Circuit arrangement for preventing the eavesdropping of talks in the case of telephone

摘要

995,025. Substation circuits. G. KRONE. Nov. 5, 1962 [Nov. 22, 1961; Jan. 11, 1962], No. 41680/62. Heading H4K. To reduce the possibility of overhearing caused by speech voltages picked up by the ringer W, which is connected across the line in series with a capacitor Cl, the line is normally shunted by a second capacitor C3 which is removed at contact UI when the cradle switch is lifted. The receiver T is normally disconnected at contact UIII, while the speech transformer is short-circuited on the primary side by contact UII. Modifications are described in which the capacitor C1 serves also for spark-quenching. A further embodiment shows main and extension substations which have individual ringers and are connected alternatively to the line. A known arrangement is mentioned which employs a glow lamp in series with a tuned ringer.