NATURE OF THE PELTIER PHENOMENON. 279 



A current which traverses a junction, heats it when it passes in 

 one direction, and cools it when it passes in the contrary one. 

 M. Becquerel showed that the direction in which there is cooling, 

 is that of the current which would produce the artificial heating of 

 the same junction. When a thermoelectrical current traverses a 

 circuit, the variations of temperature produced at the junction by the 

 current itself, tend then to become weaker, and we may say that their 

 effect is to develop an electromotive force opposed to that which 

 the current produces. That is a necessary condition ; if it did not 

 take place, an accidental current in a metallic circuit would produce 

 a difference of temperatures between the junctions, which would go 

 on increasing, and the current would maintain itself for an indefinite 

 time. 



In a circuit of two metals the hot junction of which is. at a lower 

 temperature than the neutral point, the electromotive force increases 

 with the temperature, the Peltier effect tends then to diminish the 

 temperature of this junction. Beyond the neutral point, on the 

 contrary, the electromotive force diminishes when the temperature 

 increases, and the Peltier effect would tend to increase the tem- 

 perature of the hot junction. 



The Peltier effect at the hot junction has, therefore, a different 

 sign according as the temperature of this junction is lower or higher 

 than that of the neutral point ; it follows from this that the electro- 

 motive force of contact must have changed its sign at the neutral 

 point. It is by analogous reasoning that Sir W. Thomson first 

 showed this property of the neutral point, and deduced from it the 

 necessity of the existence of electromotive forces in a homogeneous 

 conductor at variable temperatures. 



