Patent GB-521879-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M9/00
Filed: 1938-11-30
Abstract: 521,879. Signalling on trains. ASSOCIATED ELECTRIC LABORATORIES, Inc. Nov. 30, 1938, No. 34909. Convention date, Dec. 10, 1937. [Class 105] [Also in Group XXXIX] In a telephone system specially suitable for trains, a three-wire talking circuit is provided to prevent side-tone and the stations are selected by means of calling circuits using these wires in different pairs. In the system described the transmitters are of the electro-magnetic type and the receiver in the driver's cab is provided with an amplifier and is of the loudspeaking type. Two other stations are provided, one at an intermediate point and one in the rear car. The latter has an extension ininstrument at the extreme rear of the train. This can be used for answering all calls and for making calls to the driver's cab. Calls from intermediate station 200.-On the removal of the handset 246, 247, the hookswitch 240 takes up the condition shown bridging the transmitter 246 across line-wires 101, 102 and receiver 247 across line-wires 102, 103. If the line is idle and station 100 is wanted, non-locking key 230 is operated to connect the sub-station battery 250 across the wires 102, 103, and energize relay 110 in the cab. This lights calling lamp 132 and completes the talking circuit. The engine driver then throws key 131, which remains off-normal until reset, to hold 110 independently of the distant calling key. The call is released by the restoration of 131 and the handset at station 200. If station 300 is wanted, non-locking key 220 is operated to energize 320 over wires 101, 103, and so operates a buzzer 338 in the rear compartment. The key also energizes 210 which locks up and transposes the connection of the transmitter and receiver so that a both-way speaking connection is established when the handsets at both stations have been removed. This call can be answered from the extension 400 by removing the handset there, spring 421, 422 being closed when the door of its retaining compartment is opened. When the calling party hangs up, 210 is unlocked and release is completed when the other party hangs up. Calls from rear-station 300 and extension station 400.-Calls to the cab are set up as from station 200 by the removal of handset 356, 357 and operation of key 340. Calls can also be set up from the extension station by removing the handset there from its compartment, thus operating springs 421, 422 and operating key 418. This energizes 410 which energizes 110 at the cab station to light the calling lamp. To call station 200, key 330 is operated to actuate buzzer 237 over wires 101, 102. 330 also energizes 310 which locks up and transposes the connections of the transmitter and receiver. Calls from cab station 100.-To call the rear car, the driver operates the right-hand springs of switch 120 to connect battery between the wires 101, 103 and so energize relay 320, and then operates key 131 to energize 110 and complete the talking circuit at the calling end. The call may be answered if desired at the extension station 400. If the station 200 is wanted, the left-hand springs of switch 120 are actuated to connect battery to wires 101, 102 and so operate the buzzer 237.