38. We have lastly to determine the alteration in the tension of the circuit, which is produced by the chemical alteration of the extent, which has hitherto been considered. For this purpose we assume, till experience shall have taught us better, the position, that the magnitude of the electric tension between two bodies is proportional, first to the difference of their latent electroscopic forces, and secondly to a function, which we will term the coefficient of the tension, dependent on the size, position and form of the particles which act on each other at the place of contact. Not only from this hypothesis may be deduced the law which the tensions of the metals observe inter se,—nothing further being requisite than to assume the same coefficient of tension between all metals placed under similar circumstances,—but it likewise affords an explanation of the phænomenon, in accordance with which the electric tension does not merely depend on the chemical antagonism of the two bodies, but also on their relative density, and can for this reason exhibit themselves differently, even in different temperatures. For the same reasons which we have already mentioned in § 34 on the determination of the coherence which occurs between the two constituents of a mixed body, we shall likewise admit here, in the circumference of the chemically variable extent as constant, the unknown function dependent on the size, form and position of the particles in contact, and designate it by . Since now the latent electroscopic force in the disc , to which the abscissa belongs, is expressed by
and that in the disc , to which the abscissa belongs, by
the tension originating between the discs and is
consequently the sum of all the tensions produced throughout a portion exposed to chemical change
when and represent those values of , which belong to the commencement and end of the extent in question.
But the tension of the circuit undergoes, besides the change just explained, a second one, from the extremities of the che-
