Question

The conversion of a moving coil galvanometer into a voltmeter is done by
A.Introducing a large value of resistance in series
B.Introducing a resistance of small value in parallel.
C.Introducing a resistance of large value in parallel.
D.Introducing a resistance of small value in series.

Answer 1

Hint: A permanent magnet moving coil (PMMC) instrument in series with high resistance is known as voltmeter. Voltmeter is always connected in parallel with the points across which potential difference is to be measured,
Formula:
${{R}_{s}}=G(n-1)$
$n=\dfrac{V}{{{V}_{g}}}$

Complete answer:
Moving coil galvanometer is a very sensitive and delicate instrument and can give a full-scale deflection for small potential difference applied across its terminals. Hence to measure large potential difference, effective resistance of the instrument should be high. This can be achieved by connecting a high resistance in series with a moving coil galvanometer. If resistance is not connected and a high potential is applied across the terminals of PMMC then a high current will flow through the coil, which may result in burning of the instrument due to excessive heat produced. The moving coil galvanometer will be protected from such probable damage, if high resistance is connected in series with the moving coil galvanometer.
Conversion diagram of moving coil galvanometer to voltmeter is shown below

Hence we conclude that the conversion of a moving coil galvanometer into a voltmeter is done by introducing a large value of resistance in series.
The value of resistance can be found out by the formula ${{R}_{s}}=G(n-1)$
G is the resistance of galvanometer and $n=\dfrac{V}{{{V}_{g}}}$

Hence, Option(A) is correct.

Additional information:
A Galvanometer is converted into ammeter by connecting a low resistance in parallel with a galvanometer. This resistance is also known as shunt resistance. The value of resistance is so adjusted such that maximum current flows through the shunt branch. Thus galvanometer can be converted into ammeter or voltmeter as per requirement.

Note:
Remember that a high value of resistance must be connected in series with the moving coil galvanometer to increase the resistance. A small resistance that is called a shunt is connected in parallel with the galvanometer to reduce the resistance.