The length of a wire of a potentiometer is $100\mathrm{ }\mathrm{c}\mathrm{m}$ and the emf of its standard cell is $\mathrm{E}$ volt. It is employed to measured emf of a battery whose internal resistance is $0.5\mathrm{ }\mathrm{\Omega }$. If the balance point is obtained at $\mathcal{l}\mathcal{ }=\mathcal{ }30$ cm from the positive end, the emf of the battery is -