Patent GB-170600-A (GB)
Abstract: 170,600. Western Electric Co., Ltd., (Assignees of Bailey, R. S.). Oct. 25, 1920, [Convention date]. Exchange systems.-Relates to a cord circuit for private branch exchange systems, for connecting either two P.B.X. lines or a P.B.X. line and a main exchange line, and comprises several features concerning the establishment of connections with the main exchange. The jack sleeves of P.B.X. lines E1, E2 are grounded, but that of the main exchange line L1 is connected to battery, so that when a cord is plugged into the line L1 a relay 4 is energized. Battery is normally connected across the cord through back contacts of relay 4 and inductances 5, 6, but when the relay 4 is energized it disconnects battery and closes a bridge through inductances 5, 6 and a back contact of relay 7, so as to trip the machine ringing apparatus at the main exchange. The relay 4 also short-circuits the supervisory relay 8 of the answering side of the cord. The insertion of the plug P1 extinguishes the call lamp 2 and unlocks relay 1, which was energized by the ringing current. The operator listens by her key K3, completes the wanted connection to the line E2, and presses the ringing key K2. When the subscriber E2 answers the call, the supervisory relay 24 energizes relay 7, which locks itself up, prepares a circuit for relay 81, and opens the bridge 5, 6. A supervisory and feed circuit now extends from line L1 to line E2 through wires 10, 13, 15 on the tip side and 17, 20, 21, 23, 25 on the ring side. When the subscriber E2 disconnects himself, the de-energization of relay 24 lights lamp 40 and energizes relay 81, which opens the cord-circuit and connects battery across its calling end. If the P.B.X. operator delays taking down the cord, a further call from the main exchange is not transmitted to subscriber E2 but energizes a relay 42 and lamp 43, while on the other hand a further call from subscriber E2 re-energizes the relay 24 and de-energizes the relay 81 so as to restore connection with the main exchange. The main exchange may be an automatic exchange and a P.B.X. subscriber originating a call may desire either to do the dialling himself or that the operator should do it. In the former case the operator throws a key K4 which puts relays 4, 7, 81 out of action and extends a direct metallic circuit through the cord independent of the listening key K3. If, on the other hand, the dialling is to be done by the operator, the key K4 is not used, but the listening key K3 is left thrown. The calling side of the cord circuit is connected by the key K3 to wires 37, 38 and the answering side to wires 29, 34. When the operator's dial D is moved from normal, a slow-release relay 51 is energized which energizes a relay 47. Relay 47 bridges the impulse springs 56 across the calling end of the cord and energizes relay 28, which connects the answering end of the cord to relay 52, which locks relay 28 independently of relays 51, 47. After dialling, the operator should restore the key K3, but if she omits to do so the subscribers are not prevented from conversing since the calling and answering ends of the cord are now connected, for talking purposes, through the repeater 53, the answering end being supplied with current through relay 52 and the calling end being bridged through an inductance 46. When the operator goes off duty, she may leave the cord plugged between the main exchange line L1 and a P.B.X. line E2 with the key K4 thrown. Under these conditions the line E2 is a simple metallic continuation of the line L1 and can receive calls direct from the main exchange.