Current Electricity

When cell of E.M.F. '$E_1$' is connected to potentiometer wire the balancing length is '$l_1$'. Another cell of E.M.F. '$E_2$' ($E_1 > E_2$) is connected along with $E_1$ so as two cells oppose each other, the balancing length is '$l_2$'. The ratio $E_1 : E_2$ is