In the potentiometer circuit shown in the figure the internal resistance of the $6 \mathrm{V}$ battery is $1 \mathrm{\Omega }$ and length of the wire $\mathrm{AB}$ is $100 \mathrm{cm}$. When $\mathrm{AD}=60\mathrm{ }\mathrm{cm}$, the galvanometer shows no deflection. The EMF of the cell $\mathrm{C}$ is (the resistance of the wire $\mathrm{AB}$ is $2 \mathrm{\Omega }$)