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Station A the upper conductor, Fig. 170, is connected to binding post
_1_ and the lower conductor to binding post _2_, while at Station B
the upper conductor is connected to binding post _2_ and the lower
conductor to binding post _1_. The permanent wiring of this telephone
set is the same as that frequently used for a set connected to a line
having only one station, the proper ringing circuit being made by the
method of connecting up the binding posts. For example, if this
telephone set were to be used on a single station line, the binding
posts _1_ and _2_ would be connected to the two conductors of the line
as before, while binding post _3_ would be connected to post _1_
instead of being grounded.
[Illustration: Fig. 175. Circuit of Two-Party Station]
_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
set used with the system illustrated in Fig. 172 is shown in detail in
Fig. 176. The wiring of this set is arranged for local battery or
magneto working, as this method of selective ringing is more frequently
employed with magneto systems, on account of the objectionable features
which arise when applied to common-battery systems. In this figure the
line conductors are connected to binding posts _1_ and _2_, and a
ground connection is made to binding post _3_. In order that all sets
may be wired alike and yet permit the instrument to be connected for
any one of the various stations, the bell is not permanently wired to
any portion of the circuit but has flexible connections which will
allow of the set being properly connected for any desired station. The
terminals of the bell are connected to binding posts _9_ and _10_, to
which are connected flexible conductors terminating in terminals _7_
and _8_. These terminals may be connected to the binding posts _4_,
_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
D station, as required. For example, in connecting the set for Station
A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
_5_. For connecting the set for Station B terminal _7_ is connected to
binding post _5_ and _8_ to _6_. For connecting the set for Station C
terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
connecting the set for Station D terminal _7_ is connected to binding
post _4_ and _8_ to _6_.
[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]
[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]
The detailed wiring of the telephone set employed in connection with
the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
this set is arranged for a common-battery system, inasmuch as this
arrangement of signaling circuit is more especially adapted for
common-battery working. However, this arrangement is frequently
adapted to magneto systems as even with magneto systems a permanent
ground connection at a subscribers station is objectionable inasmuch
as it increases the difficulty of determining the existence or
location of an accidental ground on one of the line conductors. The
wiring of this set is also arranged so that one standard type of
wiring may be employed and yet allow any telephone set to be connected
as an A, B, C, or D station.
Harmonic Method. _Principles._ To best understand the principle of
operation of the harmonic party-line signaling systems, it is to be
remembered that a flexible reed, mounted rigidly at one end and having
its other end free to vibrate, will, like a violin string, have a
certain natural period of vibration; that is, if it be started in
vibration, as by snapping it with the fingers, it will take up a
certain rate of vibration which will continue at a uniform rate until
the vibration ceases altogether. Such a reed will be most easily
thrown into vibration by a series of impulses having a frequency
corresponding exactly to the natural rate of vibration of the reed
itself; it may be thrown into vibration by very slight impulses if
they occur at exactly the proper times.
It is familiar to all that a person pushing another in a swing may
cause a considerable amplitude of vibration with the exertion of but a
small amount of force, if he will so time his pushes as to conform
exactly to the natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.

By reactive interference is meant action whereby the transmitter
element, in emitting a wave, affects its own controlling receiver
element, thus setting up an action similar to that which occurs when
the receiver of a telephone is held close to its transmitter and
humming or singing ensues. No repeater is successful unless it is free
from this reactive interference.
[Illustration: Fig. 37. Mercury-Arc Telephone Relay]
Enough has been accomplished by practical tests of the Shreeve device
and others like it to show that the search for a method of relaying
telephone voice currents is not looking for a pot of gold at the end
of the rainbow. The most remarkable truth established by the success
of repeaters of the Shreeve type is that a device embodying so large
inertia of moving parts can succeed at all. If this mean anything, it
is that a device in which inertia is absolutely eliminated might do
very much better. Many of the methods already proposed by inventors
attack the problem in this way and one of the most recent and most
promising ways is that of Mr. J.B. Taylor, the circuit of whose
telephone-relay patent is shown in Fig. 37. In it, _1_ is an
electromagnet energized by voice currents; its varying field varies an
arc between the electrodes _2-2_ and _3_ in a vacuum tube. These
fluctuations are transformed into line currents by the coil _4_.
