Question

A permanent magnet moving coil gives full scale deflection at 40 mV potential difference and 8 mA current. What will be the required series resistance when it is used as voltmeter of range 0 200 V?

• A. 19556 ohm
• B. 20163 ohm
• C. 23884 ohm
• D. 24995 ohm

Answer 1

The correct option is D 24995 ohm

Here in figure the series combination of galvanometer(G) and shunt (S) connected parallel to some device is the voltmeter.

Now, it is given that the galvanometer has maximum deflection at $40mV$ and $8mA$.

The resistance of galvanometer$=G=\frac{40}{8}=5\Omega$

Now, we have to find resistance of shunt S, so that when potential difference across voltmeter is $200V$, potential difference across galvanometer is $40mV$ for its full deflection.

Now, if $V$ is the potential difference across voltmeter, then potential difference across galvanometer G is given by-

$V_G=\frac{G}{S+G}V$

$⟹40\times {10}^{-3}=\frac{5}{5+S}\times 200$

$⟹5+S=\frac{1000}{0.04}=25000$

$⟹S=24995\Omega$

Answer-(D)