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Rh the back, and then separated. Volta recognised that the cause of electrification was the contact pure and simple between the two dissimilar metals, and for this reason we may speak of "contact electricity" or "voltaic electricity" when we mean the kind of electrification first discovered by Volta.

Various theories have been set up to explain what may be termed the mechanism of this electrification. According to Helmholz, the molecules of a metal are endowed with the ability to attract and hold both electricities, but not with equal force. These molecular forces are different in different metals, and in consequence of these differences there takes place an actual separation between the two electricities at the boundary surface between the two metals. Other scientists (notably De la Rive) doubt the existence of such a molecular force in the metal itself, and look for the cause of electrification in the influence of an intervening link between the two metals, namely, the moisture of the atmosphere. They point out that even with the most accurate ground surfaces it is obviously impossible to make molecular contact between the two metals; that there must always be interposed a film of gases and