If the position of the nail tip is just right, the mercury blob begins to oscillate, changing shape.
An electrical double layer forms between the surface of the mercury droplet and the electrolyte solution.
When the iron tip is introduced, a redox reaction starts in which iron is oxidized to the ferric ion and the oxidizing reagent is spent (e.g. when hydrogen peroxide together with hydronium ions is reduced to water).
The mercury beating heart was first observed in the year 1800 by Alessandro Volta and William Henry.
The chemical phenomenon in the form best known today was first described by German chemist Friedlieb Ferdinand Runge, the discoverer of caffeine.