Use of an electrostatic wattmeter as a.c./d.c. comparator

Abstract

The paper describes a null method of using a quadrant electrometer for power measurement in an a.c. circuit. The instrument is conventionally connected in an a.c. load circuit, and the ‘a.c. torque’ is counter balanced by an equal and opposite ‘d.c. torque’ produced by suitably applying a steady unidirectional voltage. This type of use as an a.c./d.c. comparator gives the well known advantages of the null method over the deflectional method. A simple null method has been developed which gives a measure of the a.c. power only in terms of the reading of a d.c. voltmeter and the resistance of the quadrant shunt. Calibration of the equipment as a wattmeter is not necessary. Selected experimental results have been included to illustrate how the method can be conveniently extended to magnetic-loss measurements.