THE TELEPHONE SET
We have considered what may be called the elemental parts of a
complete telephone; that is, the receiver, transmitter, hook switch,
battery, generator, call bell, condenser, and the various kinds of
coils which go to make up the apparatus by which one is enabled to
transmit and receive speech and signals. We will now consider the
grouping of these various elements into a complete working
organization known as a telephone.
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 subscriber's station equipment, indicating the equipment that
is to be found at a subscriber's station. This, as applying to a
telephone alone, is not proper, since the subscriber's 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 subscriber's station equipment will refer to the complete
equipment at a subscriber's 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 subscriber's 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.
In some wall sets the wires leading to the bell and transmitter are
connected to the wiring of the rest of the set through the hinges of
the door, thus allowing the door to be opened and closed repeatedly
without breaking off the wires. In order to always insure positive
electrical contact between the stationary and movable parts of the
hinge a small wire is wound around the hinge pin, one end being
soldered to the stationary part and the other end to the movable part
of the hinge. In other forms of wall set the wires to the bell and the
transmitter lead directly from the stationary portion of the cabinet
to the back of the door, the wires being left long enough to have
sufficient flexibility to allow the door to be opened and closed
without injuring the wires.
At the upper portion of the box there is mounted the hook switch, this
being, in this case, of the short lever type. The lever of the hook
projects through the side of the box so as to make the hook available
as a support for the receiver. Immediately at the right of the hook
switch is mounted the induction coil, and immediately below this the
generator, its crank handle projecting through the right-hand side of
the box so as to be available for use there. The generator is usually
mounted on a transverse shelf across the middle of the cabinet, this
shelf serving to form a compartment below it in which the dry battery
of two or three cells is placed.
The wall telephone-set cabinets have assumed a multitude of forms.
When wet cells rather than dry cells were ordinarily employed, as was
the case up to about the year 1895, the magneto generator, polarized
bell, and hook switch were usually mounted in a rectangular box placed
at the top of a long backboard. Immediately below this on the
backboard was mounted the transmitter arm, and sometimes the base of
this included the induction coil. Below this was the battery box, this
being a large affair usually adapted to accommodate two and sometimes
three ordinary LeClanché cells side by side.
The dry cell has almost completely replaced the wet cell in this
country, and as a result, the general type of wall set as shown in
Figs. 142 and 143, has gradually replaced the old wet-cell type, which
was more cumbrous and unsightly. It is usual on wall sets to provide
some sort of a shelf, as indicated in Fig. 142, for the convenience of
the user in making notes and memoranda.
_Desk._ In the magneto desk-telephone sets, the so-called desk stand,
containing the transmitter, the receiver, and the hook switch, with
the standard upon which they are mounted, is shown in Fig. 144. This
desk stand evidently does not comprise the complete equipment for a
magneto desk-telephone set, since the generator, polarized bell, and
battery are lacking. The generator and bell are usually mounted
together in a box, either on the under side of the desk of the user or
on the wall within easy reach of his chair. Connections are made
between the apparatus in the desk stand proper and the battery,
generator, and bell by means of flexible conducting cords, these
carrying a plurality of conductors, as required by the particular
circuit of the telephone in question. Such a complete magneto
desk-telephone set is shown in Fig. 145, this being one of the types
manufactured by the Stromberg-Carlson Manufacturing Company.
[Illustration: Fig. 144. Desk Stand]
A great variety of arrangements of the various parts of magneto
desk-telephone apparatus is employed in practice. Sometimes, as shown
in Fig. 145, the magneto bell box is equipped with binding posts for
terminating all of the conductors in the cord, the line wires also
running to some of these binding posts.
