LAW OF INTERMEDIATE METALS. 261 



This law may be expressed as follows : 



We have already learnt that the electromotive force only depends 

 on the temperature of the two junctions ; this latter law shows that 

 the electromotive force may be expressed by the difference of two 

 terms, one of which only contains the temperature t and the other /', 

 these two terms being the values of the same function of the tem- 

 perature. 



We may then write 



IV. LAW OF INTERMEDIATE METALS (BECQUEREL). If two 

 metals A and B are separated in a circuit by one or more inter- 

 mediate metals, with all intermediate junctions kept at the same 

 temperature /, the electromotive force is the same as if the metals 

 were directly connected, and the junction raised to the same tem- 

 perature t. 



The law of intermediate metals may be expressed by the 

 equation 



For if two metals A and B are connected at the hot junction by 

 an intermediate metal C, from the law of Magnus we may suppose 

 that a point P of this third metal is at the lower temperature /, and 

 interpose, in like manner, at the cold junction, a piece of the metal 

 C kept at the temperature of this junction. We have then the two 

 couples AC and CB in the circuit between the same limits of tem- 

 perature; the electromotive force is that which would be directly 

 produced between the metals A and B. 



This law is of great practical importance; it shows that the 

 soldering at the junction of two metals has no influence on the 

 phenomena to which they give rise. 



V. PHENOMENA OF INVERSION. In the case of some thermo- 

 electric couples, the strength of the current increases continuously as 

 the temperature of the heated junction is raised, that of the cold 

 junction remaining unchanged. The couple is said to work uni- 

 formly when the electromotive force is proportional to the difference 

 of the temperatures of the two junctions. 



In most cases, on the contrary, the electromotive force of the 

 couple, after having passed through a maximum, becomes null, and 

 then changes its sign. 



