GB546466A - Improvements in or relating to a method and apparatus for measuring electrical impedances - Google Patents

Abstract

546,466. Impedance measurements. STANDARD TELEPHONES & CABLES, Ltd., and SECKER, B. Dec. 6, 1940, No. 17374/41. [Class 37] Apparatus for the direct measurement of at least one of the dimensions of an impedance value includes a single four-equal-arm bridge fed with currents at its two opposite pairs of corners in such a manner as to produce in two adjacent arms currents having a ratio equal to that between the unknown and a known impedance. As shown a bridge network comprising four equal resistance arms has one diagonal AB connected as the middle branch of a hybrid coil, the other branches being formed by the unknown impedance Z and a known resistance Ro. The primary of the hybrid coil is fed from an oscillation generator OSC through a potentiometer P1. The second diagonal of the bridge network is fed from the same source through a second transformer. By correctly choosing the transformer ratios and the values of the resistances the ratio between the current in two adjacent arms of the bridge network is made equal to the ratio of the unknown impedance Z to the known resistance Ro. The ratio of the currents is measured by measuring the voltage drops across tapped portions OX, OY of the drums OA, OB, the arm OA being in the form of a potentiometer graduated in terms of the unknown impedance. A voltmeter V is connected to a three-arm fourposition switch SW. This is first set to the first position CAL and the potentiometer P1 adjusted until a predetermined reading is obtained, this point being marked on the dial. The switch is then turned to the second position and the potentiometer OA adjusted until the same deflection is obtained. The numeric of the impedance is then read on this potentiometer. The impedance angle is determined by setting the switch to the third position in which a condenser C1 is included between the tapping X and the voltmeter V, the deflection giving the angle. The switch is then turned to the fourth position and a reading obtained. If this is higher than the marked calibrating reading the angle is positive and if lower it is negative. In a modification the hybrid coil is replaced by a bridge network, one pair of corners of the four-equal-arm bridge network being inserted in one diagonal arm. and the other pair being inserted in the other diagonal arm through an impedance.