I 



ly liavo 

 nlinc ill 



eretl with 

 metal lias 

 srhicli it is 

 lally com- 

 would bu 



JICI. 



1 of nickel, 

 ives copper 

 ,s the other 

 numbers. 



er to be ex- 

 hat copper 

 iig that the 

 of this has 

 being M,S, 



I».„0-H,,S; 

 tre given on 

 satiou heats 

 3 somethir-g 

 I 



KLECTUOMOTIVE FOUC'ES IN TIIK VOLTAIC (.'i;i,L. 



Till 



lIvAViihntical Volta Sen'*'", iii S'llplnir i>r SvljtJniri'fli'i} TTi/ilroijeii, of Mvtnllir 

 Oxidex, and jiont^iblif of Air-cuattd Mvtab. 



Metal 



Silver • 



Mercury 

 Ooi)por 

 I. fat I. . 

 Zinc I 

 Iron . I 

 Sndium . 



The series so ohtainod gives copper and iron in their proper order; 

 but it is scarcely likely to be really correct, because it asauuies that the 

 oxides of the metals are exposed to tho gas rather than the nietals 

 tl mselvcs. It is quite possible tlisit it is not very incorrect for tarnished 

 metals — i.e. metals coated with a lilm of oxide; but lor ordinarily clean 

 metals, coated, not with a film of oxide, but with a lilm of oxygen, it is 

 niitliing but a rough approximation, given because wo have no better data. 



It is to be noted that, as the fdiii of oxygen dilTuses away, tho Volta 

 efrect depending on it must diminish ; until at length tho active aliinity caus- 

 ing the chemical strain is nothing more than M,.S, or perhaps ]M,S — H ,.8. 

 A gradual falling off and ultimate even reversal of sign was observed by ^Ir. 

 Bi'owii in both HCl and H.S. In so far as actual chemical action occui's 

 ai.(i a fdm of chloride or sulpliidc forms, so far, of coui'se, also will thr> effect 

 diminisli ; because it depends essentially on the unsatisfied chemical strain, 

 not oil the accomplished chemical action. 



For a summary of the views here expi-essed see section 22. 



1'.'. Having now explained why I believe the main part of the Volta effect 

 to take its rise at the surface of contact between metal and medium rather 

 tlian l)etween metal and metal, it remains to consider whether this belief 

 requires one to assert that there is no true contact foi'ce at all at tho 

 junction of two metals. By no means ; thu existence of such a force i.«i 

 undoubted ; but for metals it is usually \gyj small and may be neglected 

 iu eoiuparison with the Volta force, though, strictly speaking, what is 

 observed electroscopically is a mixture of tho two. It is the true contact 

 force which gives rise to the Peltier effect, and its variation with tempera- 

 ture (assisted by the Thomson effect) causes thermo-electric currents. 

 A contact force exists, as Thomson has shown, not only at the junction of 

 two different metals, but also between parts of the same metal at different 

 temperatures. 



In another place • I have endeavoured to gain some insight into tho 

 nature of this true contact force and to suggest its cause. This has been done 

 hv many others ; but I may be permitted to repeat my own notion — vaguo 

 ar d incomplete though it avowedly is. Molecules of matter do not move 

 in independence of electricity ; at any rate, the converse is certainly true — 

 electricity does not move independently of matter. Electricity, in flowing 

 through a wire, meets with resistance ; there is something analogous to 

 friction between the matter and the electricity, and the opposing force is 

 precisely proportional to the strength of tho current. This much is 



' Phil. Mag. December (suppl.). 187G, ' On a Mechanical Illustration o£ Thermo- 

 electric riieuomena.' 



