A new expanded scale A-C voltmeter

IN MANY POWER SYSTEM voltage measurements the measurement problem consists of detecting and indicating accurately a variation of plus or minus one volt or less from a given reference voltage. The use of the conventional zero-left, a-c, moving-iron, or electrodynamic voltmeter does not provide enough of a deflection to read with any degree of certainty, since, for example, a one volt variation from 120 volts on a 250-degree-scale concentric-type switchboard voltmeter represents approximately 3/64 inch of arc. There is then a need for an expanded scale voltmeter that produces a scale length per volt within the range of normal voltage variation of at least four times that of the conventional voltmeter.