Detailed mechanism of oscillation of the "mercury beating heart" was investigated for the iron-triggered and the aluminium-triggered cases by probing the electrode potential, electrocapillarity and the shape of the mercury surface. Electrochemical reductions primarily of dissolved oxygen on the mercury surface and corrosion reactions of the active metals such as Fe or Al drove the potential of mercury alternately up and down. The range of potential oscillation was found to be on the positive side of the electrocapillary maximum of mercury in the case of Fe-triggered oscillation in acidic solutions, and on the negative side in the case of Al-triggered oscillation in basic solutions. The ultimate cause of the mercury oscillation was shown to be fluctuating surface charge density without involving a film-formation on the mercury surface or oxidation of mercury, at variance with some views in the literature.