Abstract: An a.c. bridge has a first arm with standard resistance, a second arm with a standard reactance element, a third arm having terminals for connection to an unknown impedance, and a fourth arm comprising a resistive circuit having first and second variable resistive elements. An a.c. supply voltage is coupled between the junction of the first arm with the second arm and the junction of the third arm with the fourth arm. First and second phase-sensitive rectifiers are coupled between the junction of the first arm with the third arm and a variable tap on the second resistive element in the fourth arm. The first rectifier is synchronized to a first a.c. reference signal in phase quadrature with the voltage across the unknown impedance, and the second rectifier is synchronized to a second a.c. reference signal in-phase with the voltage across the unknown impedance. Indicators are coupled to the outputs of the rectifiers for indicating the senses of their output signals.

Abstract: An a.c. circuit including a thermistor used as a variable resistance device. A d.c. current source is provided for varying the temperature, and hence the resistance, of the thermistor and a DVM is provided for giving an indication of the resistance thereof and/or for controlling the temperature varying current source in order to set the resistance of the thermistor to a defined resistance. The thermistor can form part of an a.c. bridge and the temperature varying current source can be arranged to adjust the resistance of the thermistor until the bridge is balanced.