The Young's modulus of brass and steel are $10 \times {10}^{10} {\mathrm{Nm}}^{-1} \mathrm{and} 2 \times {10}^{11} {\mathrm{Nm}}^{-2}$ respectivley. A brass wire and a steel wire of the same length are extended by 1 mm under the same force. The radii of the brass and steel wire are ${\mathrm{R}}_{\mathrm{B}} \mathrm{and} {\mathrm{R}}_{\mathrm{s}} \mathrm{respectively}. \mathrm{Then}$