A volt meter measures the potential difference between two points. If its internal resitance was low, a lot of current would flow and this would reduce the potential difference between the points and immediately give you the wrong answer.

If the internal resistance is high, and it is usually very very high indeed, then very little or no current will flow through the meter. That enables it to give a more accurate answer.

Ideally, the internal resitance should be infinity high, so that no current what so ever can flow through it. In cheap voltmeters, it is harder to make the internal resistance infinity high and so with very high voltages, a larger amount of current than normal might flow and the voltage reported is then wrong.

There are several types of voltmeter - potentiometers, valve voltmeters and the moving coil galvanometer. They operate in slightly different ways, as you would expect.

The moving coil galvanometer is the commonest sort of voltmeter.

If the internal resistance is high, and it is usually very very high indeed, then very little or no current will flow through the meter. That enables it to give a more accurate answer.

Ideally, the internal resitance should be infinity high, so that no current what so ever can flow through it. In cheap voltmeters, it is harder to make the internal resistance infinity high and so with very high voltages, a larger amount of current than normal might flow and the voltage reported is then wrong.

There are several types of voltmeter - potentiometers, valve voltmeters and the moving coil galvanometer. They operate in slightly different ways, as you would expect.

The moving coil galvanometer is the commonest sort of voltmeter.