Audio-frequency power measurements by dynamometer wattmeters

Dynamometer wattmeters can be designed and constructed for the measurement of power at frequencies up to about 20kc/s. The achievement of an accuracy at the higher audio frequencies comparable to that which is possible at power frequencies is dependent first upon certain precautions being taken with regard to the screening of the load and the wattmeter, and secondly upon the possibility of an accurate determination of the frequency-dependent errors of the wattmeter. The screening necessary is described, and an account is given of the methods adopted at the N.P.L. to calibrate wattmeters. For any dynamometer a frequency is eventually reached at which a significant instrument deflection is obtained with voltage only or current only applied to the terminals. This is considered to be the useful upper limit of frequency for that instrument.