Second Law

Faraday's second law compares different electrolytic cells rather than analysing just one: when the SAME quantity of electric charge is passed through the solutions of several different electrolytes (typically arranged in series in a single circuit, so that identical current necessarily flows through every cell), the masses of the different substances liberated at their respective electrodes are directly proportional to their electrochemical equivalents.

For three cells containing NiSO₄, CuSO₄ and CoSO₄ connected in series, with masses $m_{Ni}$, $m_{Cu}$ and $m_{Co}$ deposited when charge Q passes through all three, the second law states

$m_{Ni} \propto Z_{Ni}, \quad m_{Cu} \propto Z_{Cu}, \quad m_{Co} \propto Z_{Co} \qquad \Rightarrow \qquad \dfrac{m_{Ni}}{Z_{Ni}} = \dfrac{m_{Cu}}{Z_{Cu}} = \dfrac{m_{Co}}{Z_{Co}}$ …