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The deflection in a moving-coil galvanometer falls from 50 divisions to 10 divisions, when a shunt of 12 Ω is connected with it. The resistance of the galvanometer coil is

Asked in GUJCET 2008 · Galvanometer in a balanced circuit

• (1) 6 Ω
• (2) 48 Ω
• (3) 24 Ω
• (4) 12 Ω

Step-by-step solution

Why the other options are wrong

• (1) Half the shunt. No reading of the current-division rule gives it: (I_g)/I=S/(S+G)=1/5 fixes G at 48 Ω. Sanity check: G=S((θ₁)/(θ₂)-1)=12×4.
• (3) Twice the shunt. G=S((θ₁)/(θ₂)-1) gives 2S only when the deflection falls to one third. Here it falls to one fifth, so the bracket is 4 and not 2.
• (4) G=S is the half-deflection result, true when the shunt halves the deflection. This deflection drops to one fifth, so G=4S.

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