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

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 = $\frac{V_{AB}}{I_g}=\frac{18}{4X{10}^{-3}}=\frac{18X{10}^3}{4}=4.5X{10}^3$

$\therefore R=4500-15=4485\Omega$