Abstract :
The output e.m.f. given by a conventional thermal convertor is not proportional to the square of the heater current, and wattmeters using two such convertors cannot therefore be used for the precise measurement of power. The causes of non-compliance with a square law are examined, and a means of compensation is given to provide a convertor system in which the output e.m.f. is within 0.1% of the calculated square-law value. A wattmeter has been constructed in which negative-feedback amplifiers are used to supply the currents to two compensated convertors. The input signal voltages to the amplifiers are obtained from the external circuit by means of a precision wide-frequency-band voltage transformer, a 4-terminal non-inductive current shunt and a high-resistance voltage divider. The insertion loss in the main current circuit is 0.1 volt at rated current, and the voltage-divider resistance is either 1000 or 5000 ohms per volt. The overall instrument error for all conditions of load and power factor is 0.1% over the frequency range 200 c/s to 10 kc/s, and 0.3% for the range 50 c/s to 30 kc/s.
