Fig. 313. Battery Supply with Impedance Coils and Condensers
[View full size illustration.]

Cord Circuit. Battery Supply. Were it not for the necessity of providing for cord-circuit signals in common-battery switchboards, the common-battery cord circuit would be scarcely more complex than that for magneto working. Stripped of all details, such as signals, ringing and listening keys, and operator's equipment, cord circuits of three different types are shown in Figs. 311, 312, and 313. These merely illustrate the way in which the battery is associated with the cord circuits and through them with the line circuits for supplying current for talking purposes to the subscribers. It is thought that this matter will be clear in view of the discussion of the methods by which current is supplied to the subscribers' transmitters in common-battery systems as discussed in Chapter XIII. While the arrangements in this respect of Figs. 311, 312, and 313 illustrate only three of the methods, these three are the ones that have been most widely and successfully used.

Supervisory Signals. The signals that are associated with the cord circuits are termed supervisory signals because of the fact that by their means the operator is enabled to supervise the condition of the lines during times when they are connected for conversation. The operation of these supervisory signals may be best understood by considering the complete circuits of a simple switchboard and must be studied in conjunction with the circuits of the lines as well as those of the cords.

Fig. 314. Simple Common-Battery Switchboard
[View full size illustration.]

Complete Circuit. Such complete circuits are shown in Fig. 314. The particular arrangement indicated is that employed by the Kellogg Company, and except for minor details may be considered as typical of other makes also. Two subscribers' lines are shown extending from Station A and Station B, respectively, to the central office. The line wires are shown terminating in jacks in the same manner as indicated in Figs. 307, 308, and 309, and their circuits are normally continued from these jacks to the ground on one side and to the line relay and battery on the other. The jack in this case has three contacts adapted to register with three corresponding contacts in each of the plugs. The thimble of the jack in this case forms no part of the talking circuit and is distinct from the two jack springs which form the line terminals. It and the auxiliary contact 1 in each of the plugs with which it registers, are solely for the purpose of co-operating in the control of the supervisory signals.

The tip and sleeve strands of the cord are continuous from one plug to the other except for the condensers. The two batteries indicated in connection with the cord circuit are separate batteries, a characteristic of the Kellogg system. One of these batteries serves to supply current to the tip and sleeve strand of the cord circuit through the two windings 3 and 4, respectively, of the supervisory relay connected with the answering side of the cord circuit, while the other battery similarly supplies current through the windings 5 and 6 of the supervisory relay associated with the calling side of the cord circuit. The windings of these relays, therefore, act as impedance coils and the arrangement by which battery current is supplied to the cord circuits and, therefore, to the lines of the connected subscribers, is seen to be the combined impedance coil and condenser arrangement discussed in Chapter XIII.

As soon as a plug is inserted into the jack of a line, the line relay will be removed from the control of the line, and since the two strands of the cord circuit now form continuations of the two line conductors, the supervisory relay will be substituted for the line relay and will be under control of the line. Since all of the current which passes to the line after a plug is inserted must pass through the cord-circuit connection and through the relay windings, and since current can only flow through the line when the subscriber's receiver is off its hook, it follows that the supervisory relays will only be energized after the corresponding plug has been inserted into a jack of the line and after the subscriber has removed his receiver. Unlike the line relays, the supervisory relays open their contacts to break the local circuits of the supervisory lamps 7 and 8 when the relay coils are energized, and to close them when de-energized; but the armatures of the supervisory relays do alone control the circuits of the supervisory lamps. These circuits are normally held open in another place, that is, between the plug contacts 1 and the jack thimbles. It is only, therefore, when a plug is inserted into a jack and when the supervisory relay is de-energized, that the supervisory lamp may be lighted. When a plug is inserted into a jack and when the corresponding supervisory relay is de-energized, the circuit may be traced from ground at the cord-circuit batteries through the left-hand battery, for instance, through lamp 7, thence through the contacts of the supervisory relay to the contact 1 of the plug, thence through the thimble of the jack to ground. When a plug is inserted into the jack, therefore, the necessary arrangements are completed for the supervisory lamp to be under the control of the subscriber. Under this condition, whenever the subscriber's receiver is on its hook, the circuit of the line will be broken, the supervisory relay will be de-energized, and the supervisory lamp will be lighted. When, on the other hand, the subscriber's receiver is off its hook, the circuit of the line will be complete, the supervisory relay will be energized, and the supervisory lamp will be extinguished.

Salient Features of Supervisory Operation. It will facilitate the student's understanding of the requirements and mode of operation of common-battery supervisory signals in manual systems, whether simple or multiple, if he will firmly fix the following facts in his mind. In order that the supervisory signal may become operative at all, some act must be performed by the operator—this being usually the act of plugging into a jack—and then, until the connection is taken down, the supervisory signal is under the control of the subscriber, and it is displayed only when the subscriber's receiver is placed on its hook.

Cycle of Operations. We may now trace through the complete cycle of operations of the simple common-battery switchboard, the circuits of which are shown in Fig. 314. Assume all apparatus in its normal condition, and then assume that the subscriber at Station A removes his receiver from its hook. This pulls up the line relay and lights the line lamp, the pilot relay also pulling up and lighting the common pilot lamp which is not shown. In response to this call, the operator inserts the answering plug and throws her listening key L.K. The operator's talking set is thus bridged across the cord circuit and she is enabled to converse with the calling subscriber. The answering supervisory lamp 7 did not light when the operator inserted the answering plug into the jack, because, although the contacts in the lamp circuit were closed by the plug contact 1 engaging the thimble of the jack, the lamp circuit was held open by the attraction of the supervisory relay armature, the subscriber's receiver being off its hook. Learning that the called-for subscriber is the one at Station B, the operator inserts the calling plug into the jack at that station and presses the ringing key R.K., in order to ring the bell. The act of plugging in, it will be remembered, cuts off the line-signaling apparatus from connection with that line. As the subscriber at Station B was not at his telephone when called and his receiver was, therefore, on its hook, the insertion of the calling plug did not energize the supervisory relay coils 5 and 6, and, therefore, that relay did not attract its armature. The supervisory lamp 8 was thus lighted, the circuit being from ground through the right-hand cord-circuit battery, lamp 8, back contacts of the supervisory relay, third strand of the cord to contact 1 of the calling plug, and thence to ground through the thimble of the jack. The lighting of this lamp is continued until the party at Station B responds by removing his receiver from its hook, which completes the line circuit, energizes relay windings 5 and 6, causes that relay to attract its armature, and thus break the circuit of the lamp 8. Both supervisory lamps remain out as long as the two subscribers are conversing, but when either one of them hangs up his receiver the corresponding supervisory relay becomes de-energized and the corresponding lamp lights. When both of the lamps become illuminated, the operator knows that both subscribers are through talking and she takes down the connection.