Nernst equation:

When the applied e.m.f. exceed the decomposition potential the DME becomes depolarized as Zn²⁺ will become Zn (0) forming amalgam and there is a reversible electrode reaction.  Since long as the electrode is ideally depolarized, passage of current does not cause the potential to deviate from its reversible value and is determined by Nernst equation.

 E = E⁰ + 0.059/2 log[Zn ²⁺ ]

At the point C the current no longer increases linearly with applied potential but approaches a steady limiting value at the point D; no increase in current is observed at higher cathode potential until a second compound able to depolarize the indicator electrode is present in the solution.  Then from the point C onwards the DME once again becomes depolarized.  Since of this polarized and depolarized nature of the electrode that is caused by electrode reaction the curve so acquire is known as polarogram.