A precision a.c./d.c. comparator for power and voltage measurements

In recent years there has been a marked increase in the demand for more accurate, commercial a.c. power-measuring instruments. The demand is due largely to changing industrial conditions brought about by legislation, although it already existed for the manufacturer whose a.c. standardizing apparatus was only comparable in sensitivity and accuracy with that of the commercial apparatus he produced. The paper gives a brief review of the sources of error common to dynamometer wattmeters, and emphasizes some of the inherent errors which have been less publicized. A new instrument for measurement of a.c. power and voltage by direct comparison with a standard d.c. potentiometer is described, the principle of operation being a null method based on the balancing of two torques. A summary is given showing the errors inherent in the new instrument and those which have been eliminated by the method of measurement. The stability of accuracy of the new instrument is maintained by self-standardization, and a means of checking the accuracy of the associated apparatus is made available.