A better arrangement for accomplishing the automatic switching on the
part of the generator is to make no use of the crank shaft as a part
of the conducting path as is the case in both Figs. 74 and 75, but to
make the crank shaft, by its longitudinal movement, impart the
necessary motion to a switch spring which, in turn, is made to engage
or disengage a corresponding contact spring. An arrangement of this
kind that is in common use is shown in Fig. 76. This needs no further
explanation than to say that the crank shaft is provided on its end
with an insulating stud _1_, against which a switching spring _2_
bears. This spring normally rests against another switch spring _3_,
but when the generator crank shaft moves to the right upon the turning
of the crank, the spring _2_ disengages spring _3_ and engages spring
_4_, thus completing the circuit of the generator armature. It is seen
that this operation accomplishes the breaking of one circuit and the
making of another, a function that will be referred to later on in
this work.
[Illustration: Fig. 76. Generator Cut-in Switch]
Pulsating Current. Sometimes it is desirable to have a generator
capable of developing a pulsating current instead of an alternating
current; that is, a current which will consist of impulses all in one
direction rather than of impulses alternating in direction. It is
obvious that this may be accomplished if the circuit of the generator
be broken during each half revolution so that its circuit is completed
only when current is being generated in one direction.
Such an arrangement is indicated diagrammatically in Fig. 77. Instead
of having one terminal of the armature winding brought out through the
frame of the generator as is ordinarily done, both terminals are
brought out to a commuting device carried on the end of the armature
shaft. Thus, one end of the loop representing the armature winding is
shown connected directly to the armature pin _1_, against which bears
a spring _2_, in the usual manner. The other end of the armature
winding is carried directly to a disk _3_, mounted _on_ but insulated
_from_ the shaft and revolving therewith. One-half of the
circumferential surface of this disk is of insulating material _4_ and
a spring _5_ rests against this disk and bears alternately upon the
conducting portion _3_ or the insulating portion _4_, according to the
position of the armature in its revolution. It is obvious that when
the generator armature is in the position shown the circuit through it
is from the spring _2_ to the pin _1_; thence to one terminal of the
armature loop; thence through the loop and back to the disk _3_ and
out by the spring _5_. If, however, the armature were turned slightly,
the spring _5_ would rest on the insulating portion _4_ and the
circuit would be broken.
[Illustration: Fig. 77. Pulsating-Current Commutator]
[Illustration: Fig. 78. Generator Symbols]

This instrument was ordinarily mounted in a wooden box together with
the induction coil, which is shown in the upper portion of the figure.
The Blake transmitter has passed almost entirely out of use in this
country, being superseded by the various forms of granular
instruments, which, while much more powerful, are not perhaps capable
of producing quite such clear and distinct articulation.
The great trouble with the single-contact transmitters, such as the
Blake, was that it was impossible to pass enough current through the
single point of contact to secure the desired power of transmission
without overheating the contact. If too much current is sent through
such transmitters, an undue amount of heat is generated at the point
of contact and a vibration is set up which causes a peculiar humming
or squealing sound which interferes with the transmission of other
sounds.
Multiple Electrode. To remedy this difficulty the so-called
multiple-electrode transmitter was brought out. This took a very great
number of forms, of which the one shown in Fig. 39 is typical. The
diaphragm shown at _1_, in this particular form, was made of thin pine
wood. On the rear side of this, suspended from a rod _3_ carried in a
bracket _4_, were a number of carbon rods or pendants _5_, loosely
resting against a rod _2_, carried on a bracket _6_ also mounted on
the rear of the diaphragm. The pivotal rod _3_ and the rod _2_,
against which the pendants rested, were sometimes, like the pendant
rods, made of carbon and sometimes of metal, such as brass. When the
diaphragm vibrated, the intimacy of contact between the pendant rod
_5_ and the rod _2_ was altered, and thus the resistance of the path
through all of the pendant rods in multiple was changed.
[Illustration: Fig. 39. Multiple-Electrode Transmitter]
A multitude of forms of such transmitters came into use in the early
eighties, and while they in some measure remedied the difficulty
encountered with the Blake transmitter, _i.e._, of not being able to
carry a sufficiently large current, they were all subject to the
effects of extreme sensitiveness, and would rattle or break when
called upon to transmit sounds of more than ordinary loudness.
