A differential thermocouple voltmeter

An a-c voltmeter has been developed to indicate directly the percentage difference between an unknown voltage and the settings of the instrument dials. It is accurate to 0.05% from 5 cps (cycles per second) to 10 kc within the voltage range from 1 to 700 volts. Two 10-ma (milliampere) thermoelements are used; one in series with an a-c decade resistor, and the other energized from a constant-voltage d-c source, a Zener diode. A built-in galvanometer, calibrated in % of input voltage, indicates the difference between the two thermocouple-output emfs (electromotive forces), and has a resolution of more than 0.01%. The instrument was designed for rapidly calibrating other voltmeters, but it can be used also for a-c-d-c difference measurements - frequency response tests - to 0.02%.