AC rectification before PMMC
The reason you can't use a PMMC with AC is because a PMMC, since it's a moving mass, cant measure the instantaneous current, but rather measures the average current. The average current of an AC waveform is, of course, 0, so your meter will just read 0 the whole time. In fact, I'm pretty sure it's not possible to directly use a moving coil meter of any type for AC.

The following was assuming a different type of meter, but the OP is using a moving coil meter. (This is due to the hysteresis of iron. When you magnetise and then demagnetise iron, you lose energy (represented by the purple area below). When you magnetise iron, it wants to stay magnetised. This affects the current waveform, turning it from a nice ac wave into a different shape (a more pointy one, if I remember correctly).

edit: removed incorrect info)