In the magneto-telephone set illustrated the box is made large enough
to accommodate only the generator and call bell, and the batteries are
mounted elsewhere, as in a drawer of the desk, while in other cases
there is no other equipment but that shown in the cut, the batteries
being mounted within the magneto bell box itself. In still other
cases, the polarized bell is contained in one box, the generator in
another, the batteries in the drawer of the desk, the induction coil
being mounted either in the base of the desk stand, in the bell box,
or in the generator box. In such cases all of the circuits of the
various scattered parts are wired to a terminal strip, located at some
convenient point, this strip containing terminals for all the wires
leading from the various parts and for the line wires themselves. By
combining the various wires on the terminals of this terminal strip,
the complete circuits of the telephone are built up. In still other
cases the induction coil is mounted on the terminal strip and separate
wires or sets of wires are run to the polarized bell and generator, to
the desk stand itself, and to the batteries. These various
arrangements are subject largely to the desire or personal ideas of
the manufacturer or user. All of them work on the same principle so
far as the operation of the talking and signaling circuits is
concerned.
[Illustration: Fig. 145. Magneto Desk Set]
Circuits of Magneto-Telephone Sets. Magneto telephones, whether of
the wall or desk type, may be divided into two general classes, series
and bridging, according to whether the magnet of the bell is included
in series or bridge relation with the telephone line when the hook is
down.
_Series._ In the so-called series telephone line, where several
telephones are placed in series in a single line circuit, the
employment of the series type of telephone results in all of the
telephone bells being in series in the line circuit. This means that
the voice currents originating in the telephones that are in use at a
given time must pass in series through the magnets of the bells of the
stations that are not in use. In order that these magnets, through
which the voice currents must pass, may interfere to as small a degree
as possible with the voice currents, it is common to employ
low-resistance magnets in series telephones, these magnets being wound
with comparatively few turns and on rather short cores so that the
impedance will be as small as possible. Likewise, since the generators
are required to ring all of the bells in series, they need not have a
large current output, but must have sufficient voltage to ring through
all of the bells in series and through the resistance of the line. For
this reason the generators are usually of the three-bar type and
sometimes have only two bars.
In Fig. 146 are shown, in simplified form, the circuits of an ordinary
series telephone. The receiver in this is shown as being removed from
the hook and thus the talking apparatus is brought into play. The line
wires _1_ and _2_ connect respectively to the binding posts _3_ and
_4_ which form the terminals of the instrument. When the hook is up,
the circuit between the binding posts _3_ and _4_ includes the
receiver and the secondary winding of the induction coil, together
with one of the upper contacts _5_ of the switch hook and the hook
lever itself. This completes the circuit for receiving speech. The
hook switch is provided with another upper contact _6_, between which
and the contact _5_ is connected the local circuit containing the
transmitter, the battery, and the primary of the induction coil in
series. The primary and the secondary windings are connected together
at one end and connected with the switch contact _5_, as shown. It is
thus seen that when the hook is up the circuit through the receiver is
automatically closed and also the local circuit containing the
primary, the battery, and the transmitter. Thus, all the conditions
for transmitting and receiving speech are fulfilled.
[Fig. 146. Circuit of Series Magneto Set]
When the hook is down, however, the receiving and transmitting
circuits are broken, but another circuit is completed by the
engagement of the hook-switch lever with the lower hook contact _7_.
Between this contact and one side of the line is connected the
polarized ringer and the generator. With the hook down, therefore, the
circuit may be traced from the line wire _1_ to binding post _3_,
thence through the generator shunt to the call bell, and thence
through the lower switching contact _7_ to the binding post _4_ and
line wire _2_. The generator shunt, as already described in Chapter
VIII, normally keeps the generator shunted out of circuit. When,
however, the generator is operated the shunt is broken, which allows
the armature of the generator to come into the circuit in series with
the winding of the polarized bell. The normal shunting of the
generator armature from the circuit of the line is advantageous in
several ways. In the first place, the impedance of the generator
winding is normally cut out of the circuit so that in the case of a
line with several stations the talking or voice currents do not have
to flow through the generator armatures at the stations which are not
in use. Again, the normal shunting of the generator tends to save the
generator armature from injury by lightning.
[Illustration: Fig. 147. Circuit of Series Magneto Set.]
