Question

A galvanometer has a resistance of $50\Omega$ and its full-scale deflection current is $50\mu \text{A}$. What shunt resistance should be added so that the ammeter can have a range of $0-5\text{mA}$ ?

• A. $0.2\Omega$
• B. $0.5\Omega$
• C. $1.2\Omega$
• D. $1.5\Omega$

Answer 1

The correct option is B $0.5\Omega$

Given:
$G=50\Omega ;I_g=50\mu \text{A}$

The upper limit gives the maximum current to be measured which is $I=5\text{mA}$.

Shunt resistance $S$ can be calculated by

$S=\frac{I_{g}G}{I-I_g}=\frac{50\times {10}^{-6}\times 50}{5\times {10}^{-3}-50\times {10}^{-6}}$

taking approximate value

$S\approx \frac{50\times {10}^{-6}\times 50}{5\times {10}^{-3}}\approx 0.5\Omega$

Hence, option $\left(b\right)$ is correct.