A Voltmeter is an instrument which is used to measure the potential difference between two points of an electric circuit directly in volt (or milli-volt or micro-volt). Hence, it is connected in parallel across those two points of the circuit, between which the potential difference Voltmeter is to be measured. When it is connected.
Since, the resistance of coil of galvanometer of its own is low, hence, to convert a galvanometer into a voltmeter, high resistance R is series is connected with the galvanometer.
Here, ig = V/R + G
where R + G = RV = resistance of voltmeter
