Explain the use of a galvanometer as a voltmeter.

(N/A) galvanometer can be used to measure the potential difference (voltage) across a given section of a circuit.
$A$ voltmeter must be connected in parallel with the component across which the potential difference is to be measured.
To ensure accurate measurement,the voltmeter must draw a very small current from the circuit; otherwise,it would significantly disturb the original potential difference.
To achieve this,a very large resistance $R$ is connected in series with the galvanometer.
This arrangement is schematically depicted in the figure. Note that the total resistance of the voltmeter is $R_{V} = R_{G} + R \approx R$ (since $R$ is very large).
By connecting this high resistance in series,the galvanometer is converted into a voltmeter with a high input resistance,which minimizes the current drawn from the circuit.