The resistance of the metre bridge AB in given figure is 4$\Omega$. With a cell of emf $\epsilon$= 0.5 V and rheostat resistance R_h = 2 $\Omega$ the null point is obtained at some point J. When the cell is replaced by another one of emf $\epsilon$= ${\epsilon }_2$ the same null point J is found for R_h = 6 $\Omega$.The emf ${\epsilon }_2$ is.