The contraction and relaxation of the mercury is due to changes in the surface tension of, and charge on, the mercury drop. When the mercury is not in contact with the iron nail, it's surface becomes oxidized and the positive mercury ions repel each other, causing the mercury pool to relax (ie. lower its surface tension). The relaxation causes the mercury to come into contact with the iron nail, which has a sufficiently negative electrochemical potential to reduce the mercury ions to mercury metal. The surface tension of the mercury increases and the pool contracts, causing the contact with the nail to be broken. The mercury surface reacts with the acid and becomes oxidized again, thereby, completing one cycle of the oscillation.
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