A moving coil galvanometer of resistance 100Ω shows a full scale deflection for a current of 1 mA. The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA, is
Q. A moving coil galvanometer of resistance 100Ω shows a full scale deflection for a current of 1 mA. The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA, is ____ Ω.
(A) 25
(B) 2.5
(C) 0.5
(D) 10

Correct Answer: 25 Ω

Explanation

Resistance of galvanometer, G = 100 Ω

Full scale deflection current of galvanometer,

Ig = 1 mA = 0.001 A

Required full scale current of ammeter,

I = 5 mA = 0.005 A

To convert a galvanometer into an ammeter, a shunt resistance S is connected in parallel such that:

S = (Ig × G) / (I − Ig)

Substitute the given values:

S = (0.001 × 100) / (0.005 − 0.001)

S = 0.1 / 0.004

S = 25 Ω

Hence, the resistance required to convert the galvanometer into an ammeter is 25 Ω.

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