Furthermore, the presence of such large masses of material, which it
was necessary to throw into vibration by the sound waves, was
distinctly against this form of transmitter. The inertia of the moving
parts was so great that clearness of articulation was interfered with.
Granular Carbon. The idea of employing a mass of granular carbon,
supported between two electrodes, one of which vibrated with the sound
waves and the other was stationary, was proposed by Henry Hunnings in
the early eighties. While this idea forms the basis of all modern
telephone transmitters, yet it did not prevent the almost universal
adoption of the single-contact form of instrument during the next
decade.
Western Electric Solid-Back Transmitter. In the early nineties,
however, the granular-carbon transmitter came into its own with the
advent and wide adoption of the transmitter designed by Anthony C.
White, known as the _White_, or _solid-back_, transmitter. This has
for many years been the standard instrument of the Bell companies
operating throughout the United States, and has found large use
abroad. A horizontal cross-section of this instrument is shown in Fig.
40, and a rear view of the working parts in Fig. 41. The working parts
are all mounted on the front casting _1_. This is supported in a cup
_2_, in turn supported on the lug _3_, which is pivoted on the
transmitter arm or other support. The front and rear electrodes of
this instrument are formed of thin carbon disks shown in solid black.
The rear electrode, the larger one of these disks, is securely
attached by solder to the face of a brass disk having a rearwardly
projecting screw-threaded shank, which serves to hold it and the rear
electrode in place in the bottom of a heavy brass cup _4_. The front
electrode is mounted on the rear face of a stud. Clamped against the
head of this stud, by a screw-threaded clamping ring _7_, is a mica
washer, or disk _6_. The center portion of this mica washer is
therefore rigid with respect to the front electrode and partakes of
its movements. The outer edge of this mica washer is similarly clamped
against the front edge of the cup _4_, a screw-threaded ring _9_
serving to hold the edge of the mica rigidly against the front of the
cup. The outer edge of this washer is, therefore, rigid with respect
to the rear electrode, which is fixed. Whatever relative movement
there is between the two electrodes must, therefore, be permitted by
the flexing of the mica washer. This mica washer not only serves to
maintain the electrodes in their normal relative positions, but also
serves to close the chamber which contains the electrodes, and,
therefore, to prevent the granular carbon, with which the space
between the electrodes is filled, from falling out.
[Illustration: Fig. 40. White Solid-Back Transmitter]

Continuing the circuit through the winding itself, it passes to the
center pin projecting from the left-hand end of the armature shaft;
thence to the spring _4_ which rests against this pin; and thence to
the terminal wire _3_.
Normally, this path is shunted by what is practically a short circuit,
which may be traced from the terminal _2_ through the frame of the
generator to the crank shaft _5_; thence to the upper end of the
spring _4_ and out by the terminal wire _3_. This is the condition
which ordinarily exists and which results in the removal of the
resistance and the impedance on the armature winding from any circuit
in which the generator is placed, as long as the generator is not
operated.
An arrangement is provided, however, whereby the crank shaft _5_ will
be withdrawn automatically from engaging with the upper end of the
spring _4_, thus breaking the shunt around the armature circuit,
whenever the generator crank is turned. In order to accomplish this
the crank shaft _5_ is capable of partial rotation and of slight
longitudinal movement within the hub of the large gear wheel. A spring
7 usually presses the crank shaft toward the left and into engagement
with the spring _4_. A pin _8_ carried by the crank shaft, rests in a
V-shaped notch in the end of the hub _6_ and as a result, when the
crank is turned the pin rides on the surface of this notch before the
large gear wheel starts to turn, and thus moves the crank shaft _5_ to
the right and breaks the contact between it and the spring _4_. Thus,
as long as the generator is being operated, its armature is connected
in the circuit of the line, but as soon as it becomes idle the
armature is automatically short-circuited. Such devices as this are
termed _automatic shunts_.
In still other cases it is desirable to have the generator circuit
normally open so that it will not affect in any way the electrical
characteristics of the line while the line is being used for talking
In this case the arrangement is made so that the generator will
automatically be placed in proper circuit relation with the line when
it is operated.
