Internal resistance of cells \(=1 \,\Omega\)
D. C main supply voltage \(V=220\, \mathrm{V}\)
External resistance \(\mathrm{R}=?\)
\(r=\left(\frac{E-V}{V}\right) R\)
\(\mathrm{1}=\left(\frac{20}{220}\right) \times \mathrm{R}\)
\(\mathrm{R}=11 \,\Omega\)