The resistance of the metre bridge AB in given figure is $4 \Omega$. With a cell of emf $\varepsilon=0.5 \mathrm{~V}$ and rheostat resistance $\mathrm{Rh}=2 \Omega$ the null point is obtained at some point J . When the cell is replaced by another one of emf $\varepsilon=\varepsilon_2$ the same null point $J$ is found for $R_h=6 \Omega$. The emf $\varepsilon_2$ is