GB591495A - Improvements in telephone substation circuits of the anti-side-tone type - Google Patents

Abstract

591,495. Anti-side-tone circuits. WESTERN ELECTRIC CO., Inc. June 2, 1944, No. 10687. Convention date, Oct. 24, 1942. [Class 40 (iv)] In a substation circuit of the anti-side-tone type, the impedance of the balancing means is changed in accordance with the direct current flowing in the line, so that the balance is automatically adjusted for lines of differing lengths. The variation in impedance may be by a non-linear resistance variable by applied voltage or by heating. In the arrangement shown in Fig. 3, the non-linear resistance 60, comprising two parallel opposed copper oxide elements, is connected across the balancing arm X, C1 and the value of X is chosen to balance a line of maximum anticipated length. If a shorter line is connected to terminals L1, L2, the direct current flowing is greater and this causes the resistance of V to decrease to a point where the combination of X and V again balances the line. Fig. 4 shows an arrangement in which the direct current from the line L1, L2 is passed through a heating coil H, the variation in whose temperature affects the sensitive resistances E1, E2. Various other arrangements are described giving the selective insertion or cutting out of inductive or capacitive branches instead of a simple resistance. Suggested varistors are copper oxide and silicon carbide and suggested thermistors are silver sulphide, uranium oxide and mixtures of metallic oxides.