893.  The use of metallic contact in a single pair of plates, and the cause of its great superiority above contact made by other kinds of matter, become now very evident.  When an amalgamated zinc plate is dipped into dilute sulphuric acid, the force of chemical affinity exerted between the metal and the fluid is not sufficiently powerful to cause sensible action at the surfaces of contact, and occasion the decomposition of water by the oxidation of the metal, although it is sufficient to produce such a condition of the electricity (or the power upon which chemical affinity depends) as would produce a current if there were a path open for it (916. 956.); and that current would complete the conditions necessary, under the circumstances, for the decomposition of the water.

894.  Now the presence of a piece of platina touching both the zinc and the fluid to be decomposed, opens the path required for the electricity.  Its direct communication with the zinc is effectual, far beyond any communication made between it and that metal, (i.e. between the platina and zinc,) by means of decomposable conducting bodies, or, in other words, electrolytes, as in the experiment already described (891.); because, when they are used, the chemical affinities between them and the zinc produce a contrary and opposing action to that which is influential in the dilute sulphuric acid; or if that action be but small, still the affinity of their component parts for each other has to be overcome, for they cannot conduct without suffering decomposition; and this decomposition is found experimentally to re-act back upon the forces which in the acid tend to produce the current (904. 910. &c.), and in numerous cases entirely to neutralize them.  Where direct contact of the zinc and platina occurs, these obstructing forces are not brought into action, and therefore the production and the circulation of the electric current and the concomitant action of decomposition are then highly favoured.

895.  It is evident, however, that one of these opposing actions may be dismissed, and yet an electrolyte be used for the purpose of completing the circuit between the zinc and platina immersed separately into the dilute acid; for if, in fig. 73, the platina wire be retained in metallic contact with the zinc plate a, at x, and a division of the platina be made elsewhere, as at s, then the solution of iodide placed there, being in contact with platina at both surfaces, exerts no chemical affinities for that metal; or if it does, they are equal on both sides.  Its power, therefore, of forming a current in opposition to that dependent upon the action of the acid in the vessel c, is removed, and only its resistance to decomposition remains as the obstacle to be overcome by the affinities exerted in the dilute sulphuric acid.

896.  This becomes the condition of a single pair of active plates where metallic contact is allowed.  In such cases, only one set of opposing affinities are to be overcome by those which are dominant in the vessel c; whereas, when metallic contact is not allowed, two sets of opposing affinities must be conquered (894.).