GB508998A - - Google Patents

Abstract

508,998. Resistance and current measurements. PATIN, A. Dec. 5, 1938, No. 35414. Convention date, Jan. 14. [Class 37] [Also in Group XXXVIII] A resetting device for instruments in which only a small deflecting torque is produced comprises a current source such as a charged electrolytic or other condenser which, in the event of interruption of the circuit, generates an electro-magnetic couple to restore the moving system to zero position. In the self-balancing Wheatstone bridge shown in Fig. 1, one arm consists of a variable resistance 1 such as a resistance thermometer, the other arms being formed by fixed, resistances 2, 6, 7 and a potentiometer resistance 4 engaged by a slider 8 carried by the movable coil 5 of the bridge galvanometer. This coil is so wound that it turns in a direction to balance the bridge and also carries a pointer to indicate the value of the resistance 1. The coil 5 has no spring control, but in order to return it to zero a condenser 13, in series with resistances 14, 15, is charged from the voltage source between the points 16, 17 and discharges through the coil 5 in such a direction as to return it to zero in case the applied voltage disappears or in case of fracture of one of the wires 10, 11, 12. In a modification, Fig. 2, the bridge diagonal 18 has no resistance and the rotary coil system is formed by two coils 19, 20 which are mechanically connected and move the slider 21. Instead of a resistance thermometer, a smoke-gas testing apparatus may be employed in which the resistance depends on the composition of the air. In another modification the two arms 1, 2, Fig. 1, of the bridge are formed by a potentiometer resistance. A crossed-coil instrument is shown in Fig. 4, in which the moving system comprises two coils 34, 35, a condenser 40 being connected across the coil 34.