The Development of an Automatic-Reversing Differential Thermal Wattmeter

A differential thermal wattmeter for the precision measurement of power at audio frequencies is described. Automatic reversing has eliminated the zero instability of the conventional thermal wattmeter and has made possible an automatic ac/dc transfer of power at any power factor through the simultaneous superposition of ac and dc to achieve a null. A crest factor capability of four is incorporated in the design to make the instrument capable of precision measurement of power with very distorted waveforms. The wattmeter has a linear scale, a resolution of 10 ppm, and a direct reading accuracy of 30 ppm in the 50-1000-Hz frequency band. The wattmeter can be used up to 20 kHz with reduced accuracy.