A galvanometer of resistance 20 Ω gives full scale deflection

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A galvanometer of resistance 20 Ω gives full scale deflection with a current of 0.004 A. To convert it into an ammeter of range 1 A, the required shunt resistance should be

1. 0.38Ω
2. 0.21Ω
3. 0.08Ω
4. 0.05Ω

Correct Option: C

Maximum current which can pass through galvanometer, I_g = 0.004A

Let R be the resistance of shunt.
We know potential drop across AB = Potential drop across CD
R (I – I_g) = I_g (20)
⇒ R (1 – 0.004) = 0.004 × 20 ⇒ R = 0.08 Ω