Question 39. The deflection in a moving coil galvanometer falls from 25 divisions to 5 divisions when a shunt of 24 Ω is applied. The resistance of the galvanometer coil is :
(1) 12 Ω
(2) 96 Ω
(3) 48 Ω
(4) 100 Ω
Answer: (2)
Explanation:
Let the current corresponding to one division be x.
Initial deflection is 25 divisions.
So the initial current through the galvanometer is 25x.
After applying the shunt, the deflection becomes 5 divisions.
So the current through the galvanometer is 5x.
The remaining current flows through the shunt.
Current through the shunt is 25x-5x=20x.
Let the resistance of the galvanometer coil be G.
For parallel combination, the potential drop across galvanometer and shunt is equal,
5x\times G=20x\times24.
Cancel x from both sides, 5G=480.
G=96 Ω.
Therefore, the resistance of the galvanometer coil is 96 Ω.