Patent GB-213276-A (GB)
Title: Improvements in or relating to intercommunicating telephone systems
IPC: H04M5/04
Filed: 1924-03-21
Abstract: 213,276. Allmanna Telefonaktiebolaget L. M. Ericsson. March 23, 1923, [Convention date]. Intercommunication systems in which a wanted station is selected by a push button which is then locked electromagnetically, are provided with test relays which control the locking magnets so that if the button of a busy station is pressed it is not locked, and if two buttons are pressed at once neither is locked. Separate test relays are provided for local and central office calls so that a station in communication with central office may communicate with a local station without releasing the trunk. The Figure shows the circuits at one station. Outgoing calls over trunks CL<1>, CL<2>. Depression of the key CK<1> connects test relay AR to the c-wire of the selected trunk CL<1>, 1, 3, 4, AR, 5, c. If the trunk is idle, relay AR energizes, connects the receiver T and transformer IS to the a and b-wires, short-circuits its high resistance winding W<2> to busy the trunk, and energizes the looking magnet CM<1>. The a and b-wires arebridged by a coil SS<1>. Removal of the hand-set connects battery to the microphone M. When the hand-set is replaced, the hook switch HS in moving to its lower position breaks the contact 1, but subsequently slides over the pin HT to allow the contact to be re-made. The momentary interruption releases magnet CM<1> and the 'button CK<1> then releases relay AR. As the circuit for relay AR goes over normal contacts of all the push buttons, it cannot energize if more than one are depressed at once. If button CK<2> is depressed before the hand-set has been replaced, relay AR is released at contact 3 and releases button CK<1>, and if button CK<2> is kept depressed. relay AR reenergizes and energizes magnet CM<2>. If the trunk CL<1> had been busy, relay AR would have been unable to energize in parallel with the low resistance winding of the AR relay of the station using the trunk. Local calls. Depression of button LK<2> completes a test circuit 1, 27, 18, 19, 20, BR, 21, awire of line LL<2>, and resistance P and contact 22 of the wanted station. If the wanted station is idle, relay BR energizes, locks up over resistance Q, short-circuits its winding W<2> to busy the line and energizes the magnet LM<2>. The calling push B is now operated to ring the bell GS at the wanted station over the b-wire. Release &c. are the same as for central office calls. When a station in communicaation with central office wishes to talk to a local station, he makes the local call in the ordinary way, relay BR switching over the instruments to the local line. Communication over the trunk is re-established by momentarily pressing key O which releases relay BR and the operated magnet LM. Incoming calls. Only one of the stations can be rung by the central office. A bell WS which is disconnected by relay WSR when the trunk is taken into use by the local station, is provided for that purpose. The call is answered by pressing the button CK belonging to the calling trunk. If central office wants some other local station that station is advised by the answering station and connects himself to the calling trunk by his CK button. The answering station presses his key O to release himself and allow the relay AR of the wanted station to operate. At the wanted station, lamp DL glows until the relay AR operates.