(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.
The vortex error is that moving-iron instrument is used in AC circuit. For ammeter in the metal parts form vortex by magnetic field, but the vortex effect the magnetic field,so it is bring the vortex error.
The hysteresis error is that moving-iron instrument is used in DC circuit. It is bring by the core,shield and feromagnet round measuring machine.
The hysteresis error is the error produced when the electromagnetic system instrument is used in the AC circuit. For the ammeter, the alternating magnetic field of the metal parts in the instrument is used to generate eddy current, and the mutual inductance between the coil and the coil has a degaussing effect on the magnetic field of the coil, which results in the deflection of the instrument and hence the error of the eddy current. The hysteresis error is the error produced when the electromagnetic system instrument is used in the DC circuit. It is caused by the magnetic hysteresis of iron core, magnetic shield and ferromagnetic material near the measuring mechanism.