Cyclopedia of Telephony and Telegraphy Volume Ii Part 18

=Marking of Apparatus.= All apparatus should be marked with permanent and clear labels. That private exchange switchboard is best at which an almost uninformed operator could sit and operate it at once. It is not difficult to lay out a scheme of labels which will enable such a board to be operated without any detailed instructions being given.

=Desirable Features.= The board should contain means of connecting certain of the local private exchange lines to the central-office trunks when the board is unattended. Also, it is desirable that it should contain means whereby any local private exchange line may be connected to the trunk so that its station will act as an ordinary subscriber's station. Whether the trunks of the private exchange lead to a manual or an automatic equipment, it often is desired to connect a local line through in that way, either so that the calling person may make his calls without the knowledge of the private exchange operator, because he wishes to make a large number of calls in succession, or because for some other reason he prefers to transact his business directly with or through the exchange than to entrust it to his operator.

CHAPTER x.x.xV

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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 b.u.t.tons for doing the same thing. These may be referred to as the plug type and the push-b.u.t.ton type, respectively.

[Ill.u.s.tration: 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 r.e.t.a.r.dation 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 b.u.t.tons 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 b.u.t.ton; 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.

[Ill.u.s.tration: Fig. 443. Push-b.u.t.ton Wall Set]

_Kellogg Push-b.u.t.ton Type._ Fig. 443 shows a Kellogg wall-type intercommunicating set employing the push-b.u.t.ton method of selecting, and Fig. 444 shows the internal arrangement of this set.

[Ill.u.s.tration: Fig. 444. Push-b.u.t.ton Wall Set]

_Western Electric System._ The method of operation of the push-b.u.t.ton key employed in the intercommunicating system of the Western Electric Company is well shown in Fig. 445. When the b.u.t.ton 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 b.u.t.ton a.s.sumes 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 b.u.t.ton which releases the key so that it a.s.sumes its normal position as shown in the left-hand cut. When a b.u.t.ton 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 pa.s.ses 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 b.u.t.ton 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.

[Ill.u.s.tration: Fig. 445. Push-b.u.t.ton 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-b.u.t.ton arrangement of switches.

Two batteries are employed, one for ringing and the other for talking, talking current being fed to the lines through r.e.t.a.r.dation 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-b.u.t.ton type and as in the system just discussed, one pressure of the finger on one b.u.t.ton 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 b.u.t.ton down at one station, the home b.u.t.ton 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.

[Ill.u.s.tration: Fig. 446. Push-b.u.t.ton Wall Set]

[Ill.u.s.tration: Fig. 447. Push-b.u.t.ton Action, Monarch System]

Three views of the wall set of this system are shown in Fig. 446, which ill.u.s.trates 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-b.u.t.ton systems, each push-b.u.t.ton key has three positions, as shown in Fig. 447. The first b.u.t.ton shows all the springs open, the normal position of the key. The second b.u.t.ton is in the half-way or talking position with all the springs, except the ringing spring, in contact. The third b.u.t.ton 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 b.u.t.ton 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 b.u.t.tons as other stations are added.

[Ill.u.s.tration: Fig. 448. Push-b.u.t.ton 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 b.u.t.ton is equipped with a shoulder, which normally is above the round of the ladder. When the b.u.t.ton 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 pa.s.sing 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 b.u.t.ton 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 b.u.t.ton for each other station and a "home" b.u.t.ton. The salient feature of the system is that before a connection may be established, the b.u.t.ton at the calling station corresponding to the station called and also the home b.u.t.ton 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.

[Ill.u.s.tration: Fig. 449. Monarch Intercommunicating System]

Referring to Fig. 449, when the station called depresses the home b.u.t.ton 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 b.u.t.ton 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 b.u.t.ton 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 pus.h.i.+ng his trunk push b.u.t.ton. 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-b.u.t.ton 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 b.u.t.ton 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.

[Ill.u.s.tration: Fig. 450. Junction Box]

[Ill.u.s.tration: 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 b.u.t.ton, 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 b.u.t.ton, 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 b.u.t.tons at each station. These tap connections are best made by means of junction boxes which contain terminals for all the conductors.