[Illustration: Fig. 75. Generator Cut-in Switch]
A common arrangement for doing this is shown in Fig. 75, wherein the
spring _1_ normally rests against the contact pin of the armature and
forms one terminal of the armature circuit. The spring _2_ is adapted
to form the other terminal of the armature circuit but it is normally
insulated from everything. The circuit of the generator is, therefore,
open between the spring _2_ and the shaft _3_, but as soon as the
generator is operated the crank shaft is bodily moved to the left by
means of the =V=-shaped notch in the driving collar _4_ and is thus
made to engage the spring _2_. The circuit of the generator is then
completed from the spring _1_ through the armature pin to the armature
winding; thence to the frame of the machine and through shaft _3_ to
the spring _2_. Such devices as this are largely used in connection
with so-called “bridging” telephones in which the generators and bells
are adapted to be connected in multiple across the line.

Manufacturing Company, of Buffalo, New York. The circuits of such sets
do not differ materially from those of the ordinary desk telephone
set.
[Illustration: Fig. 160. Kellogg Common-Battery Desk Set]
[Illustration: Fig. 161. Dean Common-Battery Set]
Circuits of Common-Battery Telephone Sets. The complete circuits of
the Kellogg desk-stand arrangement are shown in Fig. 160, the
desk-stand parts being shown at the left and the bell-box parts at
the right. As is seen, but two conductors extend from the former to
the latter. A simplified theoretical sketch is also shown in the upper
right-hand corner of this figure.
The details of the common-battery telephone circuits of the Dean
Electric Company are shown in Fig. 161. This involves the use of the
balanced Wheatstone bridge. The only other thing about this circuit
that needs description, in view of what has previously been said about
it, is that the polarized bell is placed in series with a condenser so
that the two sides of the circuit may be insulated from each other
while the telephone is not in use, and yet permit the passage of
ringing current through the bell.
[Illustration: Fig. 162. Monarch Common-Battery Wall Set]
The use of the so-called direct-current receiver has brought about a
great simplification in the common-battery telephone circuits of
several of the manufacturing companies. By this use the transmitter
and the receiver are placed in series across the line, this path being
normally opened by the hook-switch contacts. The polarized bell and
condenser are placed in another bridge path across the line, this path
not being affected by the hook-switch contacts. All that there is to
such a complete common-battery telephone set, therefore, is a
receiver, transmitter, hook switch, bell, condenser, and cabinet, or
other support.
The extreme simplicity of the circuits of such a set is illustrated in
Fig. 162, which shows how the Monarch Telephone Manufacturing Company
connect up the various parts of their telephone set, using the
direct-current receiver already described in connection with Fig. 54.
[Illustration: VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]
CHAPTER XV
NON-SELECTIVE PARTY-LINE SYSTEMS
A party line is a line that is for the joint use of several stations.
It is, therefore, a line that connects a central office with two or
more subscribers stations, or where no central office is involved, a
line that connects three or more isolated stations with each other.
The distinguishing feature of a party line, therefore, is that it
serves more than two stations, counting the central office, if there
is one, as a station.
Strictly speaking, the term _party_ line should be used in
contradistinction to the term _private_ line. Companies operating
telephone exchanges, however, frequently lease their wires to
individuals for private use, with no central-office switchboard
connections, and such lines are, by common usage, referred to as
“private lines.” Such lines may be used to connect two or more
isolated stations. A _private_ line, in the parlance of telephone
exchange working, may, therefore, be a _party_ line, as inconsistent
as this may seem.
A telephone line that is connected with an exchange is an exchange
line, and it is a party line if it has more than one station on it. It
is an individual line or a single party line if it has but a single
station on it. A line which has no central-office connection is called
an “isolated line,” and it is a party line if it has more than two
stations on it.
The problem of mere speech transmission on party lines is comparatively easy, being scarcely more complex than that involved in private or
single party lines. This is not true, however, of the problem of
signaling the various stations. This is because the line is for the
common use of all its patrons or subscribers, as they are termed, and
the necessity therefore exists that the person sending a signal, whether
operator or subscriber, shall be able in some way to inform a person at
the desired station that the call is intended for that station. There
are two general ways of accomplishing this purpose.
