THERMOELECTRIC POWER. 271 



279. The expression for the electromotive force of the couple 

 AB between the temperatures / and f 19 is 



E! (AB) = f (/)<//= f 

 jij A 



it is therefore represented by the area of the quadrilateral comprised 

 between the curves AX and BX, and the ordinates corresponding to 

 the two temperatures ^ and /. 



If, while the cold junction remains at a constant temperature t lt 

 the temperature / of the hot junction is increased, the electromotive 

 force increases with the corresponding area, until the temperature 

 attains the value t n1 which corresponds to the point of meeting of 

 the two curves. 



At this temperature t n the thermoelectrical power of the two 

 metals is zero ; this is called the neutral point. When the tem- 

 perature exceeds that of the neutral point, the electromotive force 

 decreases, for it is now represented by the difference of the two 

 triangular areas which have their apex at I; it becomes zero, as does 

 the strength, at a temperature / 2 such that 



area M 2 IN 2 = area MjINp 



Lastly, as soon as the temperature of the hot junction exceeds 

 / 2 > the electromotive force becomes negative and inversion takes 

 place. Hence the temperature of inversion depends on the tem- 

 perature of the cold junction. 



280. The previous results become very simple when the curves 

 AX and BX are straight lines. The figure M^NN^ is then a 

 trapezium, the surface of which has the value 



We have further 



2 



= const. = #, 



and, therefore, 



