278 THERMOELECTRIC CURRENTS. , 



At two symmetrical points, M and M', the difference of the 

 final temperatures t and /', corresponds exactly to the Thomson 

 effect. As the heating is greater in the part in front of the median 

 region AA', where the temperature is a maximum, there is a kind 

 of electrical convection of heat in a direction opposite to that of the 

 current, and this convection is proportional to the strength of the 

 current. 



Sir W. Thomson found in this way that for iron the electrical 

 convection of heat is negative (that is to say, in the direction 

 opposite to the current), and that the convection is positive^ but far 

 feebler, for copper. 



M. le Roux extended these same observations to a great number 

 of metals ; he proved that the effect is proportional to the strength 

 of the current, and ascertained that it is almost zero for lead, so that 

 from this point of view lead is sensibly neutral. 



286. NATURE OF THE PELTIER PHENOMENON. We may now 

 discuss, with more accuracy, the Peltier phenomenon. 



The electromotive force of contact between two metals is 

 expressed in Sir W. Thomson's theory by the general formula 



If the course of the couple is uniform, we have, for the two 

 temperatures T T and T, 



E = A(T-T 1 ), 



from which follows 



H = AT. 



The electromotive force of contact between two metals is then 

 proportional to the absolute temperature, and the Peltier effect 

 should follow the same law. At the temperatures of 25 and 100, 

 for instance, we shall have 



=i ( 75 _ T , T 



' 



H 25 273 + 25 298 4 



According to M. le Roux's experiments, the bismuth-copper 

 couple exactly satisfies this condition. 