(_1_) The first and simplest of these ways is to make no provision for
ringing any one bell on the line to the exclusion of the others, and
thus allow all bells to ring at once whenever any station on the line
is wanted. Where this is done, in order to prevent all stations from
answering, it is necessary, in some way, to convey to the desired
station the information that the call is intended for that station,
and to all of the other stations the information that the call is not
intended for them. This is done on such lines by what is called “code
ringing,” the code consisting of various combinations of long and
short rings.
(_2_) The other and more complex way is to arrange for selective
ringing, so that the person sending the call may ring the bell at the
station desired, allowing the bells at all the other stations to
remain quiet.
[Illustration: Fig. 163. Grounded-Circuit Series Line]

In 1893, Oliver Heaviside proposed that the inductance of telephone
lines be increased above the amount natural for the inter-axial
spacing, with a view to counteracting the hurtful effects of the
capacity. His meaning was that the increased inductance–a harmful
quality in a circuit not having also a harmfully great capacity–would
act oppositely to the capacity, and if properly chosen and applied,
should decrease or eliminate distortion by making the lines effect on
fundamentals and harmonics more nearly uniform, and as well should
reduce the attenuation by neutralizing the action of the capacity in
dissipating energy.
There are two ways in which inductance might be introduced into a
telephone line. As the capacity whose effects are to be neutralized
is distributed uniformly throughout the line, the counteracting
inductance must also be distributed throughout the line. Mere increase
of distance between two wires of the line very happily acts both to
increase the inductance and to lower the capacity; unhappily for
practical results, the increase of separation to bring the qualities
into useful neutralizing relation is beyond practical limits. The
wires would need to be so far above the earth and so far apart as to
make the arrangement commercially impossible.
Practical results have been secured in increasing the distributed
inductance by wrapping fine iron wire over each conductor of the line.
Such a treatment increases the inductance and improves transmission.
The most marked success has come as a result of the studies of
Professor Michael Idvorsky Pupin. He inserts inductances in series
with the wires of the line, so adapting them to the constants of the
circuit that attenuation and distortion are diminished in a gratifying
degree. This method of counteracting the effects of a distributed
capacity by the insertion of localized inductance requires not only
that the requisite total amount of inductance be known, but that the
proper subdivision and spacing of the local portions of that
inductance be known. Professor Pupins method is described in a paper
entitled “Wave Transmission Over Non-uniform Cables and Long-Distance
Air Lines,” read by him at a meeting of the American Institute of
Electrical Engineers in Philadelphia, May 19, 1900.
NOTE. United States Letters Patent were issued to Professor Pupin
on June 19, 1900, upon his practical method of reducing
attenuation of electrical waves. A paper upon “Propagation of
Long Electric Waves” was read by Professor Pupin before the
American Institute of Electrical Engineers on March 22, 1899, and
appears in Vol. 15 of the Transactions of that society. The
student will find these documents useful in his studies on the
subject. He is referred also to “Electrical Papers” and
“Electromagnetic Theory” of Oliver Heaviside.
Professor Pupin likens the transmission of electric waves over
long-distance circuits to the transmission of mechanical waves over a
string. Conceive an ordinary light string to be fixed at one end and
shaken by the hand at the other; waves will pass over the string from
the shaken to the fixed end. Certain reflections will occur from the
fixed end. The amount of energy which can be sent in
this case from the shaken to the fixed point is small, but if the
string be loaded by attaching bullets to it, uniformly throughout its
length, it now may transmit much more energy to the fixed end.
[Illustration: MAIN ENTRANCE AND PUBLIC OFFICE, SAN FRANCISCO HOME

CHAPTER II
ELECTRICAL REPRODUCTION OF SPEECH
The art of telephony in its present form has for its problem so to
relate two diaphragms and an electrical system that one diaphragm will
respond to all the fundamental and harmonic vibrations beating upon it
and cause the other to vibrate in exact consonance, producing just
such vibrations, which beat upon an ear.
The art does not do all this today; it falls short of it in every
phase. Many of the harmonics are lost in one or another stage of the
process; new harmonics are inserted by the operations of the system
itself and much of the volume originally available fails to reappear.
The art, however, has been able to change commercial and social
affairs in a profound degree.
