Precision electrothermic voltmeter

AN ELECTROTHERMIC VOLTMETER of the thermocouple type, with an adjustable resistor in series with an insulated-heater, evacuated thermoelement has been developed at the National Bureau of Standards primarily for standardizing voltmeters over a wide voltage range at audio frequencies. In conjunction with a suitable d-c potentiometer, it can be used for measuring alternating voltages from 0.4 to 400 volts at frequencies from 20 to 20,000 cycles per second, with an accuracy of 0.01 per cent. The voltmeter was intended specifically for use solely as a transfer instrument. Such instruments are designed to have ideally the same response on alternating current as on direct current, so that when calibrated with direct current they become standards which, in effect, transfer the units of measurement (which are maintained by d-c standards) to alternating current. By making sufficiently frequent calibrations, only the transfer performance or frequency influence becomes significant, and instruments intended for such use may be designed and constructed for the best possible transfer performance and for high sensitivity, at the expense of other characteristics.