260 THERMOELECTRIC CURRENTS. 



metal has either a temporary or permanent dissymmetry in its physical 

 properties, on either side of the heated part. 



272. LAWS OF THERMOELECTRICAL CURRENTS. Without dis- 

 cussing the experiments which demonstrate these special points, and 

 which have served to establish the laws of the phenomenon, we shall 

 confine ourselves to giving the laws themselves. 



I. LAW OF VOLTA. There is never a current in any metallic 

 circuit all of whose points are at the same temperature. 



For the algebraical sum of all the electromotive forces of con- 

 tact is necessarily zero since the metals obey the law of successive 

 contacts (189). 



II. LAW OF MAGNUS. In any homogeneous circuit there is never 

 a permanent current, whatever may be the shape of the conductor, and 

 whatever the variations of temperature which exist between the different 

 points of the circuit. 



This law leads to the conclusion, either that the variation of 

 temperature from one point to another determines no difference of 

 potential between these two points, or that this difference, if it 

 exists, only depends on the temperatures themselves, and not at all 

 on the law of variation. 



From the hottest part of the circuit to the coldest, we find, 

 in fact, by two different paths, the same fall of temperature, 

 but with variations entirely independent on either side. If there 

 are variations of potential in the circuit, the sum of these 

 variations is null; hence between the two temperatures t and /', 

 the total variation of the potential must be the same on each 

 side. 



It follows from the law of Magnus that the electromotive force 

 only depends on the temperature of the two junctions, and not 

 at all on the distribution of temperatures in the conductors which 

 separate them. 



We shall represent by Ef(AB) the electromotive force of the 

 two metals A and B when the junctions are at the temperatures / 

 and /', the current going from A to B across the hottest junction at 

 the temperature /'. This electromotive force is a function of the 

 two temperatures t and /'. 



III. LAW OF SUCCESSIVE TEMPERATURES (BECQUEREL). For a 

 given couple the electromotive force corresponding to any two tempera- 

 tures t and t' of the two junctions, is equal to the sum of the electro- 

 motive forces, which correspond to the temperatures t and on the one 

 hand, and B and t' on the other, being a temperature between the 

 two former. 