CHAPTER V
TRANSMITTERS
Variable Resistance. As already pointed out in Chapter II, the
variable resistance method of producing current waves, corresponding
to sound waves for telephonic transmission, is the one that lends
itself most readily to practical purposes. Practically all telephone
transmitters of today employ this variable-resistance principle. The
reason for the adoption of this method instead of the other possible
ones is that the devices acting on this principle are capable, with
great simplicity of construction, of producing much more powerful
results than the others. Their simplicity is such as to make them
capable of being manufactured at low cost and of being used
successfully by unskilled persons.

The relation between the windings of the induction coil in this
practice are such that the secondary winding contains many more turns
than the primary winding. Changes in the circuit of the primary
winding produce potentials in the secondary winding correspondingly
higher than the potentials producing them. These secondary potentials
depend upon the _ratio_ of turns in the two windings and therefore,
within close limits, may be chosen as wished. High potentials in the
secondary winding are admirably adapted to transmit currents in a
high-resistance line, for exactly the same reason that long-distance
power transmission meets with but one-quarter of one kind of loss when
the sending potential is doubled, one-hundredth of that loss when it
is raised tenfold, and similarly. The induction coil, therefore,
serves the double purpose of a step-up transformer to limit line
losses and a device for vastly increasing the range of change in the
transmitter circuit.
Fig. 13 is offered to remind the student of the action of an induction
coil or transformer in whose primary circuit a direct current is
increased and decreased. An increase of current in the local winding
produces an impulse of _opposite_ direction in the turns of the
secondary winding; a decrease of current in the local winding produces
an impulse of _the same_ direction in the turns of the secondary
winding. The key of Fig. 13 being closed, current flows upward in the
primary winding as drawn in the figure, inducing a downward impulse of
current in the secondary winding and its circuit as noted at the right
of the figure. On the key being opened, current ceases in the primary
circuit, inducing an upward impulse of current in the secondary
winding and circuit as shown. During other than instants of opening
and closing (changing) the local circuit, no current whatever flows in
the secondary circuit.
[Illustration: Fig. 13. Induction-Coil Action]
It is by these means that telephone transmitters draw direct current
from primary batteries and send high-potential alternating currents
over lines; the same process produces what in Therapeutics are called
“Faradic currents,” and enables also a simple vibrating contact-maker
to produce alternating currents for operating polarized ringers of
telephone sets.

Several important points must be borne in mind in the design of the
hook switch. The spring provided to lift the hook must be sufficiently
strong to accomplish this purpose and yet must not be strong enough to
prevent the weight of the receiver from moving the switch to its other
position. The movement of this spring must be somewhat limited in
order that it will not break when used a great many times, and also it
must be of such material and shape that it will not lose its
elasticity with use. The shape and material of the restoring spring
are, of course, determined to a considerable extent by the length of
the lever arm which acts on the spring, and on the space which is
available for the spring.
The various contacts by which the circuit changes are brought about
upon the movement of the hook-switch lever usually take the form of
springs of German silver or phosphor-bronze, hard rolled so as to have
the necessary resiliency, and these are usually tipped with platinum
at the points of contact so as to assure the necessary character of
surface at the points where the electric circuits are made or broken.
A slight sliding movement between each pair of contacts as they are
brought together is considered desirable, in that it tends to rub off
any dirt that may have accumulated, yet this sliding movement should
not be great, as the surfaces will then cut each other and, therefore,
reduce the life of the switch.
Contact Material. On account of the high cost of platinum, much
experimental work has been done to find a substitute metal suitable
for the contact points in hook switches and similar uses in the
manufacture of telephone apparatus. Platinum is unquestionably the
best known material, on account of its non-corrosive and
heat-resisting qualities. Hard silver is the next best and is found in
some first-class apparatus. The various cheap alloys intended as
substitutes for platinum or silver in contact points may be dismissed
as worthless, so far as the writers somewhat extensive investigations
have shown.
In the more recent forms of hook switches, the switch lever itself
does not form a part of the electrical circuit, but serves merely as
the means by which the springs that are concerned in the switching
functions are moved into their alternate cooperative relations. One
advantage in thus insulating the switch lever from the
current-carrying portions of the apparatus and circuits is that, since
it necessarily projects from the box or cabinet, it is thus liable to
come in contact with the person of the user. By insulating it, all
liability of the user receiving shocks by contact with it is
eliminated.
