Device for automatically signalling incoming calls on normal already engaged telephones

Claim 1

Apparatus for automatically signalling incoming calls to a telephone already connected to another telephone through the normal communication network of a telephone exchange, the normal communication network at the exchange including: a line selector switch having a movable arm and a plurality of switch positions; and a series circuit connected across a power source and comprising a resistance element with a shunt relay contact thereacross, a selector relay coil operable to open and close said shunt relay contact, and one of a plurality of preselector coils selectable by said line selector switch; said apparatus comprising, for each telephone, auxiliary circuitry located at the exchange, said auxiliary circuitry comprising: a supplemental resistance element series connected in one lead of the power supply lines extending to each telephone; at least one supplementary relay contact shunting said supplemental resistance element; a thermal switch coil placed in said series circuit; a low voltage relay coil series connected with a thermal switch contact actuated by said thermal switch coil, the low voltage relay coil and thermal switch contact being connected between the arm of said linE selector switch and one of the power lines of said power source; whereby the opening of said supplemental relay contact in response to actuation of said low voltage relay, which in turn is actuated by a third telephone being connected to the connection already made between said telephone and said other telephone, alters the current in said power supply lines and thus effects signalling of an incoming call.