INTERCOMMUNICATING SYSTEMS
=Definition.= The term "intercommunicating" has been given to a
specialized type of telephone system wherein the line belonging to each
station is extended to each of the other stations, resulting in all
lines extending to all stations. Each station is provided with apparatus
by means of which the telephone user there may connect his own telephone
with the line of the station with which he wishes to communicate,
enabling him to signal and talk with the person at that station.
=Limitations.= The idea is simple. Each person does his own switching
directly, and no operator is required. It is easy to see, however, that
the system has limitations. The amount of line wire necessary in order
to run each line to each station is relatively great, and becomes
prohibitive except in exchanges involving a very small number of
subscribers, none of which is remote from the others. Again, the amount
of switching apparatus required becomes prohibitive for any but a small
number of stations. As a result, twenty-five or thirty stations are
considered the usual practical limit for intercommunicating systems.
=Types.= An intercommunicating system may be either magneto or
common-battery, according to whether it uses magneto or common-battery
telephones. The former is the simpler; the latter is the more generally
used.
[Illustration: WESTERN ELECTRIC COMPANY BATTERY ROOM AT MONMOUTH,
ILLINOIS]
=Simple Magneto System.= The schematic circuit arrangement of an
excellent form of magneto intercommunicating system is given in Fig.
441. In this, five metallic circuit lines are led to as many stations,
an ordinary two-contact open jack being tapped off of each line at each
station. A magneto bell of the bridging type is permanently bridged
across each line at the station to which that line belongs. The
telephone at each station is an ordinary bridging magneto set except
that its bell is, in each case, connected to the line as just stated.
Each telephone is connected through a flexible cord to a two-contact
plug adapted to fit into any of the jacks at the same station.
The operation is almost obvious. If a person at Station _A_ desires to
call Station _E_, he inserts his plug into the jack of line _E_ at his
station and turns his generator crank. The bell of Station _E_ rings
regardless of where the plug of that station may be. The person at
Station _E_ responds by inserting his own plug in the jack of line _E_,
after which the two parties are enabled to converse over a metallic
circuit. It makes no difference whether the persons, after talking,
leave these plugs in the jacks or take them out, since the position of
the plug does not alter the relation of the bell with the line.
[Illustration: Fig. 441. Magneto Intercommunicating System]
This system has the advantage of great simplicity and of being about as
"fool proof" as possible. It is, however, not quite as convenient to use
as the later common-battery systems which require no turning of a
generator crank.
=Common-Battery Systems.= In the more popular common-battery systems two
general plans of operation are in vogue, one employing a plug and jacks
at each station for switching the "home" instrument into circuit with
any line, and the other employing merely push buttons for doing the
same thing. These may be referred to as the plug type and the
push-button type, respectively.
[Illustration: Fig. 442. Plug Type of Common-Battery Intercommunicating
System]
_Kellogg Plug Type._ The circuits of a plug type of intercommunicating
system, as manufactured by the Kellogg Company, are shown in Fig. 442.
While only three stations are shown, the method of connecting more will
be obvious.
This system requires as many pairs of wires running to all stations as
there are stations, and in addition, two common wires for ringing
purposes. The talking battery feed is through retardation coils to each
line. When all the hooks are down, each call bell is connected between
the lower common wire and the tip side of the talking circuit individual
to the corresponding station. The ringing buttons at each station are
connected between the tip of the plug at that station and the upper
common wire. As a result, when a person at one station desires to call
another, it is only necessary for him to insert his plug in the jack of
the desired station and press his ringing button; the circuit being
traced from one pole of the ringing battery through the upper common
ringing wire, ringing key of the station making the call, tip of plug,
tip conductor of called station's line, bell of called station, and back
to the ringing battery through the lower common ringing wire.
[Illustration: Fig. 443. Push-Button Wall Set]
_Kellogg Push-Button Type._ Fig. 443 shows a Kellogg wall-type
intercommunicating set employing the push-button method of selecting,
and Fig. 444 shows the internal arrangement of this set.
