268 Prof. Oliver Lodge on the Seat of the Electromotive 



As a matter of historical fact, this is exactly the basis on 

 which Avenarius, in I860, discovered the law of E.M.F. in a 

 thermoelectric circuit. He knew Yolta-efFects very well : he 

 surmised that they might vary with temperature, say as a 

 quadratic function, 



V AB = a + bt + ct 2 ; 



he considered it obvious, as every one did before Thomson 

 (and as Ayrton and Perry do still), that the E.M.F. of a ther- 

 moelectric circuit depends on the junctions alone ; and he 

 made the natural jump of supposing that 



E=Y 1 -Y 2 



This expression he verified by experiment so far as the vari- 

 able part is concerned ; and inform it stands correct at the 

 present day. 



What, then, is wrong with this theory? According to 

 Ayrton and Perry, nothing ! not even the neglecting of the 

 then unknown Thomson effect, for its introduction will merely 

 modify the value of the constant c } leaving all the equations 

 unaltered. 



According to my view, the only thing wrong is the suppo- 

 sition that the Y involved in E has anything whatever to do 

 with the totally distinct phenomenon observed by Yolta : 

 though this is enough to put all the constants quite wrong. 



According to Prof. Tait, from whatever point of view he 

 regards the error (I think he regards it mainly as a neglect of 

 Thomson-effects), the falsity of the theory is so serious as to 

 deprive Avenarius of every claim to credit, even to the credit 

 of guessing at a formula and experimentally verifying it 

 within certain limits of accuracy. Or perhaps Prof. Tait 

 means that the credit of such an achievement as that is so 

 small as to be negligible. I am unable to agree with this 

 opinion ; but I certainly have thought that the confounding 

 of thermoelectric E with a temperature-variation of Yolta- 

 effect was an error so serious, however natural, as to diminish 

 very considerably the brilliancy of his happy, though unsub- 

 stantially founded, guess. 



But now that in the year 1.886 Professors Ayrton and Perry, 

 learned in the doctrines of Sir William Thomson, support and 

 contend for this identical view, I beg to modify this opinion, 

 and to withdraw everything that I have said adverse to Pro- 

 fessor Avenarius's remarkable paper, written at Kiew in 1863. 

 For, since the matter is at present one of controversy, its truth 

 or untruth must be regarded as still sub juclice, and not to be 



