\(R = \rho \frac{l}{A}\, \Rightarrow \,H = \frac{{{V^2}t\,A}}{{\rho \,l}}\). \(\frac{{{t_2}}}{{{t_1}}} = \frac{{{l_2}}}{{{l_1}}}\)
\( \Rightarrow \) \(\frac{{{t_2}}}{{15}} = \frac{{\frac{2}{3}{l_1}}}{{{l_1}}}\, \Rightarrow {t_2} = 10\,\min \)
(ઇલેક્ટ્રોનનો વિદ્યુતભાર $=1.6 \times 10^{-19}\, C$ લો.)