Question

A galvanometer having internal resistance $10\Omega$ requires 0.01 A for a full scale deflection. To convert this galvanometer to a voltmeter of full-scale deflection at 120V, we need to connect a resistance of

• A. $11990\Omega$ in series
• B. $11990\Omega$ in parallel
• C. $12010\Omega$ in series
• D. $12010\Omega$ in parallel

Answer 1

Answer: (A)

$11990\Omega$ in series

Since the galvanometer needs to show full scale deflection at $120V$ with current $0.01A$ in the circuit, the equivalent resistance of the circuit, in accordance with the Ohm's law must be $\frac{120V}{0.01A}=12000\Omega$.

Hence $10\Omega +R=12000\Omega$

$⟹R=11990\Omega$