What is a voltmeter? How can a galvanometer be converted into a voltmeter? Explain.

Explanation

A moving-coil galvanometer can be convertecd into a voltmeter by increasing its effective resistance by connecting a high resistance `R_(s)` in series with the galvanometer. The series resistance protects the meter and increases the range of the meter.
Let G be the resistance of the galvanometer coil and `I_(g)` the current required for full-scale deflection.
Let V be the maximum p.d. to be measured. Then, the series resistance `R_(s)` should be such be such that the current `I_(g)` passes through the galvanometer when the p.d. V is applied across the instrument.

In the series combination, the p.d. V gets divided across the galvanometer and the resistance `R_(s)`:
`V=I_(g)G+I_(g)R_(s)=I_(g)(G+R_(s))`
`therefore R_(s)=(V)/(I_(g))-G`
This gives the required value of the series resistance. The scale of the galvanometer is then calibrated so as to read the potential difference in volt directly.