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

Options

A. `60 +- 0.15 Omega`
B. `135 +- 0.56 Omega`
C. `60 +- 0.25 Omega`
D. `135 +- 0.23 Omega`

Answer

C

Explanation

(c) In case of a meter bridge
`R/l = X/(100-l) Here X = 90 Omega, l= 40.0 cm`
`:. R = (Xl)/(100-l)` ,brgt For finding the value of R
`R = (90 xx 40)/60 = 60 Omega`
For finding the value of `DeltaR`
`(DeltaR)/R = (Deltal)/l + (Delta(100-l))/(100-l)`
`:. (DeltaR)/60 = 0.1/40 + 0.1/60`
`:. DeltaR = 0.25 Omega`
Therefore, `R = (60+- 0.25)Omega`.