CBSE Class 12 · 2010 · Current ElectricityShort Answer

\textbf{Principle:} when a steady current flows through a wire of uniform cross-section, the potential drop across any portion is proportional to its length, $V=k\ell$. \textbf{Internal resistance:} balance the cell's EMF with the external key open ($E\propto {\ell }_1$). Then close the key to put a known $R$ across the cell and balance its terminal voltage ($V\propto {\ell }_2$). Since $\frac{E}{V}=\frac{{\ell }_1}{{\ell }_2}$ and $V=\frac{ER}{R+r}$,