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In a metre-bridge experiment a null point is obtained at 40.0 cm with a known standard resistance of 90 ohm in the right gap. The scale has a least count of 1 mm. Determine the unknown resistance with its uncertainty.

1. 60 +/- 0.15 ohm
2. 135 +/- 0.56 ohm
3. 60 +/- 0.25 ohm
4. 135 +/- 0.23 ohm

Correct answer: 60 +/- 0.25 ohm

Solution

R = 90*(40/60) = 60 ohm and dR = 60*(1/40 + 1/60)*0.1 = 0.25 ohm, giving 60 +/- 0.25 ohm.

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