With an unknown resistance X in the left gap and a resistance R in the right gap of a meter bridge, the neutral point is obtained at a distance of 75cm from the left end. The shift of null point if the unknown resistance is shunted by an equal resistance X and keeping same resistance in right gap is

Options

A. 25 cm towards right
B. 25 cm towards left
C. 15 cm towards right
D. 15 cm towards left

Answer

D

Explanation

(i) `(X)/(R)=(75)/(25) :.X=3R`
(ii) Now, `X'=(X)/(2)=(3R)/(2)`
`(X')/(R)=(l_(x))/(100-l_(x))`
`3/2= (l_(x))/(100-l_(x))`
`300-3l_(x)=2l_(x) " " 5l_(x)=300`
`:.l_(x)=60cm`
(iii) Shift `=75-60=15` towards left.