JEE Advanced | 2014Current Electricity and Electrical InstrumentHard

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, as shown in the figure. The least count of the scale used in the metre bridge is 1 mm. The unknown resistance is
       

Options

A.60 ± 0.15 Ω
B.135 ± 0.56 Ω
C.60 ± 0.25 Ω
D.135 ± 0.23 Ω

Solution

  (Balanced Wheatstone)
For R, ⇒ R =

For ᐃR,
ln R = ln x + ln(100 - x)


ᐃR = 60
ᐃR = 0.25 Ω
⇒ R = 60 ± 0.25 Ω

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