concerning Volta's Contact Force. 373 



the statement will not be objected to in substance though its 

 purposely crude wording maybe criticised), that the E.M.F.s 

 concerned in Yolta contact electricity, as well as in ordinary 

 voltaic electricity, are caused in a chemical manner. In 

 saying this I am not intending any controversy, I am only 

 trying to state. The best plan perhaps at this stage is to be 

 more explicit. 



1 hold that Volta forces can be calculated, as regards tbeir 

 major part, from the differential energy of combination of the 

 metals with oxygen (or other active surrounding element), 

 and I have justified this, more or less completely, by nume- 

 rical calculations (B. A. Report, 1884 ; Phil. Mag. April 1885, 

 § 17, pp. 272, 274, &c.) based upon the great fundamental 

 voltaic-cell paper of Lord Kelvin in 1851. 



Lord Kelvin, on the other hand, holds that the Yolta force 

 can be calculated as regards its major part from the chemical 

 affinity of metals for each other, or, to be specific, from the 

 energy of combination of zinc with copper in the formation 

 of brass. It is true that for the calculation to be made in 

 this order, an estimate of molecular dimensions must be postu- 

 lated, whereas his original calculation treated the order of 

 calculation inversely; but this is of no immediate consequence, 

 and no one at the present date has any serious compunction 

 at including the order of ordinary molecular dimensions 

 among data practically known. 



Yery well then, unless this statement be objected to, I say 

 that the opposite camps are involved both of them in contact 

 views, and both of them in chemical views. It is not a 

 question of whether physical contact or some form of chemical 

 action is operative ; it is a question of which of several con- 

 tacts is the really effective one, and what kind of chemical 

 action or chemical affinity is the active cause. 



Is it the contact and the chemical affinity across the metal- 

 metal junctions, or is it the contact and chemical affinity 

 across the metal-air junctions ? 



The opposite camps are thus metallic versus dielectric. 



I say oxygen. Lord Kelvin scouts oxygen and says brass. 

 He also says aether, and I am willing to say aether, too, in 

 some sense later on, if the evidence permits ; but meanwhile, 

 it is more commonplace and safer to say oxygen : for un- 

 doubtedly oxygen is there. 



Lord Kelvin does not deny the possibility, perhaps the 

 probability, of some E.M.F. at "the metal-air or metal-vacuum 

 junction, which contributes to the total Yolta effect, but I 

 apprehend that he does not believe its contribution to be 

 great. Nor, on the other hand, do I deny the existence 



