A d.c. instrument that can be made suitable for a.c. measurements by using a rectifier to convert the alternating current to be measured into a unidirectional current. A common arrangement is a bridge circuit formed from four diodes and a moving-coil instrument, M (see diagram). The indicated value on the instrument is usually the root-mean-square value of a sinusoidal a.c. input waveform; rectifier instruments are subject to waveform error if used with nonsinusoidal input waveforms.

Rectifier instrument used as an a.c. voltmeter