A potentiometer wire $\mathrm{AB}$ having length $L$ and resistance $12r$ is joined to a cell $\mathrm{D}$ of emf $\epsilon$ and internal resistance $r$. A cell $\mathrm{C}$ having EMF $\epsilon /2$ and internal resistance $3r$ is connected. The length $\mathrm{AJ}$, at which the galvanometer, as shown in the figure, shows no deflection is