Question

A galvanometer has a resistance of $20\Omega$ and reads full-scale when $0.2\text{V}$ is applied across it. To convert it into a $10\text{A}$ ammeter, the galvanometer coil should have a

• A. $0.01\Omega$ resistor connected across it in parallel.
• B. $0.02\Omega$ resistor connected across it in parallel.
• C. $200\Omega$ resistor connected in series with it
• D. $2000\Omega$ resistor connected in series with it

Answer 1

The correct option is B $0.02\Omega$ resistor connected across it in parallel.

Given:
$G=20\Omega ;I=10\text{A};V=0.2\text{V}$

So, $I_g=\frac{V}{G}=\frac{0.2}{20}=0.01\text{A}$

Let the resistance connected in parallel be $S$, then

$S=\frac{I_{g}G}{(I-I_g)}$

$\Rightarrow S=\frac{0.01\times 20}{10-0.01}=0.02002$

$\therefore S=0.02\Omega$

Hence, option (b) is the correct answer.