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Question:

During an experiment with a metre bridge, the galvanometer shows a null point when the jockey is pressed at 40.0 cm using a standard resistance of 90 Ω. The least count of the scale used in the metre bridge is 1 mm. The unknown resistance is?

60±0.25Ω

135±0.23Ω

60±0.15Ω

135±0.56Ω

Solution:

Wheatstone meter bridge formula, R₁/R₂ = l₁/l₂
∴ R₁/90 = 40/60
R₁ = 60Ω
Now, we also have, dR/R = (L₁ + L₂)/(L₁.L₂).dL
dR/60 = (60 + 40)/(60.40)0.1
∴ dR = 0.25Ω