Wall Telephone Hooks. _Kellogg._ A typical form of hook switch, as
employed in the ordinary wall telephone sets, is shown in Fig. 83,
this being the standard hook of the Kellogg Switchboard and Supply
Company. In this the lever _1_ is pivoted at the point _3_ in a
bracket _5_ that forms the base of all the working parts and the means
of securing the entire hook switch to the box or framework of the
telephone. This switch lever is normally pressed upward by a spring
_2_, mounted on the bracket _5_, and engaging the under side of the
hook lever at the point _4_. Attached to the lever arm _1_ is an
insulated pin _6_. The contact springs by which the various electrical
circuits are made and broken are shown at _7_, _8_, _9_, _10_, and
_11_, these being mounted in one group with insulated bushings between
them; the entire group is secured by machine screws to a lug
projecting horizontally from the bracket _5_. The center spring _9_
is provided with a forked extension which embraces the pin _6_ on the
hook lever. It is obvious that an up-and-down motion of the hook lever
will move the long spring _9_ in such manner as to cause electrical
contact either between it and the two upper springs _7_ and _8_, or
between it and the two lower springs _10_ and _11_. The hook is shown
in its raised position, which is the position required for talking.

Electrolytic hazards depend not on the heating effects of currents but
on their chemical effects. The same natural law which enables primary
and secondary batteries to be useful provides a hazard which menaces
telephone-cable sheaths and other conductors. When a current leaves a
metal in contact with an electrolyte, the metal tends to dissolve into
the electrolyte. In the processes of electroplating and electrotyping,
current enters the bath at the anode, passes from the anode through
the solution to the cathode, removing metal from the former and
depositing it upon the latter. In a primary battery using zinc as the
positive element and the negative terminal, current is caused to pass,
within the cell, from the zinc to the negative element and zinc is
dissolved. Following the same law, any pipe buried in the earth may
serve to carry current from one region to another. As single-trolley
tractiosystems with positive trolley wires constantly are sending
large currents through the earth toward their power stations, such a
pipe may be of positive potential with relation to moist earth at some
point in its length. Current leaving it at such a point may cause its
metal to dissolve enough to destroy the usefulness of the pipe for its
intended purpose.
Lead-sheathed telephone cables in the earth are particularly exposed
to such damage by electrolysis. The reasons are that such cables often
are long, have a good conductor as the sheath-metal, and that metal
dissolves readily in the presence of most aqueous solutions when
electrolytic differences of potential exist. The length of the cables
enables them to connect between points of considerable difference of
potential. It is lack of this length which prevents electrolytic
damage to masses of structural metal in the earth.
Electrical power is supplied to single-trolley railroads principally
in the form of direct current. Usually all the trolley wires of a city
are so connected to the generating units as to be positive to the
rails. This causes current to flow from the cars toward the power
stations, the return path being made up jointly of the rails, the
earth itself, actual return wires which may supplement the rails, and
also all other conducting things in the earth, these being principally
lead-covered cables and other pipes. These conditions establish
definite areas in which the currents tend to leave the cables and
pipes, _i.e._, in which the latter are positive to other things. These
positive areas usually are much smaller than the negative areas, that
is, the regions in which currents tend _to enter_ the cables form a
larger total than the regions in which the currents tend _to leave_
the cables. These facts simplify the ways in which the cables may be
protected against damage by direct currents leaving them and also they
reduce the amount, complication, and cost of applying the corrective
and preventive measures.
All electric roads do not use direct current. Certain simplifications
in the use of single-phase alternating currents in traction motors
have increased the number of roads using a system of
alternating-current power supply. Where alternating current is used,
the electrolytic conditions are different and a new problem is set,
for, as the current flows in recurrently different directions, an area
which at one instant is positive to others, is changed the next
instant into a negative area. The protective means, therefore, must be
adapted to the changed requirements.
CHAPTER XIX
PROTECTIVE MEANS
Any of the heating hazards described in the foregoing chapter may
cause currents which will damage apparatus. All devices for the
protection of apparatus from such damage, operate either to stop the
flow of the dangerous current, or to send that flow over some other
path.

Conventional Symbols. In Fig. 82 are shown six conventional symbols
of polarized bells. The three at the top, consisting merely of two
circles representing the magnets in plan view, are perhaps to be
preferred as they are well standardized, easy to draw, and rather
suggestive. The three at the bottom, showing the ringer as a whole in
side elevation, are somewhat more specific, but are objectionable in
that they take more space and are not so easily drawn.
[Illustration: Fig. 82. Ringer Symbols]
Symbols _A_ or _B_ may be used for designating any ordinary polarized
ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
biased ringer. If the bell is designed to operate only on positive
impulses, then the plus sign is placed opposite the symbol, while a
minus sign so placed indicates that the bell is to be operated only by
negative impulses.