Conversion from Sound Waves to Vibration of Diaphragm. However
produced, by the voice or otherwise, sounds to be transmitted by
telephone consist of vibrations of the air. These vibrations, upon
reaching a diaphragm, cause it to move. The greatest amplitude of
motion of a diaphragm is, or is wished to be, at its center, and its
edge ordinarily is fixed. The diaphragm thus serves as a translating
device, changing the energy carried by the molecules of the air into
localized oscillations of the matter of the diaphragm. The waves of
sound in the air advance; the vibrations of the molecules are
localized. The agency of the air as a medium for sound transmission
should be understood to be one in which its general volume has no need
to move from place to place. What occurs is that the vibrations of the
sound-producer cause alternate condensations and rarefactions of the
air. Each molecule of the air concerned merely oscillates through a
small amplitude, producing, by joint action, shells of waves, each
traveling outward from the sound-producing center like rapidly growing
coverings of a ball.
Conversion from Vibration to Voice Currents. Fig. 1 illustrates a
simple machine adapted to translate motion of a diaphragm into an
alternating electrical current. The device is merely one form of
magneto telephone chosen to illustrate the point of immediate
conversion. _1_ is a diaphragm adapted to vibrate in response to the
sounds reaching it. _2_ is a permanent magnet and _3_ is its armature.

No definite limit may be set to apply to all conditions but it may be
safely stated that under ordinary circumstances no more than ten
stations should be placed on a non-selective line. Twenty stations
are, however, common, and sometimes forty and even fifty have been
connected to a single line. In such cases the confusion which results,
even if the talking and the ringing efficiency are tolerable, makes
the service over such overloaded lines unsatisfactory to all
concerned.
CHAPTER XVI
SELECTIVE PARTY-LINE SYSTEMS
The problem which confronts one in the production of a system of
selective ringing on party lines is that of causing the bell of any
chosen one of the several parties on a circuit to respond to a signal
sent out from the central office without sounding any of the other
bells. This, of course, must be accomplished without interfering with
the regular functions of the telephone line and apparatus. By this is
meant that the subscribers must be able to call the central office and
to signal for disconnection when desired, and also that the
association of the selective-signaling devices with the line shall not
interfere with the transmission of speech over the line. A great many
ways of accomplishing selective ringing on party lines have been
proposed, and a large number of them have been used. All of these ways
may be classified under four different classes according to the
underlying principle involved.
Classification. (_1_) _Polaritysystems are so called because they
depend for their operation on the use of bells or other responsive
devices so polarized that they will respond to one direction of
current only. These bells or other devices are so arranged in
connection with the line that the one to be rung will be traversed by
current in the proper direction to actuate it, while all of the others
will either not be traversed by any current at all, or by current in
the wrong direction to cause their operation.
(_2_) The _harmonic_ systems have for their underlying principle the
fact that a pendulum or elastic reed, so supported as to be capable of
vibrating freely, will have one particular rate of vibration which it
may easily be made to assume. This pendulum or reed is placed under
the influence of an electromagnet associated with the line, and owing
to the fact that it will vibrate easily at one particular rate of
vibration and with extreme difficulty at any other rate, it is clear
that for current impulses of a frequency corresponding to its natural
rate the reed will take up the vibration, while for other frequencies
it will fail to respond.

In general it may be said that the packing difficulty has been almost
entirely eliminated, not by the employment of remedial devices, such
as those often proposed for stirring up the carbon, but by preventing
the trouble by the design and manufacture of the instruments in such
forms that they will not be subject to the evil.
Carrying Capacity. Obviously, the power of a transmitter is
dependent on the amount of current that it may carry, as well as on
the amount of variation that it may make in the resistance of the path
through it. Granular carbon transmitters are capable of carrying much
heavier current than the old Blake or other single or multiple
electrode types. If forced to carry too much current, however, the
same frying or sizzling sound is noticeable as in the earlier types.
This is due to the heating of the electrodes and to small arcs that
occur between the electrodes and the granules.
One way to increase the current-carrying capacity of a transmitter is
to increase the area of its electrodes, but a limit is soon reached in
this direction owing to the increased inertia of the moving electrode,
which necessarily comes with its larger size.
