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In the shown circuit the galvanometer shows a deflection theta0 when key K1 is closed while K2 stays open. When K2 is then also closed and R2 is set to 5 ohm, the deflection drops to theta0/5. Ignoring the internal resistance of the cell, the galvanometer resistance equals:

1. 12 ohm
2. 25 ohm
3. 5 ohm
4. 22 ohm

Correct answer: 22 ohm

Solution

With the shunt the galvanometer carries 1/5 of its original current, and applying the current-division relation with R2 = 5 ohm gives G = 22 ohm.

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