Thermo-electric Couples 



247 



gent 0. If 2c is negative, the line will slope downward from 

 left to right. 



If the junctions are at U and h, the shaded area, fig. 2, repre- 

 sents the electromotive force around the circuit, hence for metals 

 M' and M" fig. 3, the shaded area represents the electromotive 

 force when the junctions are at U and U. The ordinate, Tq of 

 the point where the lines cross represents the neutral tempera- 

 ture for those metals. It is customary to represent the thermo- 

 electric power lines of all metals on one sheet, from such a sheet 

 the direction and values of electromotive forces for given tem- 

 peratures and given metals, as well as neutral points, may be 

 determined. 



Peltier effect. Peltier, in 1834, observed that this thermo- 

 electric effect is reversible, that when a current is sent around a 

 circuit whose junctions are at the same temperatures, heat is 



Heat 

 generated 



Eeat 

 absorbed 



absorbed at one junction, and generated at the other. This may 

 be explained by assuming that there is a sudden change of poten- 

 tial at the junctions, which is independent of the battery, and 

 that the current does work in traversing this fall of potential. 



Thomson effect (figs. 4 and 5). Lord Kelvin, while trying to 

 apply the reversible cycle of Carnot to the Peltier effect, con- 

 cluded that were the Peltier effect the only reversible effect in 

 the circuit, the electromotive force of the couple would be directly 

 proportional to the difference in temperature of the junctions, 

 which is contrary to observed facts. This led him, in 1851, to 

 predict the existence of other effects, which he found, and which 

 were' named after him, the Thomson effect. 



