GB355182A - Improvements in switching arrangements for telephone systems - Google Patents

Abstract

355,182. Exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON, 33, Kungsgatan, Stockholm.-(Assignees of Vigren, S. D. ; 41, Banergatan, Stockholm.) March 19, 1931, No. 8513. Convention date, March 21, 1930. [Class 40 (iv).] The microphone at a prepayment station is associated with one or more rectifiers, and the direction of line current is reversed during the period between the completion of the wanted connection and the operation of the prepayment device to render the microphone ineffective. Relays A, B energize when the subscriber calls, lighting the lamp AL, which is extinguished by the energization of relay BR when the operator plugs in. The operator sets up the call and throws the ringing key EOR, whereupon relay SVR energizes over the sleeve circuit and locks, reversing the line current to disable the microphone M. When the prepayment device D is operated e.g. by the dropping of a coin, the insertion of the resistance R releases relay A, and relay SVR falls back due to the short-circuiting of its holding winding. In a modification applicable also to automatic working, the current reversal is dependent on the reply of the called subscriber. As shown two rectifiers L1, L2 are used, but it may be sufficient to use a single rectifier in series or in parallel with the microphone M.