Unlike dry cell,the mercury cell has a constant cell potential throughout its useful life. Why?

(N/A) The cell potential of a mercury cell remains constant throughout its useful life because the overall cell reaction does not involve any ions whose concentration could change.
In a mercury cell,$Zn(Hg)$ is used as the anode and $HgO$ with carbon paste is used as the cathode. The electrode reactions are as follows:
Cathode: $HgO_{(s)} + H_2O_{(l)} + 2e^- \rightarrow Hg_{(l)} + 2OH^-_{(aq)}$
Anode: $Zn(Hg) + 2OH^-_{(aq)} \rightarrow ZnO_{(s)} + H_2O_{(l)} + 2e^-$
Overall cell reaction: $Zn(Hg) + HgO_{(s)} \rightarrow ZnO_{(s)} + Hg_{(l)}$
Since the overall reaction involves only solids and liquids,the concentration of the species remains constant,resulting in a steady cell potential of $1.35 \ V$.