[Illustration: Fig. 444. Push-Button Wall Set]
_Western Electric System._ The method of operation of the push-button
key employed in the intercommunicating system of the Western Electric
Company is well shown in Fig. 445. When the button is depressed all the
way down, as shown in the center cut of Fig. 445, which represents the
ringing position of the key, contact is made with the line wires of the
station called, and ringing current is placed on the line. When the
pressure is released, the button assumes an intermediate position, as
shown in the right-hand cut, which represents the talking position of
the key and in which the ringing contacts _1_ and _2_ are open, but
contact with the line for talking purposes is maintained. The key is
automatically held in this intermediate position by locking plate _3_
until this plate is actuated by the operation of another button which
releases the key so that it assumes its normal position as shown in the
left-hand cut. When a button is depressed to call a station, it first
connects the called station's line to the calling station through the
two pairs of contacts _4_ and _5_ and then connects the ringing battery
to that line by causing the spring _1_ to engage the contact _2_. The
ringing current then passes through the bell at the called station,
through the back contacts of the switch hook at that station, over one
side of the line, and through the "way-down" contact _1_ of the button
at the calling station, thence over the other side of the battery line
back to the ringing battery, operating the bell at the called station.
[Illustration: Fig. 445. Push-Button Action, Western Electric System]
The circuits of the Western Electric system are similar to those of Fig.
442, but adapted, of course, to the push-button arrangement of switches.
Two batteries are employed, one for ringing and the other for talking,
talking current being fed to the lines through retardation coils to
prevent interference or cross-talk from other stations which might be
connected together at the same time.
_Monarch System._ As the making of connections in an intercommunicating
system is entirely in the hands of the user, it is desirable that the
operation be simple and that carelessness on the part of the user result
in as few evil effects as possible. For instance, the leaving of the
receiver off its hook will, in many systems, result in such a drain on
the battery as to greatly shorten its life.
The system of the Monarch Company has certain distinctive features in
this respect. It is of the push-button type and as in the system just
discussed, one pressure of the finger on one button clears the station
of previous connections, rings the station called, and establishes a
talking connection between the caller's telephone and the line desired.
In addition to this, the system is designed to eliminate battery waste
by so arranging the circuits that the battery current does not flow
through either called or calling instrument until a complete connection
is made--the calling button down at one station, the home button down at
the called station, and both receivers off the hook. It does not hurt
the batteries, therefore, if one neglects to hang up his receiver.
[Illustration: Fig. 446. Push-Button Wall Set]
[Illustration: Fig. 447. Push-Button Action, Monarch System]
Three views of the wall set of this system are shown in Fig. 446, which
illustrates how both the door and the containing box are separately
hinged for easy access to the apparatus and connecting rack. As in the
Western Electric and Kellogg push-button systems, each push-button key
has three positions, as shown in Fig. 447. The first button shows all
the springs open, the normal position of the key. The second button is
in the half-way or talking position with all the springs, except the
ringing spring, in contact. The third button shows the springs all in
contact, the condition which exists when ringing a station.
The mechanical construction of the key is shown in Fig. 448. Each button
has a separate frame upon which the springs are mounted. Any one of the
frames with its group of contact springs may be removed without
interfering with either the electrical or the mechanical operation of
the others. This is a convenient feature, making possible the
installation of as few stations as are needed at first, and the
subsequent addition of buttons as other stations are added.
[Illustration: Fig. 448. Push-Button Keys]
The restoring feature is a horizontal metal carriage, in construction
very much like a ladder--one round pressing against each key frame, due
to the tension on the carriage exerted by a single flat spring. The
plunger of each button is equipped with a shoulder, which normally is
above the round of the ladder. When the button is operated, this
shoulder presses against a round of the carriage forcing it over far
enough so that the shoulder can slip by. The upper surface of the
shoulder is flat, and on passing below the pin, allows the carriage to
slip back into its normal position and the pin rests on the top of the
shoulder holding the plunger down. This position places the talking
springs in contact. The ringing springs are open until the plunger is
pressed all the way down, then the ringing contact is made. When the
pressure is released, the plunger comes back to the half-way or talking
position, leaving the ringing contacts open again.
When another button is pressed, the same operation takes place and, by
virtue of the carriage being temporarily displaced, the original key is
left free to spring back to its normal position.
Each station is provided with a button for each other station and a
"home" button. The salient feature of the system is that before a
connection may be established, the button at the calling station
corresponding to the station called and also the home button of the
station called must be depressed, if it is not already down. The home
key at any station, when depressed, transposes the sides of the line
with respect to the talking apparatus. The home key also has a spring
which changes the normal connection of the line at that station from the
negative to the positive side of the talking battery. Unless, therefore,
a connection between two stations is made through the calling key at one
station and the home key at the other, no current can flow even though
both receivers are off their hooks, because in that case no connection
will exist with the positive side of the battery. This relation is shown
in Fig. 449, which gives a simplified circuit arrangement for two
connected stations.
