Middle of the Nineteenth Century. 265 



of temperature, which is the same in all parts of the conductor 

 where the cross-section is the same ; but he did not succeed in 

 connecting the thermal phenomena quantitatively with the 

 strength of .the current a failure which was due chiefly to the 

 circumstance that his attention was fixed on the rise of 

 temperature rather than on the amount of the heat evolved. 

 But incidentally the investigation led to an important discovery 

 namely, that when a current was passed in succession through 

 two conductors made of dissimilar metals, there was an evolution 

 of heat at the junction ; and that this depended on the direction of 

 the current ; for if the junction was heated when the current 

 flowed in one sense, it was cooled when the current flowed in the 

 opposite sense. This Peltier effect, as it is called, is quite distinct 

 from the ordinary Joulian liberation of heat, in which the 

 amount of energy set free in the thermal form is unaffected by 

 a reversal of the current ; the Joulian effect is, in fact, propor- 

 tional to the square of the current-strength, while the Peltier 

 effect is proportional to the current-strength directly. The 

 Peltier heat which is absorbed from external sources when a 

 current i flows for unit time through a junction from one metal 

 B to another metal A may therefore be denoted by 



where T denotes the absolute temperature of the junction. The 

 function n^ (T) is found to be expressible as the difference of 

 two parts, of which one depends on the metal A only, and the 

 other on the metal B only ; thus we can write 



In 1851 a general theory of thermo-electric phenomena was 

 constructed on the foundation of Seebeck's* and Peltier's dis- 

 coveries by W. Thomson.f Consider a circuit formed of two 



* Cf. pp. 92, 93. 



t Proc. R.S. Edinb. iii (1851), p. 91 ; Phil. Mag. iii (1852), p. 529 : Kelvin's 

 Math, and Phys. Paper*, i, p. 316. Cf. also Trans. R. S. Edinb. xxi (1854), 

 p. 123, reprinted in Papers, i, p. 232 : and Phil. Trans., 1856, reprinted in Papers, 

 ii, p. 189. 



