contact with an aqueous solution. A small quantity of oxide is formed; this dissolves, and ions are then able to deposit on the metal, which thus becomes positively charged, whilst the solution receives a negative charge. The opposite effect is produced when a metal of high solution pressure is immersed in a solvent. The ions then pass from the metal, leaving this negatively charged and communicating a positive charge to the liquid.
The Volta Effect. — The above method of viewing the process explains the Volta effect for combinations of metals and liquids. If we are concerned with two liquids, the charges are due to the dissimilar mobilities of the ions (see p. 218). However, the Volta effect is also produced between metals and insulators, as, e,g., varnish. In this case the varnish may be conceived as a medium (a solvent) in which traces of metal oxides or salts dissolve. "When air is the insulator, it is simplest to imagine that the metal reacts with the oxygen ions (of the air), and is thus oxidised, whereby the metal becomes negatively electrified and the air positively.
Now, if we have, as in Volta*s original experiment, two metals, A and B, in the air, these are oxidised to different extents according to their " chemical affinities " for oxygen. As a consequence of this the potential difference between the metal A and the air will be different from that between the metal B and the air. In other words, there is a certain potential difference between the two metals, so long as they are not in metallic contact, and the potential difference is such that the more eaaily oxidisable metal is negatively electrified.
If the two metals be joined by a wire, the difference of potential disappears by positive electricity passing to the more easily oxidisable metal, and negative electricity passing to the more " noble " metal. If the metals be in the form of plates, and if they be brought close together, so that the distance between them is small, a condenser is produced, as in Volta's experiment, and therefore the electricities
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