The more complete circuits of a series magneto telephone are shown in
Fig. 147. In this the line binding posts are shown as _1_ and _2_. At
the bottom of the telephone cabinet are four other binding posts
marked _3_, _4_, _5_, and _6_. Of these _3_ and _4_ serve for the
receiver terminals and _5_ and _6_ for the transmitter and battery
terminals. The circuits of this diagram will be found to be
essentially the same as those of Fig. 146, except that they are shown
in greater detail. This particular type of circuit is one commonly
employed where the generator, ringer, hook switch, and induction coil
are all mounted in a so-called magneto bell box at the top of the
instrument, and where the transmitter is mounted on an arm just below
this box, and the battery in a separate compartment below the
transmitter. The only wiring that has to be done between the bell box
and the other parts of the instrument in assembling the complete
telephone is to connect the receiver to the binding posts _3_ and _4_
and to connect the battery and transmitter circuit to the binding
posts _5_ and _6_.
_Bridging._ In other cases, where several telephones are placed on a
single-line circuit, the bells are arranged in multiple across the
line. For this reason their magnets are wound with a very great number
of turns and consequently to a high resistance. In order to further
increase the impedance, the cores are made long and heavy. Since the
generators on these lines must be capable of giving out a sufficient
volume of current to divide up between all of the bells in multiple,
it follows that these generators must have a large current output, and
at the same time a sufficient voltage to ring the bells at the
farthest end of the line. Such instruments are commonly called
bridging instruments, on account of the method of connecting their
bells across the circuit of the line.
[Illustration: Fig. 148. Circuit of Bridging Magneto Set]
The fundamental characteristic of the bridging telephone is that it
contains three possible bridge paths across the line wires. The first
of these bridge paths is through the talking apparatus, the second
through the generator, and the third through the ringer. This is shown
in simplified form in Fig. 148. The talking apparatus is associated
with the two upper contacts of the hook switch in the usual manner and
needs no further description. The generator is the second separate
bridge path, normally open, but adapted to be closed when the
generator is operated, this automatic closure being performed by the
movement of the crank shaft. The third bridge contains the polarized
bell, and this, as a rule, is permanently closed. Sometimes, however,
the arrangement is such that the bell path is normally closed through
the switch which is operated by the generator crank shaft, and this
path is automatically broken when the generator is operated, at which
time, also, the generator path is automatically closed. This
arrangement brings about the result that the generator never can ring
its own bell, because its switch always operates to cut out the bell
at its own station just before the generator itself is cut into the
circuit.
In Fig. 149 is shown the complete circuit of a bridging telephone.
The circuit given in this figure is for a local-battery wall set
similar in type to that shown in Figs. 142 and 143. A simplified
diagrammatic arrangement is shown in the lower left-hand corner of
this figure, and from a consideration of this it will be seen that the
bell circuit across the line is normally completed through the two
right-hand normally closed contacts of the switch on the generator.
When, however, the generator is operated these two contacts are made
to disengage each other while the long spring of the generator switch
engages the left-hand spring and thus brings the generator itself into
the circuit.
[Illustration: Fig. 149. Circuit of Bridging Magneto Set]
Of the three binding posts, _1_, _2_, and _3_, at the top of Fig. 149,
_1_ and _2_ are for connecting with the line wires, while _8_ is for a
ground connection, acting in conjunction with the lightning arrester
mounted at the top of the telephone and indicated at _4_ in Fig. 149.
This has no function in talking or ringing, and will be referred to
more fully in Chapter XIX. Suffice it to say at this point that these
arresters usually consist of two conducting bodies, one connected
permanently to each of the line binding posts, and a third conducting
body connected to the ground binding post. These three conducting
bodies are in close proximity but carefully insulated from each other;
the idea being that when the line wires are struck by lightning or
subjected otherwise to a dangerous potential, the charge on the line
will jump across the space between the conducting bodies and pass
harmlessly to ground.
NOTE. The student should practice making simplified diagrams from
actual wiring diagrams. The difference between the two is that
one is laid out for ease in understanding it, while the other is
laid out to show the actual course of the wires as installed.