Some specific types of ringers are designed to operate only on a given
frequency of current. That is, they are so designed as to be
responsive to currents having a frequency of sixty cycles per second,
for instance, and to be unresponsive to currents of any other
frequency. Either symbols _E_ or _F_ may be used to designate such
ringers, and if it is desired to indicate the particular frequency of
the ringer this is done by adding the proper numeral followed by a
short reversed curve sign indicating frequency. Thus 50~ would
indicate a frequency of fifty cycles per second.
CHAPTER IX
THE HOOK SWITCH
Purpose. In complete telephone instruments, comprising both talking
and signaling apparatus, it is obviously desirable that the two sets
of apparatus, for talking and signaling respectively, shall not be
connected with the line at the same time. A certain switching device
is, therefore, necessary in order that the signaling apparatus alone
may be left operatively connected with the line while the instrument
is not being used in the transmission of speech, and in order that the
signaling apparatus may be cut out when the talking apparatus is
brought into play.

One of these transmitters, embodying these same features but with
modified details, is shown in Fig. 42, this being the new transmitter
manufactured by the Western Electric Company. In this the bridge of
the original White transmitter is dispensed with, the electrode
chamber being supported by a pressed metal cup _1_, which supports the
chamber as a whole. The electrode cup, instead of being made of a
solid block as in the White instrument, is composed of two portions, a
cylindrical or tubular portion _2_ and a back _3_. The cylindrical
portion is externally screw-threaded so as to engage an internal screw
thread in a flanged opening in the center of the cup _1_. By this
means the electrode chamber is held in place in the cup _1_, and by
the same means the mica washer _4_ is clamped between the flange in
this opening and the tubular portion _2_ of the electrode chamber. The
front electrode is carried, as in the White transmitter, on the mica
washer and is rigidly attached to the center of the diaphragm so as to
partake of the movement thereof. It will be seen, therefore, that this
is essentially a White transmitter, but with a modified mounting for
the electrode chamber.
A feature in this transmitter that is not found in the White
transmitter is that both the front and the rear electrodes, in fact,
the entire working portions of the transmitter, are insulated from the
exposed metal parts of the instrument. This is accomplished by
insulating the diaphragm and the supporting cup _1_ from the
transmitter front. The terminal _5_ on the cup _1_ forms the
electrical connection for the rear electrode, while the terminal _6_,
which is mounted _on_ but insulated _from_ the cup _1_ and is
connected with the front electrode by a thin flexible connecting
strip, forms the electrical connection for the front electrode.

Bearing in mind that the calculations of Table IV are all based upon
the “diameter over insulation,” which it states at the outset for each
of four different kinds of covering, it is evident what is meant by
“turns per linear inch.” The columns referring to “turns per square
inch” mean the number of turns, the ends of which would be exposed in
one square inch if the wound coil were cut in a plane passing through
the axis of the core. Knowing the distance between the head, and the
depth to which the coil is to be wound, it is easy to select a size of
wire which will give the required number of turns in the provided
space. It is to be noted that the depth of winding space is one-half
of the difference between the core diameter and the complete diameter
of the wound coil. The resistance of the entire volume of wound wire
may be determined in advance by knowing the total cubic contents of
the winding space and multiplying this by the ohms per cubic inch of
the selected wire; that is, one must multiply in inches the distance
between the heads of the spool by the difference between the squares
of the diameters of the core and the winding space, and this in turn
by .7854. This result, times the ohms per cubic inch, as given in the
table, gives the resistance of the winding.
There is a considerable variation in the method of applying silk
insulation to the finer wires, and it is in the finer sizes that the
errors, if any, pile up most rapidly. Yet the table throughout is
based on data taken from many samples of actual coil winding by the
present process of winding small coils. It should be said further that
the table does not take into account the placing of any layers of
paper between the successive layers of the wires. This table has been
compared with many examples and has been used in calculating windings
in advance, and is found to be as close an approximation as is
afforded by any of the formulas on the subject, and with the further
advantage that it is not so cumbersome to apply.
_Winding Calculations._ In experimental work, involving the winding of
coils, it is frequently necessary to try one winding to determine its
effect in a given circuit arrangement, and from the knowledge so
gained to substitute another just fitted to the conditions. It is in
such a substitution that the table is of most value. Assume a case in
which are required a spool and core of a given size with a winding of,
say No. 25 single silk-covered wire, of a resistance of 50 ohms.
