GB556607A - Improvements in or relating to electrical measuring apparatus - Google Patents

Abstract

556,607. Inductance measurements. KAPELLA, Ltd., and REASON, R. E. March 6, 1942, No. 2963. [Class 37] [Also in Group XXXVIII] A measuring apparatus comprises two variable inductances so controlled in accordance with the quantity to be measured that a change in such quantity causes one inductance to increase and the other to decrease, an A.C. source, and a metering circuit including a D.C. meter and two rectifiers which are respectively in circuit with the two variable inductances and through which the meter is differentially energized, each inductance and the associated part of the metering circuit being energized in parallel from the A.C. source through an impedance, characterized in that the impedance of each variable inductance bears to the associated impedance of the metering circuit a ratio not greater than unity or not less than 1: 3. As shown in Fig. 1 a detector member which moves in accordance with the quantity to be measured is directly connected to an armature A common to two detector coils B1, B2 and arranged so that movement from its balanced position increases the inductance of one coil and decreases that of the other, the inductances being equal in the balanced position. The coils B1, B2 are connected in series with impedances C1, C2 across similar secondary windings D1, D2 of a transformer whose primary D is fed from an A.C. source E. Full-wave rectifiers F1, F2 are connected in parallel with the coils B1, B2 and have loading impedances G1, G2 connected across their output terminals. One of the output terminals of one rectifier F1 is directly connected to the output terminal of like polarity on the other rectifier F2 and a D.C. meter H is connected between the other two rectifier terminals. To prevent inaccuracies in the meter reading due to variations in the rectifier impedances, the impedance of each detector coil B1, B2 has a value between that of the rectifier circuit across it and one-third thereof, the ratio preferably being about one-half. In a modification the output terminals of one rectifier are respectively connected to the output terminals of opposite polarity of the other rectifier, the meter being connected between these connections. The loading impedances G1, G2 are omitted, the meter itself serving as a load for the rectifiers. In another arrangement, Fig. 3, the A.C. source L is connected to the input terminals of the bridge formed by the two detector coils M1, M2 and the two series impedances N1, N2, half-wave rectifiers O1, O2 in series with loading impedances PI, P2 being connected across the detector coils M1, M2. The meter Q is connected between the junctions of the rectifiers and their loading impedances. The two detector coils M1, M2 are controlled by a single armature as shown in Fig. 1 and the ratio of the impedance of each detector coil to that of the metering circuit across it lies between unity and one-third and is preferably about one-half. Specification 541,628, [Group XXXVIII], is referred to.