A cell of emf E having an internal resistance r is connected to an external resistance R. The potential difference V across the resistance R varies with R as shown in figure by the curve:  

 

When a cell with internal resistance r is connected to an external resistance R, 

Potential difference across R is 

 $\mathrm{V} = \mathrm{\epsilon }-\mathrm{ir}$

Also, Current $\mathrm{i} = \frac{\mathrm{\epsilon }}{\mathrm{r}+\mathrm{R}}$

From the above 2 equations, :

$\mathrm{V} = \mathrm{\epsilon }-\frac{\mathrm{\epsilon r}}{\mathrm{r}+\mathrm{R}}= \frac{\mathrm{\epsilon R}}{\mathrm{r}+\mathrm{R}}=\frac{\mathrm{\epsilon }}{{\displaystyle \frac{\mathrm{r}}{\mathrm{R}}}+1}$

The curve will be an increasing curve with decreasing slope.