
$\frac{X}{R}=\frac{l_{1}}{100-l_{1}}$
$X=R\left(\frac{l_{1}}{100-l_{1}}\right)$
$12=18\left(\frac{l_{1}}{100-l_{1}}\right)$
$1200=30 l_{1}$
$l_{1}=40 \mathrm{cm}$
When $R=8$ ohm,
$12=8\left(\frac{l_{2}}{100-l_{2}}\right)$
$1200=20 l_{2}$
$l_{2}=60 \mathrm{cm}$
Hence, distance through $J$ is shifted is
$l=l_{2}-l_{1}=20 \mathrm{cm}$




statement$-1$ : The temperature dependence of resistance is usually given as $R=R_{0}(1+\alpha \Delta t)$. The resistance of a wire changes from $100\; \Omega$ to $150\; \Omega$ when its temperature is increased from $27^{\circ} C$ to $227^{\circ} C$. This implies that $\alpha=2.5$ $\times 10^{-3} /{ }^{\circ} C$
statement$-2\;: R=R_{0}(1+\alpha \Delta t)$ is valid only when the change in the temperature $\Delta T$ is small and $\Delta R=\left(R-R_{0}\right) < < R_{0}$


