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In a metre bridge the resistors are arranged as shown and the null point occurs at a balancing length l1 = 40 cm. An unknown resistance x is now added in series with P, and the new balance point, measured from the same end, shifts to 80 cm. The value of x is ____ ohm.

1. 1 ohm
2. 2 ohm
3. 4 ohm
4. 8 ohm

Correct answer: 8 ohm

Solution

The first balance gives P/Q = 40/60 = 2/3; after adding x, (P+x)/Q = 80/20 = 4, leading to x = 8 ohm.

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