350 Mr. 0. J. Lodo-c on Thermo-electric Phenomena. 



o 



observes, of the very same form as the one subsequently given 

 by Professor Tait (1870-71) to express the value of this elec- 

 tromotive force. A few words of explanation are therefore 

 necessary from me. 



The work of Professor Avenarius in 1863, as detailed in his 

 memoir*, " Die Thermoelektricitat, ihrem Ursprunge nach, als 

 identisch mit der Contactelektricitat betrachtet," consisted, 

 first of all, in arriving at this expression by means of two hy- 

 potheses, very reasonable and probable at first sight, but really 

 inconsistent with each other, and one of them false ; and 

 secondly, in bringing the expression so obtained to the test of 

 experiment, and verifying it in a complete and satisfactory 

 manner between certain limits of temperature. This second 

 part of his work is unassailable ; and as it was performed several 

 years before the similar more extensive experiments of Pro- 

 fessor Tait, it constitutes, so far as I know, the original dis- 

 covery of the actual numerical laws of the electromotive force 

 of a thermo-electric pair. 



Regarded, then, as an empirical formula expressing with 

 experimental accuracy the electromotive force of a thermo- 

 electric circuit, it is correct ; but considered as a formula de- 

 duced from and embodying a certain theory in contact-elec- 

 tricity (which theory one would naturally have supposed to 

 be verified by the verification of the formula), it is erroneous, 

 and has led Professor Avenarius himself to erroneous! results. 

 In a subsequent communication to Poggendorff's Annalen% 

 in 1873, to which he now refers me, he admits this to some 

 extent ; and in this paper he makes full use of the laws dis- 

 covered thermodynamically by Sir William Thomson in 1851, 

 and shows that his own expression is in agreement with them 

 if the " specific heat of electricity in a metal " is assumed to be 

 proportional to its absolute temperature. Professor Tait had 

 pointed this out in a very similar manner a few years pre- 

 viously § ; but the original publication of the substance of the 

 1873 paper took place in the Reports of the University of 

 Kiew for 1870, a copy of which Professor Avenarius has been 

 good enough to send me, as I was unable to find them in the 

 library of the British Museum. Hence it appears that the 

 priority in this also rests with him. 



* Pogg. Ann. vol. cxix. 



t See, for instance, Pogg. Ann. vol. cxxii., where the Volta contact- 

 force between two metals at any temperature is supposed to be deduced 

 with the help of thermo-electric measurements, the two distinct pheno- 

 mena of voltaic and thermal electromotive force being mixed up and con- 

 fused together. 



\ Vol. cxlix. "Ein Beitrag zur Theorie der Thermostrome." 



§ Proc. Roy. Soc. Edinb. 1870-71. 



