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

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.