[Illustration: Fig. 449. Monarch Intercommunicating System]
Referring to Fig. 449, when the station called depresses the home button
the talking circuit is then completed after the hook switch is raised.
This is because the talking battery is controlled by the home key.
Conductors from both the negative and the positive sides of the battery
enter this key. In the normal position of the springs, the negative side
of the battery is in contact with the master spring in the home key and
through these springs the negative battery is applied to all the calling
keys, and from there on to the hook switch. When, however, the home
button is operated, the spring which carries the negative battery to
the home key is opened, and the spring which carries the positive
battery is closed. This puts the positive battery on at the hook switch
instead of the negative battery, as in its normal condition.
In this system it is seen that a separate pair of line wires is used for
each station, and in addition to these, two common pairs are run to all
stations, one for ringing and one for talking battery connections.
=For Private Branch Exchanges.= So far the intercommunicating system has
been discussed only with respect to its use in small isolated plants. It
has a field of usefulness in connection with city exchange work, as it
may be made to serve admirably as a private branch exchange. Where this
is done, one or more trunk lines leading to an office of the city
exchange are run through the intercommunicating system exactly as a
local line in that system, being tapped to a jack or push button at
every station. A person at any one of the stations may originate a call
to the main office by inserting his plug in the trunk jack, or pushing
his trunk push button. Also any station, within hearing or sight of the
trunk-line signal from the main office, may answer a main-office call in
the same way. In order that the convenience of a private branch exchange
may be fully realized, however, it is customary to provide an
attendant's station at which is placed the drop or bell on which the
incoming trunk signal is received. The duty of this attendant during
business hours is to answer trunk calls from the main office and finding
out what party is desired, call up the proper station on the
intercommunicating system. The party at that station may then connect
himself with the trunk.
The practice of the Dean Company, for instance, is as follows in regard
to trunking between intercommunicating systems and main offices with
common-battery equipment. The attendant's station telephone cabinet
contains, besides the push-button keys for local and trunk connections,
a drop signal and release key, together with relays in each trunk
circuit. The latter are used to hold the trunks until the desired party
responds.
The main-exchange trunk lines, besides terminating at the attendant's
station, are wired through the complete intercommunicating system so
that any intercommunicating telephone can be connected direct to the
central office by depressing the trunk key, which is provided with a
button of distinctive color. The pressing of the trunk key allows the
telephone to take its current from the main-office storage battery and
to operate the main-office line and supervisory signals direct, without
making it necessary to call on the attendant to set up the connection.
[Illustration: Fig. 450. Junction Box]
[Illustration: Fig. 451. Typical Arrangement of Intercommunicating
System]
Incoming calls from the common-battery main office to the
intercommunicating system are all handled by the attendant. The
main-office operator signals the intercommunicating system by ringing,
the same as for a regular subscriber's line. This will operate a drop in
the attendant's station cabinet, and through an armature contact, give a
signal on a low-pitched buzzer. This alarm buzzer operates only when the
main exchange is ringing and, therefore, does not require that the drop
shutter be restored immediately. An extra key may be provided for an
extension night-alarm bell, for use where the attendant also does work
in a room separate from that containing the attendant's station
telephone equipment.
The attendant operator answers the main-line signal by pressing the
proper trunk button, as designated by the operated drop on the
attendant's cabinet. The answering of the trunk connects a locking relay
across the circuit so that the attendant may call the desired party on
the intercommunicating system without having to hold the trunk manually.
The party desired is then notified which trunk to use and the attendant
operator hangs up her receiver, no further attention being necessary on
her part.
The trunk-holding relay is automatically released when the desired party
(with the telephone receiver off the hook) depresses the proper trunk
button, thus clearing the trunk line of all bridged apparatus and making
the talking circuit the same as in the regular type of private
branch-exchange switchboard.
The most convenient way of installing the wires of an intercommunicating
system is to run a cable containing the proper number of pairs to
provide for the ultimate number of stations to all the stations, tapping
off from the conductors in the cable to the jacks or push buttons at
each station. These tap connections are best made by means of junction
boxes which contain terminals for all the conductors.
Such a junction box, with the through cable and the tap cable in place,
is illustrated in Fig. 450. A schematic lay-out of the various parts of
a Dean intercommunicating system, provided with an attendant's station
and with trunks to a city office, is given in Fig. 451.