Patent GB-166700-A (GB)
Title: Improvements in or relating to intercommunication telephones
IPC: H04M9/00
Filed: 1920-04-20
Abstract: 166,700. Palmer, H. J., and Telephone Manufacturing Co., Ltd. April 20, 1920. Intercommunication systems. - In systems in which separate signalling and speaking batteries are multipled to each station, the signalling battery D is cut out and the speaking battery C brought into circuit at the calling and called stations by relays B, which are energized when the selector A at the called station is moved to the contact of the calling station. These relays operate also to complete the talking connections and to lock the selectors. The circuits of a single station are shown in Fig. 1. It will be seen that normally the positive poles of the batteries C, D are open-circuited at contacts G', J<3>, H<2>, while the negative poles are connected in circuit through contacts G<1>, H<1>. At a calling station, however, this condition of things is reversed by the depression of a ringing-key F, which closes contacts G<2>, J<3>, H<2> and opens G<1>, H<1>. Thus only the positive poles of the two batteries are operatively connected at a calling station and only the negative poles at a called station. The key F is constructed so that when it is released it stays in an intermediate position, opening the ringing contact J<3> but not restoring the other contacts. The key is eventually restored to its normal position by a lever J when the telephone is replaced on its hook or cradle. A call is made by lifting the telephone from its hook (which opens contact L' and closes L<2>), turning the selector A to the contact of the wanted station, and pressing the ringing-key F. By these operations a circuit is closed from the positive pole of battery D, through contacts J<3>, J<4> of the key F, indicator E<1>, and tatler E<2>, to the selector A, and thence over the home-line of the wanted station to the terminal HL at that station, through the contact L<1> and buzzer E to the negative pole of battery D. At the called station the call is answered by lifting the telephone (which opens the circuit of the buzzer E at the contact L<1>) and turning the selector A. When the selector reaches the contact of the calling station, a circuit is closed from the negative pole of battery D, contacts H<1>, H, upper contact of springs A<1>, coil B, upper contact of springs A<2>, contacts G<3>, G<4>, to the selector A, and thence over the home-line of the calling station to the terminal HL at that station, through contact L<2>, coil B, upper contact of springs A<1>, contacts H, H<2> to the positive pole of battery D. The relays B at both stations are energized in this circuit and shift the springs A<1>, A<2>, A<3>, with the following consequences :-(1) the selectors at both stations are locked, (2) the signalling battery D at both stations is entirely disconnected, making it impossible for a third station to cut in, (3) the speaking battery C is connected in place of the battery D, and (4) the talking circuits are completed, the telephone instruments, which have hitherto been on open circuit, being brought into connection. The circuit is now as follows :-at the calling station, from the positive pole of battery C, through springs G<2>, G, lower contact of springs A', coil B, to junction c; at the called station, from the negative pole of battery C, through springs G<1>, G, lower contact of springs A<1>, coil B, to junction c; and between the junctions c at the two stations, two parallel circuits each including the bottom contact of springs A<2>, transmitter T and selector A of one station, and the home-line and contact L<2> of the other station. The invention is applicable to systems in which the selector at the called station operates automatically, and to systems, such as that described in Specification 162,748, affording alternative connection to a public exchange.