GB252990A - Improvements in wheatstone bridges for the measurement of electrical capacity - Google Patents

Abstract

252,990. Campbell, A. Nov. 14, 1925. Addition to 244,596. Capacities and power-factors, measuring.-The variable mutual inductance claimed in the parent Specification is employed in an alternating-current wheatstone bridge for the measurement of the capacity and powerfactor of a condenser. In two arms of the bridge are placed similar inductive coils H, K, one or both of which are secondary coils of the mutual inductance. In series with the coils H, K are equal non-inductive resistances P, Q, which may be fixed or simultaneously variable. Equal ratio resistances R, S are connected as usual, and between their junction F and the slider of a potentiometer rheostat B is connected a source A of alternating current and the primary windings T, U of the mutual inductance. The condenser C to be tested is connected in parallel with the resistance Q and balance is restored by adjusting the mutual inductance until a null indication is obtained in the telephone of vibration galvanometer G. Then approximately the value of the capacity C = L/Q<2>, where L is twice the value of the mutual inductance. The values of P, Q may be inversely proportional to the square roots of the desired multiplying factors, for example #1000, 10, #10, 1 so as to give multiplying factors of 0.001, 0.01, 0.1, and 1 respectively. The potentiometer resistance B provides a fine adjustment of the resistance balance, and may be provided with a scale from the readings of which the power-factor may be deduced. In order to compensate for the slight increase of inductance in one arm and decrease in the other due to adjustment of the slider of the resistance B, a compensating variable inductance or a coil in the primary circuit of the mutual inductance may be mechanically coupled with the slider. Alternatively the potentiometer resistance B mav be replaced by a constant inductance rheostat in one arm of the bridge. When two condensers are to be compared, one is placed across each of the resistances P, Q, the difference of their capacities being measured. The ratio arms R, S may be unequal, the ratios of the resistances P, Q being correspondingly altered.