[ ^81 ] 



XXXVI. On the Dynamical Theory of Heat.— Vart VI. Thermo - 

 electric Currents*. By William Thomson, M.A., Professor 

 of Natural Philosophy in the University of Glasgow. 



[Continued from p. 225.] 

 §§ 112-124. Genei-al Equations of Thermo-electric Ctirrents in 

 non-crystalline Linear Conductors. 

 112. ^T^HE only reversible thermal effect of electric currents 

 A which experiment has yet demonstrated, is that which 

 Peltier hasdiscovered in the passage of electricity from one metal to 

 another. Besides this, we may conceive that in oae homogeneous 

 metal formed into a conductor of varying section, different thermal 

 effects may be produced by a current in any part, according as 

 it passes in the direction in which the section increases, or in 

 the contrary direction ; and with greater probability we may sup- 

 pose, that a current in a conductor of one metal unequally heated 

 may produce different thermal effects, according as it passes from 

 hot to cold, or from cold to hot. But Magnus has shown by careful 

 experiments, that no application of heat can sustain a current in a 

 circuit of one homogeneous non-crystalline metal, however vary- 

 ing in section ; and from this it is easy to conclude, by equations 

 (7) and (9), that there can be no reversible thermal effect due to 

 the passage of a current between parts of a homogeneous metallic 

 conductor having different sections. Now it is clear that no 

 circumstances, except those which have just been mentioned, can 

 possibly give rise to different thermal effects in any part of a 



* Foot note on the third paragraph of § 109—" And (2), That JB, which 

 cannot vanish in any case, is the absolute numerical measure of the gal- 

 vanic resistance of the principal conductor itself" — contained in previous 

 article (p. 224). 



Principle of Mechanical Action on Electromotive Forces and Galvanic 

 Resistances. 



This conclusion was first given by Joule in his first paper, which was 

 communicated to the Royal Society, December 17, 1840, "On the Pro- 

 duction of Heat by Voltaic Electricity" (see 'Proceedings' of that date). 

 The paper was published in the Philosophical Magazine, vol. xix. p. 260. 

 See also " On the Calorific Effects of Magneto- electricity, and the Mecha- 

 nical Value of Heat," by the same author (Phil. Mag. vol. xxiii. 1843), 

 where the principles of mechanical action in the electric generation of 

 heat are more fully developed. 



The conclusion stated in the text was also given by Helmholtz in his 

 Erhaltung tier Kraft, Berlin, 1847 (translated in Taylor's New Scientific 

 Memoirs). It was given by the author of the present paper with various 

 numerical a|)plications regarding the electromotive forces of electro-che- 

 mical arrangements and the resistances of metallic conductors in absolute 

 units, in two papers in the Philosophical Magazine, December 1851, "On 

 the Mechanical Theory of Electrolysis," and "On the Applications of the 

 Principle of Mechanical Effect," &c. 



Phil. Mag. S. 4. Vol. 11. No, 72. April 1856. U 



