What is voltmeter ? Conversion of Galvanometer into Voltmeter .

Voltmeter : A voltmeter is an instrument used to measure the potential difference across the two ends of a circuit element. 

Conversion of galvanometer into voltmeter : 

A galvanometer can be converted into a voltmeter by connecting a large resistance in series to the galvanometer.

Let G and R be the resistance of a Galvanometer and a conductor connected in series with it respectively.

Let V volt be the potential difference to be measured by the voltmeter.Let $I_{g}$ be the current flowing in the circuit .

Now potential difference between points A and B is given by :

$$V = I_{g}R + I_{g}G$$

$$V = I_{g}(R+G)$$

$$\therefore \ R + G = \frac{V}{I_{g}}$$

$$R = \frac{V}{I_{g}}-G$$

Effective Resistance of the voltmeter : 

Effective value of resistance of the galvanometer is given by :

$$R^{'} = (R+G)$$

Which is very High

Thus , voltmeter is a high resistance device , Resistance of an ideal voltmeter is infinite.

Note : a voltmeter is always connected in parallel to the circuit component across which voltage is to be measured.