The resistance of a galvanometer is 30 Ω. It gives full scale deflection for 2 mA. It is to be converted into an ammeter to measure current 0.3 A. The value of the shunt required is approximately
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 0.2 Ω
Given:
Resistance of galvanometer, $R_g = 30\,\Omega$
Full-scale deflection current, $I_g = 2\,mA = 2 \times 10^{-3}\,A$
Total current to be measured, $I = 0.3\,A$
Let shunt resistance be $R_s$.
Current through shunt: $I_s = I - I_g = 0.3 - 0.002 = 0.298\,A$
Since the voltage across galvanometer and shunt are equal:
$I_g R_g = I_s R_s$
Hence,
$R_s = \frac{I_g R_g}{I_s}$
Substitute values:
$R_s = \frac{2 \times 10^{-3} \times 30}{0.298}$
$R_s = \frac{0.06}{0.298} = 0.201\,\Omega$
Final Answer: $R_s \approx 0.2\,\Omega$