hello, what would happen if i connect a dc voltmeter(0-30 v)to a ac voltage of 12v ac and now what would happen if i connect a ac voltmeter to a dc voltage of 12v dc

 

It would show pulsating numbers as it goes from peak voltages positive to negative peak voltages.

The AC meter would show zero volts because it reads AC values due to an internal capacitor.

 

It all depends upon the type of meter you are asking about. My Simpson Model 260 will show a shaky 0 if set for dc and connected to an AC source. If set for AC and connected to a DC source, it will read upscale for either polarity applied. The scale will not be accurate in that case. If an AC signal is riding on top of a DC level, there is a connection called "OUTPUT" which has a cap in series with the AC circuit to block the DC and allow the effect of the AC through so it can be measured. This was used in the old days to measure audio siginal level on the plate circuits of tube type amplifiers.