A "moving iron" or "moving vane" meter responds to the average value, i.e. mean of the absolute value of the current through the winding. The harmonic content of the current waveform will affect the rms value indicated, but the meter does not see the voltage across the load. A "moving coil" or "d'Arsonval" meter responds to the mean of the current or DC value, so it cannot measure AC directly, only through a rectifier of some kind.