less
quickly than the ordinary Leclanché.)

Page 187: changed two instances of "Ampére" to "Ampère" in the index
(Ampère, 55 / Ampère's law, 11)





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