A current of less magnitude than a DC meter will not harm it. Based on the construction of a DC voltmeter, it gives a reading of instantaneous values. Therefore when an AC wave passes near it, the pointer deflects at each moment of the wave. But when the wave moves as fast as 50 seconds per second, the meter will show the average value. AC meters, on the other hand, show RMS values because the value of RMS is the AC wave that gives the same effect as DC.
To achieve this, the AC wave is corrected and the meter measures the effective average of the rectified wave. In better meters, the wave may be missed before measurement and square root. In a later version, the meter can take readings of any alternate waveform.
