These are very rare in most cases, so get in there first. Make sure that the voltmeter is across the primary coil, NOT the supply to get the true potential difference. To do this investigation properly, you will need a wide range of load resistances. Here are a few hints on how to go about trying to get some meaningful results. Read carefully - they could save you a lot of time and effort.
Sensitive ammeters have a resistance.
In theory, your experiments should be accurate enough that this will make a difference, especially when measuring very low currents or when the ammeter in question is in series with a small load resistance. How you get round this is up to your initiative. Wind your own coils. School coils, if they have been repaired in use, don t necessarily have the same number of windings as the labels on them say. Make sure all the windings are in the same direction though.