Loading Effect of Voltmeter

Consider a circuit as shown below.

Now, if R>>15 then 15/R tends to 0 and (2+(15/R)) tends to 2 and therefore voltmeter reading tends to 50V.

If R<<15 then 15/R tends to infinity and (2+(15/R)) tends to infinity and therefore voltmeter reading tends to 0V.

So, depending upon the resistance value of the voltmeter with respect to resistance R₂ , voltmeter can show 0V to 50V.

It is therefore concluded that if the resistance of the voltmeter used is not large enough as compared to the resistance of the circuit or element across which it is connected, then, the voltmeter itself acts as a load for the circuit (hence called loading effect of voltmeter) and the reading shown is lower than the actual.

Therefore, to minimise the loading effect, a voltmeter with resistance value as high as possible with respect to the circuit resistance, is used.

This also explains why a voltmeter has high resistance.