496 KEroRT — 1884. 



determined before any alteration of the metallic surface wetted by the 

 liquid has occurred; but these alterations produce themselves very 

 rapidly.' 



Pellat's theoretical conclusions being short may also be here quoted, 

 and I will number them on with the others. 



(6) ' It is extremely probable that the difference of potential between 

 the electric coats which cover two metals connected metallically repre- 

 sents the true difference of potential which exists between them. No 

 reason, either theoretical or experimental, can be invoked against the 

 existence of a difference of potential between two metals in contact. 



(7) ' This last quantity has no connection with the thermo-electric 

 E.M.F. measured by the Peltier phenomenon. 



(8) ' It has only a vague and distant connection with the difference of 

 oxidisability of the metals.' 



Concerning these propositions I may remark that while Number 2 is 

 likely to annoy contact theorists (though I know they have methods of 

 explaining it away), Numbers 4 and 5 are calculated to restore their 

 equanimity. The five experimental conclusions I accept as in duty 

 bound, only permitting myself pai'tially to doubt the perfect generality of 

 Numbers 4 and 5 under all circumstances ; but the three theoretical ones I 

 am unable wholly to accept. Thus with respect to the second part of 

 Number G, I beg entirely to differ from M. Pellat if I am called on to 

 simultaneously admit Number 7. Whether one is prepared to accept any 

 of his theoretical conclusions or to reject them all depends upon how one 

 regards them. If in the way he himself intended, then I reject them all. 

 If with one's own interpretation, then I say that the second part of 6 and 

 8 are true (though for ' ordy a vague and distant ' I would substitute 

 ' no ') ; and 7 is also true if it be held to refer to the quantity first men- 

 tioned in Number G while Number 8 refers to the other quantity. Number 

 G I should also consider true if the prefix ' im ' be made to the fourth 

 word. 



10. Pellat then proceeds to explain why he considers the Peltier effect to 

 be quite distinct from, and have no relation to, the true E.M.F. of contact. 

 In explaining this he makes use of a piece of unpleasantly plausible 

 reasoning, which I myself have heard Professor Ayrton use, and which 

 when unexpectedly suggested is so painfully benumbing that it is worth 

 while to quote it and to indicate its weak point. Pellat's statement of 

 the argument is rather long, perhaps it can w T ith advantage be abbreviated. 



Two metals A and B put into contact are at different potentials, the 

 difference A/B being due to and equal to the E.M.F. of contact. There is 

 then at the junction not only the contact force E, but also the equal opposite 



force — , due to the difference of potential established. Either of these 



dn 

 forces alone would resist or aid the passage of electricity across the junction, 

 and so give rise to a Peltier effect, but both together will do nothing of the 

 sort, and so if there be any Peltier effect it must be some small residual 

 phenomenon, or it must be due to some other and totally distinct cause. 1 

 Professor Ayrton's way of putting the argument, which I think he 

 said he got from Sir William Thomson, was something like this. When 

 1 Thus it may be, suggests Pellat, due to a slight difference between E and 



— _ produced by the mere fact of a current passing; i.e. contact E.M.F. with 

 dn 



electricity at rest may be slightly different to what it is with electricity in motion. 



