The electrostatic voltmeter as a d.c./a.c. transfer instrument

The use of the electrostatic voltmeter for the measurement of a.c. voltages in terms of d.c. standards is found to give rise to errors due to departure from unity of the ratio of the d.c. sensitivity of the instrument to the a.c. sensitivity. The effect appears to be due to semi-conducting films on the surfaces of the voltmeter elements, and is believed to be absent in voltmeters with working surfaces of freshly-cleaned platinum.