Let $m(t)$ be the mass of copper that has been deposited and $r(t)$ be the 'radius' of the copper deposit after time $t$ in the electrolysis experiment described in Section 18.1. It may be shown that the current flowing, and thus, by Faraday's law, the rate of mass deposition, is proportional to $r(t)$. On the assumption that the growth forms an approximate fractal of dimension $s$, so that $m(t) \sim c r(t)^s$, give an argument to suggest that $r(t) \sim c_1 t^{1 /(s-1)}$.