During an experiment with a metre bridge, the galvanometer shows a null point when the jokey is pressed at 40 .0 cm using a standard resistance of `90Omega` , 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 pm 0.15 Omega`
B. `135 pm 0.56 Omega`
C. `60 pm 0.25 Omega`
D. `135 pm 0.23 Omega`

Answer

C

Explanation

For meter bridge
`R=X((l)/(100-l))=90((l)/(100-l))=90((40)/(60))`
`=60 Omega`
error in R : (error = least count)
`(dR)/(R)=(dx)/(x)+(dl)/(l)+(d(100-l))/((100-l))=0+(1mm)/(40cm)+(1mm)/(60cm)`
`(dR)/(60 Omega)=(1)/(4)rArr dR=0.25 Omega`.