If the large detailed circuit of Fig. 149 be compared with the small
theoretical circuit in the same figure, the various conducting
paths will be found to be the same. Such a simplified circuit does
more to enable one to grasp the fundamental scheme of a complex
circuit than much description, since it shows at a glance the general
arrangement. The more detailed circuits are, however, necessary to
show the actual paths followed by the wiring.
The circuits of desk stands do not differ from those of wall sets in
any material degree, except as may be necessitated by the fact that
the various parts of the telephone set are not all mounted in the same
cabinet or on the same standard. To provide for the necessary relative
movement between the desk stand and the other portions of the set,
flexible conductors are run from the desk stand itself to the
stationary portions of the equipment, such as the battery and the
parts contained in the generator and bell box.
[Illustration: Fig. 150. Circuit of Bridging Magneto Desk Set]
In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto
desk-telephone set, illustrated in Fig. 145. This diagram needs no
explanation in view of what has already been said. The conductors,
leading from the desk-stand group of apparatus to the bell-box group of
apparatus, are grouped together in a flexible cord, as shown in Fig.
145, and are connected respectively to the various binding posts or
contact points within the desk stand at one end and at the base of the
bell box at the other end. These flexible conductors are insulated
individually and covered by a common braided covering. They usually are
individualized by having a colored thread woven into their insulating
braid, so that it is an easy matter to identify the two ends of the
same conductor at either end of the flexible cord or cable.
[Illustration: Fig. 151. Common-Battery Wall Set]
[Illustration: Fig. 152. Common-Battery Wall Set]
Common-Battery Telephone Sets. Owing to the fact that common-battery
telephones contain no sources of current, they are usually somewhat
simpler than the magneto type. The component parts of a
common-battery telephone, whether of the wall or desk type, are the
transmitter, receiver, hook switch, polarized bell, condenser, and
sometimes an induction coil. The purpose of the condenser is to
prevent direct or steady currents from passing through the windings of
the ringer while the ringer is connected across the circuit of the
line during the time when the telephone is not in use. The
requirements of common-battery signaling demand that the ringer shall
be connected with the line so as to be receptive of a call at any time
while the telephone is not in use. The requirements also demand that
no conducting path shall normally exist between the two sides of the
line. These two apparently contradictory requirements are met by
placing a condenser in series with the ringer so that the ringer will
be in a path that will readily transmit the alternating ringing
currents sent out from the central-office generator, while at the same
time the condenser will afford a complete bar to the passage of steady
currents. Sometimes the condenser is also used as a portion of the
talking apparatus, as will be pointed out.
[Illustration: MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS
CITY, MO.]
_Wall._ In Figs. 151 and 152 are given two views of a characteristic
form of common-battery wall-telephone set, made by the
Stromberg-Carlson Manufacturing Company. The common-battery wall set
has usually taken this general form. In it the transmitter is mounted
on an adjustable arm at the top of the backboard, while the box
containing the bell and all working parts of the instrument is placed
below the transmitter, the top of the box affording a shelf for
writing purposes. In Fig. 151 are shown the hook switch and the
receiver; just below these may be seen the magnets of the polarized
bell, back of which is shown a rectangular box containing the
condenser. Immediately in front of the ringer magnets is the induction
coil.
[Illustration: Fig. 153. Stromberg-Carlson Common-Battery Wall Set]
In Fig. 153 are shown the details of the circuit of this instrument.
This figure also includes a simplified circuit arrangement from which
the principles involved may be more readily understood. It is seen
that the primary of the induction coil and the transmitter are
included in series across the line. The secondary of the induction
coil, in series with the receiver, is connected also across the line
in series with a condenser and the transmitter.
_Hotel._ Sometimes, in order to economize space, the shelf of
common-battery wall sets is omitted and the entire apparatus mounted
in a small rectangular box, the front of which carries the transmitter
mounted on the short arm or on no arm at all. Such instruments are
commonly termed hotel sets, because of the fact that their use was
first confined largely to the rooms in hotels. Later, however, these
instruments have become very popular in general use, particularly in
residences. Sometimes the boxes or cabinets of these sets are made of
wood, but of recent years the tendency has been growing to make them
of pressed steel. The steel box is usually finished in black enamel,
baked on, the color being sometimes varied to match the color of the
surrounding woodwork. In Figs. 154 and 155 are shown two views of a
common-battery hotel set manufactured by the Dean Electric Company.
