Patent GB-140561-A (GB)
Title: Improvements in and relating to inter-communication telephone systems
IPC: H04M9/00
Filed: 1919-02-19
Abstract: 140,561. Bygrave, L. C. Feb. 19, 1919. Intercommunication systems and apparatus.- In an intercommunication telephone system, a relay performs the functions of a calling-device, a busy test, and an inductance for localizing the talking currents. A third station is prevented from switching in upon an engaged subscriber, by rendering the current through its switching-relay insufficient to operate the relay. Two batteries may be employed which are connected in series for calling, while only one is employed for talking; or one battery may be used for calling and the other for talking. Busy test circuits and interrupted call circuits are provided. The selector switch may be interconnected with the calling- switch, held in its operative position by the removal of the telephone, in order that the selector switch need not be returned to its zero position after conversation, the circuit being broken by replacing the telephone. Each station may be provided with two lines to prevent inductive interference. When station No. 1, Fig. 1, wishes to call station No. 2, the selector switch C' is placed in the position shown, the telephone A<1> is removed from its hook E<1>, and the switch D<1> depressed. A calling-circuit is completed thus : line a, lead a', lower contact of switch D<1>, winding b<1> of switch B<1>, selector switch C<1>, line 2, lead b, and a parallel circuit consisting of telephone A<2> and upper contact of switch D<2>, winding b<2> of switch B<2>, and hook-switch E<2>, to line a<2>. The operation of the switch B<2> short-circuits its winding b<2>, so that its armature vibrates to send a buzzing current through the telephones A<1>, A<2>. When the telephone A<2> is removed from its hook E<2> at station No. 2, the telephones A', A<2> are connected in parallel with each other and in series with the winding b' across the supply lines a, a<2>, the winding b' acting as an impedance to localise the fluctuating talking currents. If station No. 3 attempts to call station No. 2 while stations No. 1 and No. 2 are talking, the winding b<3> of switch B<3> is connected in parallel with the winding b<1> of switch B<1> and in series with the parallelled telephones A<1>, A<2>, in which condition the current through the winding b<3> is not sufficient to operate the switch B<3>. to switch in station No. 3. In the modification shown in Fig. 2, the switch b<2> breaks or introduces a resistance into its operating-circuit, instead of short-circuiting its winding b<2>. In the modification shown in Fig. 3, each station is provided with battery F. In the modification shown in Fig. 4. the batteries F<1>, F<2> are connected in series for cailing, while the speaking circuits are placed across the battery F<2>. In the modification shown in Fig. 5, the battery F<1> is employed for calling and the battery F<1> for talking. The station battery F<2>, Fig. 6, is employed for speaking and is connected in series with the common battery F<1> for calling. To prevent inductive interference, each station may be provided with two lines as shown in Fig. 7. Additional contacts may be provided on the switches B to disconnect the talking circuits on both sides. A busy test circuit 7 is provided in the arrangement shown in Fig. 8, which is closed by the operation of switch D and broken at the upper contact of switch B if the wanted station is not engaged. An interrupted call may be provided by placing a thermal interrupter between a common calling-line and the calling-battery. The switch D, Fig. 10, is provided with a magnetic extension which retains in its depressed condition when the coil B is energized, and the selector switch C is provided with additional contacts H in the circuit of the coil B so that the switch D is released when the selector C is operated. In a modification, the switch D is held on by a latch operated by the coil B and released by short-circuiting this coil when the selector switch is operated. To avoid having to return the selector C to zero after use, the pivot of the selector is connected to a contact c, Fig. 12, of the switch D, in order that this circuit may be broken when the switch D is released by placing the telephone upon its hook. In the modification shown in Fig. 13, a relay L shunted by a non-inductive resistance or capacity M is employed, and the relay L is de-energized when the selector C is operated. The selector C may consist of press-buttons, Fig. 14, which are locked in position until another button is depressed, and which, during depression, break the circuit of the coil of switch B by the operation of the locking-bar O. The switch D may be replaced by a switch operated by the locking-bar O. Modifications of bush-button selectors are described in which two contact-springs are operated by each key, and in which, if two buttons are depressed simultaneously, only one circuit is completed.