Question

To convert a galvanometer into voltmeter we must connect a:

• A. high resistance in series
• B. low resistance in parallel
• C. high resistance in parallel
• D. low resistance in series

Answer 1

Answer: (A)

Let $G$ be the resistance of the galvanometer and $i_g$ the current, which on passing through the galvanometer, produces full-scale deflection.
Suppose $V$ is the maximum potential difference to be measured which exists between points $a$ and $b$. On connecting the galvanometer across $a$ and $b$ a current $i_g$ flows through it. Then from Ohm's law we have.

$i_g=\frac{V}{G+R}$
$G+R=\frac{V}{i_g}$
$\Rightarrow R=\frac{V}{i_g}-G$
Thus, on connecting a resistance $R$ of above value in series with the galvanometer the galvanometer will become a voltmeter of range $0$ to $V$ volts.
The value of resistance should be high enough so that it does not draw any current from the circuit.
Resistance of an ideal voltmeter is high.