Such sets are extremely neat in appearance and have the advantage of
taking up little room on the wall and the commercial advantage of
being light and compact for shipping purposes. A possible disadvantage
of this type of instrument is the somewhat crowded condition which
necessarily follows from the placing of all the parts in so confined a
space. This interferes somewhat with the accessibility of the various
parts, but great ingenuity has been manifested in making the parts
readily get-at-able in case of necessity for repairs or alterations.
[Illustration: Fig. 154. Steel Box Hotel]
[Illustration: Fig. 155. Steel Box Hotel Set]
_Desk_. The common-battery desk telephone presents a somewhat simpler
problem than the magneto desk telephone for the reason that the
generator and local battery, the two most bulky parts of a magneto
telephone, do not have to be provided for. Some companies, in
manufacturing desk stands for common-battery purposes, mount the
condenser and the induction coil or impedance coil, or whatever device
is used in connection with the talking circuit, in the base of the
desk stand itself, and mount the polarized ringer and the condenser
used for ringing purposes in a separate bell box adapted to be
mounted on the wall or some portion of the desk. Other companies mount
only the transmitter, receiver, and hook switch on the desk stand
proper and put the condenser or induction coil, or other device
associated with the talking circuit, in the bell box. There is little
to choose between the two general practices. The number of conducting
strands in the flexible cord is somewhat dependent on the arrangement
of the circuit employed.
[Illustration: Fig. 156. Common-Battery Desk Set]
[Illustration: Fig. 157. Bell for Common-Battery Desk Set.]
The Kellogg Switchboard and Supply Company is one which places all the
parts, except the polarized ringer and the associated condenser, in
the desk stand itself. In Fig. 156 is shown a bottom view of the desk
stand with the bottom plate removed. In the upper portion of the
circle of the base is shown a small condenser which is placed in the
talking circuit in series with the receiver. In the right-hand portion
of the circle of the base is shown a small impedance coil, which is
placed in series with the transmitter but in shunt relation with the
condenser and the receiver.
[Illustration: Fig. 158. Bell for Common-Battery Desk Set]
In Figs. 157 and 158 are shown two views of the type of bell box
employed by the Kellogg Company in connection with the common-battery
desk sets, this box being of pressed-steel construction and having a
removable lid, as shown in Fig. 158, by which the working parts of the
ringer are made readily accessible, as are also the terminals for the
cord leading from the desk stand and for the wires of the line
circuit. The condenser that is placed in series with the ringer is
also mounted in this same box. By employing two condensers, one in the
bell box large enough to transmit ringing currents and the other in
the base of the desk stand large enough only to transmit voice
currents, a duplication of condensers is involved, but it has the
corresponding advantages of requiring only two strands to the flexible
cord leading from the bell box to the desk stand proper.
[Illustration: Fig. 159. Microtelephone Set]
A form of desk-telephone set that is used largely abroad, but that has
found very little use in this country, is shown in Fig. 159. In this
the transmitter and the receiver are permanently attached together,
the receiver being of the watch-case variety and so positioned
relatively to the transmitter that when the receiver is held at the
ear, the mouthpiece of the transmitter will be just in front of the
lips of the user. In order to maintain the transmitter in a vertical
position during use, this necessitates the use of a curved mouthpiece
as shown. This transmitter and receiver so combined is commonly
called, in this country, the _microtelephone set_, although there
seems to be no logical reason for this name. The combined transmitter
and receiver, instead of being supported on an ordinary form of hook
switch, are supported on a forked bracket as shown, this bracket
serving to operate the switch springs which are held in one position
when the bracket is subjected to the weight of the microtelephone, and
in the alternate position when relieved therefrom. This particular
microtelephone set is the product of the L.M. Ericsson Telephone
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]