Two sources of equal E.M.F. are connected to an external resistance $\mathrm{R}$. The internal resistances of the two sources are ${\mathrm{R}}_1$ and ${\mathrm{R}}_2$ $\left({\mathrm{R}}_2>{\mathrm{R}}_1\right)$. If the potential difference across the source having internal resistance ${\mathrm{R}}_2$ is zero, then