A battery of emf 12 V and internal resistance 2 Ω is connected in series with a tangent galvanometer of resistance 4 Ω . The deflection is 60 o when the plane of the coil is along the magnetic meridian. To get a deflection of 30 o , the res

Question

A battery of emf 12 V and internal resistance 2 Ω is connected in series with a tangent galvanometer of resistance 4 Ω . The deflection is 60 o when the plane of the coil is along the magnetic meridian. To get a deflection of 30 o , the resistance to be connected in series with the tangent galvanometer is

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Correct answer: A. 12 Ω

I = k tan ⁡ θ , k being reduction factor and I ∝ 1 R ⇒ R + r R + r + R ′ = tan ⁡ θ ′ tan ⁡ θ ⇒ 4 + 2 6 + R ′ = tan ⁡ 30 o tan ⁡ 60 o ⇒ R ′ = 12 Ω

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