Question

How will the voltage sensitivity of a moving coil galvanometer change, if its current sensitivity is doubled?

• A. Double
• B. Half
• C. No change
• D. Four times

Answer 1

Answer: (C)

The current sensitivity of the galvanometer is deflection per unit current, $\frac{\phi}{I}=\frac{N A B}{k}$ .…(i)
Similarly, the voltage sensitivity is deflection per unit voltage,
$\frac{\phi}{V}=\left(\frac{N A B}{k}\right)\frac{I}{V}=\left(\frac{N A B}{k}\right)\frac{1}{R}$ ....(ii)
From (i) and (ii),
$\left(\text{Voltage sensitivity}\right)=\left(\text{current sensitivity}\right)\times \frac{1}{\left(\text{resistance}\right)}$
Now, if current sensitivity is doubled, then the resistance in the circuit will also double as it is proportional to the length of the wire.
Hence, voltage sensitivity will be,
$\text{⇒}\left(\text{Voltage sensitivity}\right)=\text{(2\times old current sensitivity)}\times \frac{1}{\text{(2\times old resistance)}}$
$\text{⇒Voltage sensitivity}=\text{old current sensitivity}\times \frac{1}{\text{old resistance}}$
Hence, voltage sensitivity will remain unchanged.