How does voltmeter measures potential difference

Voltmeter

A voltmeter is connected in parallel with the circuit element across which potential difference is to be measured. It should have a very high resistance as not to alter the circuit resistance, and hence circuit current.

The galvanometer can measure voltages upto I_GG. For larger potential differences, a suitable high resistance R (called multiplier) is connected in series.

The value of R is chosen according to the maximum voltage V that we want to measure. But the galvanometer by itself can only handle a voltage of I_gG. The remaining potential difference (V - I_gG) should be across the multiplier R. The current through it is I_g. Therefore, equating voltage drops, we get

V = I_g G + I_g

R

The resistance of the voltmeter (i.e., a galvanometer in series with a high resistance) is

R_V = G + R

Since R is high, the multiplier increases the resistance of the voltmeter, and of course, extends the voltage range (from I_gG to V).

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regards

Ramesh