The carrying capacity of transmitters may also be increased by
providing special means for carrying away the heat generated in the
variable-resistance medium. Several schemes have been proposed for
this. One is to employ unusually heavy metal for the electrode
chamber, and this practice is best exemplified in the White solid-back
instrument. It has also been proposed by others to water-jacket the
electrode chamber, and also to keep it cool by placing it in close
proximity to the relatively cool joints of a thermopile. Neither of
these two latter schemes seems to be warranted in ordinary commercial
practice.
Sensitiveness. In all the transmitters so far discussed damping
springs of one form or another have been employed to reduce the
sensitiveness of the instrument. For ordinary commercial use too great
a degree of sensitiveness is a fault, as has already been pointed out.
There are, however, certain adaptations of the telephone transmitter
which make a maximum degree of sensitiveness desirable. One of these
adaptations is found in the telephone equipments for assisting
partially deaf people to hear. In these the transmitter is carried on
some portion of the body of the deaf person, the receiver is strapped
or otherwise held at his ear, and a battery for furnishing the current
is carried in his pocket. It is not feasible, for this sort of use,
that the sound which this transmitter is to reproduce shall always
occur immediately in front of the transmitter. It more often occurs at
a distance of several feet. For this reason the transmitter is made as
sensitive as possible, and yet is so constructed that it will not be
caused to produce too loud or unduly harsh sounds in response to a
loud sound taking place immediately in front of it. Another adaptation
of such highly sensitive transmitters is found in the special
intercommunicating telephone systems for use between the various
departments or desks in business offices. In these it is desirable
that the transmitter shall be able to respond adequately to sounds
occurring anywhere in a small-sized room, for instance.
Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted
for such use. This has been termed by its makers the _acousticon
transmitter_. Like all the transmitters previously discussed, this is
of the variable-resistance type, but it differs from them all in that
it has no damping springs; in that carbon balls are substituted for
carbon granules; and in that the diaphragm itself serves as the front
electrode.
This transmitter consists of a cup _1_, into which is set a
cylindrical block _2_, in one face of which are a number of
hemispherical recesses. The diaphragm _3_ is made of thin carbon and
is so placed in the transmitter as to cover the openings of the
recesses in the carbon block, and lie close enough to the carbon
block, without engaging it, to prevent the carbon particles from
falling out. The diaphragm thus serves as the front electrode and the
carbon block as the rear electrode. The recesses in the carbon block
are about two-thirds filled with small carbon balls, which are about
the size of fine sand. The front piece _4_ of the transmitter is of
sheet metal and serves to hold the diaphragm in place. To admit the
sound waves it is provided with a circular opening opposite to and
about the size of the rear electrode block. On this front piece are
mounted the two terminals of the transmitter, connected respectively
to the two electrodes, terminal _5_ being insulated from the front
piece and connected by a thin metal strip with the diaphragm, while
terminal _6_ is mounted directly on the front piece and connected
through the cup _1_ with the carbon block _2_, or back electrode of
the transmitter.
[Illustration: Fig 46. Acousticon Transmitter]

Local currents, therefore, are generated, circulating between the two
adjacent metals, and, as a result, the zinc plate and the electrolyte
are needlessly wasted and the general condition of the cell is
impaired. This is called _local action_.
_Amalgamated Zincs._ Local action might be prevented by the use of
chemically pure zinc, but this, on account of its expense, cannot be
employed commercially. Local action, however, may be overcome to a
great extent by amalgamating the zinc, _i.e._, coating it with
mercury. The iron particles or other impurities do not dissolve in the
mercury, as does the zinc, but they float to the surface, whence the
hydrogen bubbles which may form speedily carry them off, and, in other
cases, the impurities fall to the bottom of the cell. As the zinc in
the pasty amalgam dissolves in the acid, the film of mercury unites
with fresh zinc, and so always presents a clear, bright, homogeneous
surface to the action of the electrolyte.
The process of amalgamating the zinc may be performed by dipping it in
a solution composed of
Nitric Acid 1 lb.
Muriatic Acid 2 lbs.
Mercury 8 oz.
The acids should be first mixed and then the mercury slowly added
until dissolved. Clean the zinc with lye and then dip it in the
solution for a second or two. Rinse in clean water and rub with a
brush.
Another method of amalgamating zincs is to clean them by dipping them
in dilute sulphuric acid and then in mercury, allowing the surplus to
drain off.
