When an unknown resistance $X$ is connected in the left gap of a meter bridge and a known resistance $R$ in the right gap, null point is obtained at $40\ \text{cm}$ from the left end. If a $2\ \Omega$ resistance is connected in series with $X$, the null point shifts towards right by $10\ \text{cm}$, with same resistance in right gap. The value of $X$ must be