Question

A galvanometer coil has a resistance of $15\, \Omega$ and the metre shows full scale deflection for a current of $4 \,mA$. To convert the meter into a voltmeter of range $0$ to $18\, V$, the required resistance is

• A. $5885\, \Omega$ in series
• B. $4485\, \Omega$ in series
• C. $5885\, \Omega$ in parallel
• D. $4485\, \Omega$ in parallel

Answer 1

Answer: (B)

$4485\, \Omega$ in series

Answer 2

The galvanometer can be transformed into a voltmeter by applying a high resistance in series. If the series resistance is R, then total resistance in circuit $=(15+R)\Omega$ Now $15+R=\frac{V}{I} =\frac{18}{4\times10^{-3}}$ $=\frac{18\times10^{3}}{4}$ $=4.5\times10^{3}$ $\therefore R=4500-15$ $=4485\,\Omega$