Commercial zincs, for use in voltaic cells as now manufactured,
usually have about 4 per cent of mercury added to the molten zinc
before casting into the form of plates or rods.

Swedish or Norway iron wire about .02 of an inch in diameter. The
diameter and the length of the coil, and the relation between the
number of turns in the primary and in the secondary, and the
mechanical construction of the coil, are all matters which are subject
to very wide variation in practice. While the proper relationship of
these factors is of great importance, yet they may not be readily
determined except by actual experiment with various coils, owing to
the extreme complexity of the action which takes place in them and to
the difficulty of obtaining fundamental data as to the existing facts.
It may be stated, therefore, that the design of induction coils is
nearly always carried out by “cut-and-try” methods, bringing to bear,
of course, such scientific and practical knowledge as the experimenter
may possess.
[Illustration: Fig. 107. Induction Coil]
[Illustration: Fig. 108. Section of Induction Coil]
_Use and Advantage._ The use and advantages of the induction coil in
so-called local-battery telephone sets have already been explained in
previous chapters. Such induction coils are nearly always of the open
magnetic circuit type, consisting of a long, straight core comprised
of a bundle of small annealed iron wires, on which is wound a primary
of comparatively coarse wire and having a small number of turns, and
over which is wound a secondary of comparatively fine wire and having
a very much larger number of turns. A view of such a coil mounted on a
base is shown in Fig. 107, and a sectional view of a similar coil is
shown in Fig. 108. The method of bringing out the winding terminals is
clearly indicated in this figure, the terminal wires _2_ and _4_ being
those of the primary winding and _1_ and _3_ those of the secondary
winding. It is customary to bring out these wires and attach them by
solder to suitable terminal clips. In the case of the coil shown in
Fig. 108 these clips are mounted on the wooden heads of the coil,
while in the design shown in Fig. 107 they are mounted on the base, as
is clearly indicated.
Repeating Coil. The so-called repeating coil used in telephony is
really nothing but an induction coil. It is used in a variety of ways
and usually has for its purpose the inductive association of two
circuits that are conductively separated. Usually the repeating coil
has a one to one ratio of turns, that is, there are the same number of
turns in the primary as in the secondary. However, this is not always
the case, since sometimes they are made to have an unequal number of
turns, in which case they are called _step-up _or _step-down_
repeating coils, according to whether the primary has a smaller or a
greater number of turns than the secondary. Repeating coils are almost
universally of the closed magnetic circuit type.
_Ringing and Talking Considerations._ Since repeating coils often
serve to connect two telephones, it follows that it is sometimes
necessary to ring through them as well as talk through them. By this
is meant that it is necessary that the coil shall be so designed as to
be capable of transforming the heavy ringing currents as well as the
very much smaller telephone or voice currents. Ringing currents
ordinarily have a frequency ranging from about 16 to 75 cycles per
second, while voice currents have frequencies ranging from a few
hundred up to perhaps ten thousand per second. Ordinarily, therefore,
the best form of repeating coil for transforming voice currents is not
the best for transforming the heavy ringing currents and _vice versâ_.
If the comparatively heavy ringing currents alone were to be
considered, the repeating coil might well be of heavy construction
with a large amount of iron in its magnetic circuit. On the other
hand, for carrying voice currents alone it is usually made with a
small amount of iron and with small windings, in order to prevent
waste of energy in the core, and to give a high degree of
responsiveness with the least amount of distortion of wave form, so
that the voice currents will retain as far as possible their original
characteristics. When, therefore, a coil is required to carry both
ringing and talking currents, a compromise must be effected.
_Types._ The form of repeating coil largely used for both ringing and
talking through is shown in Fig. 109. This coil comprises a soft iron
core made up of a bundle of wires about .02 inch in diameter, the ends
of which are left of sufficient length to be bent back around the
windings after they are in place and thus form a completely closed
magnetic path for the core. The windings of this particular coil are
four in number, and contain about 2,400 turns each, and have a
resistance of about 60 ohms. In this coil, when connected for local
battery work, the windings are connected in pairs in series, thus
forming effectively two windings having about 120 ohms resistance
each. The whole coil is enclosed in a protecting case of iron. The
terminals are brought out to suitable clips on the wooden base, as
shown. An external perspective view of this coil is shown in Fig. 110.