(1) For permanent-magnet moving-coil instrument AC voltage or current measurement are used to obtain the average by the rectifier, Then by the relationship between the average and the RMS make the average RMS voltage or current conversion scale. This meter with the advantage of high sensitivity, simple circuit, reading the sine wave is more accuracy but the disadvantage is non-sine wave may with low-precise.
(2) For moving-iron instrument measured AC voltage or current is the RMS voltage or current directly, but its drawback is that the serious non-linear scale, low-precision, low sensitivity, frequency and waveform of the change may bring measurement error.