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An unknown resistance R is measured with a metre bridge using a standard resistance S = 100 ohm, and the balance (null) point is found at l1 = 2.9 cm. To improve the accuracy of the measurement, the best step is to

1. change S to 3 ohm and repeat the experiment
2. measure l1 more accurately
3. change S to 1000 ohm and repeat the experiment
4. give up hope of a more accurate measurement with a metre bridge

Correct answer: change S to 3 ohm and repeat the experiment

Solution

Choosing S close to R brings the balance point near 50 cm, minimising the percentage error; here S should be near 3 ohm.

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