Assume also that the circuit regulations required that this spool
should be rewound so as to have a resistance of, say 1,000 ohms. What
size single silk-covered wire shall be used? Manifestly, the winding
space remains the same, or nearly so. The resistance is to be
increased from 50 to 1,000 ohms, or twenty times its first value.
Therefore, the wire to be used must show in the table twenty times as
many ohms per cubic inch as are shown in No. 25, the known first size.
This amount would be twenty times 7.489, which is 149.8, but there is
no size giving this exact resistance. No. 32, however, is very nearly
of that resistance and if wound to exactly the same depth would give
about 970 ohms. A few turns more would provide the additional thirty
ohms.
Similarly, in a coil known to possess a certain number of turns, the
table will give the size to be selected for rewinding to a greater or
smaller number of turns. In this case, as in the case of substituting
a winding of different resistance it is unnecessary to measure and
calculate upon the dimensions of the spool and core. Assume a spool
wound with No. 30 double silk-covered wire, which requires to be
wound with a size to double the number of turns. The exact size to do
this would have 8922. turns per square inch and would be between No.
34 and No. 35. A choice of these two wires may be made, using an
increased winding depth with the smaller wire and a shallower winding
depth for the larger wire.

The action of this receiver will be understood when it is stated that
in common-battery practice, as will be shown in later chapters, a
steady current flows over the line for energizing the transmitter. On
this current is superposed the incoming voice currents from a distant
station. The steady current flowing in the line will, in the case of
this receiver, pass through the magnet winding and establish a normal
magnetic field in the same way as if a permanent magnet were employed.
The superposed incoming voice currents will then be able to vary this
magnetic field in exactly the same way as in the ordinary receiver.
An astonishing feature of this recent development of the so-called
direct-current receiver is that it did not come into use until after
about twenty years of common-battery practice. There is nothing new in
the principles involved, as all of them were already understood and
some of them were employed by Bell in his original telephone; in fact,
the idea had been advanced time and again, and thrown aside as not
being worth consideration. This is an illustration of a frequent
occurrence in the development of almost any rapidly growing art. Ideas
that are discarded as worthless in the early stages of the art are
finally picked up and made use of. The reason for this is that in some
cases the ideas come in advance of the art, or they are proposed
before the art is ready to use them. In other cases the idea as
originally proposed lacked some small but essential detail, or, as is
more often the case, the experimenter in the early days did not have
sufficient skill or knowledge to make it fit the requirements as he
saw them.
Monarch Receiver. The receiver of the Automatic Electric Company
just discussed employs but a single electromagnet by which the initial
magnetization of the cores and also the variable magnetization
necessary for speech reproduction is secured. The problem of the
direct-current receiver has been attacked in another way by Ernest E.
Yaxley, of the Monarch Telephone Manufacturing Company, with the
result shown in Fig. 54. The construction in this case is not unlike
that of an ordinary permanent-magnet receiver, except that in the
place of the permanent magnets two soft iron cores _1-1_ are employed.
On these are wound two long bobbins of insulated wire so that the
direct current flowing over the telephone line will pass through these
and magnetize the cores to the same degree and for the same purpose as
in the case of permanent magnets. These soft iron magnet cores _1-1_
continue to a point near the coil chamber, where they join the two
soft iron pole pieces _2-2_, upon which the ordinary voice-current
coils are wound. The two long coils _4-4_, which may be termed the
direct-current coils, are of somewhat lower resistance than the two
voice-current coils _3-3_. They are, however, by virtue of their
greater number of turns and the greater amount of iron that is
included in their cores, of much higher impedance than the
voice-current coils _3-3_. These two sets of coils _4-4_ and _3-3_ are
connected in multiple. As a result of their lower ohmic resistance the
coils _4-4_ will take a greater amount of the steady current which
comes over the line, and therefore the greater proportion of the
steady current will be employed in magnetizing the bar magnets. On
account of their higher impedance to alternating currents, however,
nearly all of the voice currents which are superposed on the steady
currents, flowing in the line will pass through the voice-current
coils _3-3_, and, being near the diaphragm, these currents will so
vary the steady magnetism in the cores _2-2_ as to produce the
necessary vibration of the diaphragm.
[Illustration: Fig. 54. Monarch Direct-Current Receiver]
This receiver, like the one of the Automatic Electric Company, does
not rely on the shell in any respect to maintain the permanency of
relation between the pole pieces and the diaphragm. The cup _5_, which
is of pressed brass, contains the voice-current coils and also acts as
a seat for the diaphragm. The entire working parts of this receiver
may be removed by merely unscrewing the ear piece from the hard rubber
shell, thus permitting the whole works to be withdrawn in an obvious
manner.

