GB679680A - Improvements in or relating to alternating current bridges - Google Patents

Abstract

679,680. Impedance measuring. GENERAL ELECTRIC CO., Ltd., and THOMPSON, G. T. July 11, 1951 [April 14, 1950], No. 9243/50. Class 37. A circuit for measuring impedance comprises two T-networks, one constituted by a resistance 3, Fig. 2, inductance 4 and variable capacitance 5, and the other constituted by inductances 6, 7 and variable capacitance 8 which can be shunted by the impedance 9 to be measured. In operation, the circuit is first balanced without the shunt impedance 9 by varying condensers 5, 8 to values Cg2 and Cb2 respectively; the unknown impedance is then connected in shunt, and the circuit re-balanced by adjusting condensers 8, 5 to values Cg1, Cbl respectively. If L1, L2, L3 are the values of the inductances 4, 6, 7 respectively, and R is the value of resistance 3, then the conductance of the unknown impedance is L1 R(Cg2-Cg1)/L2L3 and the susceptance is (Cb2-Cb1). Both these expressions are independent of frequency. In practice condensers 5 and 8 have trimmers in parallel; the main condensers are pre-set and the bridge balanced with the unknown impedance in shunt by means of the trimmers; the unknown impedance is then removed, and the bridge rebalanced by varying the main condensers. The main condensers may be in conductance and susceptance. In a practical embodiment adapted for very high frequency operation, the inductances consist of short lengths of wire 41, 42, 45, Fig. 6, which with the resistance 47 are contained in a metal housing, upon which are mounted the main variable condensers 33, Fig. 3 and the trimmers 18; the condensers 33 consist of brass tubes adjustable to include more or less of plugs 30 which project from the plates 31, 32, Fig. 6; the trimmers 18 are similar. Sockets 54, 55, 49 permit connections of the co-axial type.