# In a meter bridge experiment null point is obtained at 20 cm from one end of the wire when resistance $X$ is balanced against another resistance Y. If $X < Y$, then where will be the new position of the null point from the same end, if one decides to balance a resistance of 4$X$ against $Y$?

$\begin {array} {1 1} (A)\;50\: cm & \quad (B)\;80\: cm \\ (C)\;40\: cm & \quad (D)\;70\: cm \end {array}$

We have from meter bridge experiment
$\large\frac{R_1}{R_2}$ $= \large\frac{l_1}{l_2}$ where $l_2 = ( 100 - l_1 \: cm)$
In the first case, $\large\frac{X}{Y}$ $= \large\frac{20}{80}$
In the second case, $\large\frac{4X}{Y}$ $= \large\frac{l}{100-l}$
$l = 50 \: cm$
Ans : (A)

edited Mar 14, 2014