250 A. W. Davison 



the junction is cooled. It is at once apparent how, if the elec- 

 trons are traveling from lead to zinc, the junction will be heated. 



But how may we explain thermo-electric inversion, and the 

 neutral point? 



Liebenow found that the greater the ratio: — conductivity of 

 heat to conductivity of electricity, the more readily electrons pass 

 from a metal across the hot junction. Also, that ''there is an 

 electromotive force generated within the metal which urges the 

 electrons to proceed in the direction in which heat is being prop- 

 agated." So the electromotive force of a thermo-couple is a 

 differential effect, due to more electrons being dragged along in 

 one metal than in another. 



Explanation of neutral point and inversion. In one metal, at 

 definite temperatures, the electromotive force is proportional to 

 V L/ S, where L is the heat conductivity, and S the electrical 

 conductivity. Now L does not usually vary greatly with tem- 

 perature, but it is well known that electrical conductivity does; 

 and it is this very thing which causes thermo-electric curves to be 

 other than straight lines, and which causes thermo-electric inver- 

 sion. 



Now in the majority of metals, the quantity V L/ S increases with 

 rise in temperature, but in some, as in iron, the ratio diminishes, 

 or reverses upon heating; and the couple passes through a maxi- 

 nmm, or is reversed. 



For an efficient thermo-couple, two metals are desired whose 

 ratios V L/S are as different as possible, for if as the temperature 

 changes, these ratios become equal, the wandering of the electrons 

 will be the same in both directions, and the neutral point will 

 be reached. 



THE DETERMINATION OF THE THERMO-ELECTRIC CHARACTERIS- 

 TICS OF SAMPLES OF WIRE OBTAINED FROM THE DRIVER- 

 HARRIS WIRE COMPANY 



In determining these characteristic quantities, Leeds and 

 Northup Research galvanometer No. 323, and L. & N. Standard 

 Laboratory Wheatstone Bridge No. 5034 were used. To alter 

 the sensibility of the galvanometer, so that various maximum 

 electromotive forces might be determined, the resistance box 



