Volume Ii Part 18 (1/2)
=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.