Before considering the various types it is well to state that the term
telephone is often rather loosely used. We sometimes hear the receiver
proper called a telephone or a hand telephone. Since this was the
original speaking telephone, there is some reason for so calling the
receiver. The modern custom more often applies the term telephone to
the complete organization of talking and signaling apparatus, together
with the associated wiring and cabinet or standard on which it is
mounted. The name telephone set is perhaps to be preferred to the word
telephone, since it tends to avoid misunderstanding as to exactly what
is meant. Frequently, also, the telephone or telephone set is referred
to as a subscribers station equipment, indicating the equipment that
is to be found at a subscribers station. This, as applying to a
telephone alone, is not proper, since the subscribers station
equipment includes more than a telephone. It includes the local wiring
within the premises of the subscriber and also the lightning arrester
and other protective devices, if such exist.
To avoid confusion, therefore, the collection of talking and signaling
apparatus with its wiring and containing cabinet or standard will be
referred to in this work as a telephone or telephone set. The receiver
will, as a rule, be designated as such, rather than as a telephone.
The term subscribers station equipment will refer to the complete
equipment at a subscribers station, and will include the telephone
set, the interior wiring, and the protective devices, together with
any other apparatus that may be associated with the telephone line and
be located within the subscribers premises.
Classification of Sets. Telephones may be classified under two
general headings, magneto telephones and common-battery telephones,
according to the character of the systems in which they are adapted to
work.
_Magneto Telephone._ The term magneto telephone, as it was originally
employed in telephony, referred to the type of instrument now known as
a receiver, particularly when this was used also as a transmitter. As
the use of this instrument as a transmitter has practically ceased,
the term magneto telephone has lost its significance as applying to
the receiver, and, since many telephones are equipped with magneto
generators for calling purposes, the term magneto telephone has, by
common consent, come to be used to designate any telephone including,
as a part of its equipment, a magneto generator. Magneto telephones
usually, also, include local batteries for furnishing the transmitter
with current, and this has led to these telephones being frequently
called local battery telephones. However, a local battery telephone is
not necessarily a magneto telephone and _vice versâ_, since sometimes
magneto telephones have no local batteries and sometimes local battery
telephones have no magnetos. Nearly all of the telephones which are
equipped with magneto generators are, however, also equipped with
local batteries for talking purposes, and, therefore, the terms
magneto telephone and local battery telephone usually refer to the
same thing.
_Common-Battery Telephone._ Common-battery telephones, on the other
hand, are those which have no local battery and no magneto generator,
all the current for both talking and signaling being furnished from a
common source of current at the central office.
_Wall and Desk Telephones._ Again we may classify telephones or
telephone sets in accordance with the manner in which their various
parts are associated with each other for use, regardless of what parts
are contained in the set. We may refer to all sets adapted to be
mounted on a wall or partition as _wall telephones_, and to all in
which the receiver, transmitter, and hook are provided with a standard
of their own to enable them to rest on any flat surface, such as a
desk or table, as _desk telephones_. These latter are also referred to
as portable telephones and as portable desk telephones.
In general, magneto or local battery telephones differ from
common-battery telephones in their component parts, the difference
residing principally in the fact that the magneto telephone always has
a magneto generator and usually a local battery, while the
common-battery telephone has no local source of current whatever. On
the other hand, the differences between wall telephones and desk
telephones are principally structural, and obviously either of these
types of telephones may be for common-battery or magneto work. The
same component parts go to make up a desk telephone as a wall
telephone, provided the two instruments are adapted for the same class
of service, but the difference between the two lies in the structural
features by which these same parts are associated with each other and
protected from exposure.
[Illustration: Fig. 142. Magneto Wall Set]
[Illustration: Fig. 143. Magneto Wall Set]
Magneto-Telephone Sets. _Wall._ In Fig. 142 is shown a familiar type
of wall set. The containing box includes within it all of the working
parts of the apparatus except that which is necessarily left outside
in order to be within the reach of the user. Fig. 143 shows the same
set with the door open. This gives a good idea of the ordinary
arrangement of the apparatus within. It is seen that the polarized
bell or ringer has its working parts mounted on the inside of the door
or cover of the box, the tapper projecting through so as to play
between the gongs on the outside. Likewise the transmitter arm, which
supports the transmitter and allows its adjustment up and down to
accommodate itself to the height of the user, is mounted on the front
of the door, and the conductors leading to it may be seen fastened to
the rear of the door in Fig. 143.