240 ENERGY OF CURRENTS. 



at the point P. This latter is positive if the fall is in the direction 

 of the current, and negative in the contrary case. If there is no 

 other work than that corresponding to changes of temperature, this 

 energy will appear as a disengagement of heat at P in the first case, 

 and by an absorption in the second that is, by a cooling. This is 

 the effect which is known as Peltier's phenomenon, produced at the 

 contact of the two metals. 



It may be that the localised energy IH is correlated to a chemical 

 reaction, which expends heat if H is positive, and on the other 

 hand produces heat if H is negative, so that the changes of tempera- 

 ture are then merely due to the heat disengaged in accordance with 

 Joule's law. 



248. The' converse of the conclusions which we have established 

 is evident. If, at any point of the circuit, a thermal or chemical 

 phenomenon is produced, the energy of which is proportional to 

 the strength of the current, it may be affirmed that at this point 

 there will be a sudden variation of potential positive or negative, 

 according to the sign of the work, and that the variation is indepen- 

 dent of the current. If, further, the work changes with the direction 

 of the current, we conclude from this that the corresponding variation 

 of potential is fixed, and is independent of the current. 



Let us consider this latter case ; let r be the resistance of the 

 region in which the fall of potential is manifested, and let us suppose 

 that only thermal phenomena are produced at that place. The 

 quantity of heat disengaged is made up of two parts ; one defined 

 by Joule's law is expressed by IV, and is independent of the 

 direction of the current; the other, due to the Peltier effect, has 

 the value IH, and changes its sign with the direction of the current. 

 If the current passes in one direction, the total quantity of heat 

 disengaged is 



iBfi+I^ 



V 



and if it passes in the opposite directicfh 



i r 



VTT 

 " 



In proportion as the current is diminished, the term I will 



ri 



become smaller and smaller, the Peltier effect will predominate, and 

 the reversal of the current will more and more tend to produce 

 equal effects and contrary signs. 



