406 



iiKi'OUT — 1884. 



<lcterniinc(l bufoio any alteration of tlio lactallic siirfact,' wetted hy the 

 licjiiid 1ms oucuric'd; but these ulteratiuns produeu themselvua very 

 rapidly.' 



Pellat's theoretical conoliisiojis lieing short may also bo hero quoted, 

 find I will nnmbcr them on with the others. 



((!) 'It is extremely probable that tho diirercnco of potential between 

 the eleetrie coats which cover two metals connected metallieally re[)r( . 

 lients tho true ditl'erenee of potential which exists between them. Mu 

 reason, either theoretical oi* experimental, can bo invoked against; the 

 existence of a differenee of potential between two nietids in contact. 



(7) 'This last (piantity has no connection with the thei'mo-eleetric 

 Iv.M.l"'. measnred by the Peltier phenomenon. 



(8) 'It has oidy a vajjac and distant connection with tho diflbrenco of 

 oxidisability of the metals.' 



Concerning these propositions I u\;\y remark that while Xnmber 2 is 

 likely to annoy contact theorists (thon<^h I know they have methods of 

 explaining it away), Numbers 4' and "» are calculated to restore their 

 oqaanimity. The live experimental conclusions I accept as in duty 

 bound, only ])ermittinu: myself partially to doubt the perfect generality of 

 Numbers •!• and ."» under all circumstances ; but tho three theoretical ones I 

 am unable whc'lly to accept. Thus with respect to tho second part of 

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

 himidtaneously admit Number 7. Whether one is jirepared to accept any 

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

 regards them. If in the way ho himself intended, then I reject them al'. 

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

 S aro true (though for 'otdya 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 while Number 8 refers to the other quantity. Number 

 (5 I should also consider true if the prefix ' im ' be made to the fonrtli 

 word. 



] 0. Pellat then proceeds to explnin why he considers the Peltier effect to 

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

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

 reasoning, which 1 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 with advantage be abbreviated. 



Two metals A and IJ 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 tho junction not only the contact foi'ce E, but also the equal opposite 



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

 till 



I'orces alone would resist or aid the passage of electricity across the junction, 



and so erive rise to a Peltier efiPect, but both tofjether will do nothing of the 



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



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



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

 said ho got from Sir Wilham Thomson, was something like this. When 



' Thus it may be, sugyosts IVllat, duo (o a slight dilfcronce between K and 



— - produced by the mere fact of a current passing:; i.e. contact E.M.F. wi"" 



t/ii 

 tlcctricity at rest may 1)l' slightly different to what it is with electricity in motien. 



