A galvanmeter has a coil of resistance 200 Ω with a full scale deflection at 20 μA. The value of resistance to be added to use it as an ammeter of range (0–20) mA is:
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A galvanmeter has a coil of resistance 200 Ω with a full scale deflection at 20 μA. The value of resistance to be added to use it as an ammeter of range (0–20) mA is:

G1 Physics · Accepted Answer

Given: G = 200 Ω I(g) = 20 μA = 20 × 10⁻⁶ A Ammeter range: I = 20 mA = 20 × 10⁻³ A For shunt S: I = I(g)[1 + G/S] Therefore: 20 × 10⁻³ = 20 × 10⁻⁶[1 + 200/S] 1000 = 1 + 200/S 999 = 200/S S = 200/999 ≈ 0.20 Ω