A potentiometer wire $\mathrm{PQ}$ of $1\mathrm{m}$ length is connected to a standard cell ${\mathrm{E}}_1$. Another cell ${\mathrm{E}}_2$ of emf $1.02 \mathrm{V}$ is connected with a resistance '$\mathrm{r}$' and switch $\mathrm{S}$ (as shown in figure). With switch $\mathrm{S}$ open, the null position is obtained at a distance of $49 \mathrm{cm}$ from$\mathrm{Q}$. The potential gradient in the potentiometer wire is :