Why Is The Internal Resistance Of A Volt Meter Is Very High?

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.

An ideal voltmeter should theoretically have infinite resistance. Only is the resistance internal to the voltmeter is extremely high it will measure the correct voltage drop across the real circuit terminals. Voltmeters are always connected in parallel to the circuits under test. Hence any current running through the voltmeter will add on to the overall current in the circuit under test. This will affect the voltage being measured. A perfect voltmeter that has an infinite resistance, will virtually draws no current from the circuit under test. However, one cannot have an infinite resistance voltmeter in practical life and hence we should use a voltmeter with very high resistance. For example if the internal resistance of a circuit is 300 M ohms and the internal resistance of the voltmeter has a resistance of only 10 ohms, and it is connected in parallel to the circuit,  this will affect the voltage measured drastically and we would obtain an incorrect reading.