80 Mr. A. Stansfield on some Improvements in 



logarithmic formula depends on the straightness of the 



1 -pfr curve. 

 ai 



The curves which are plotted in fig. 8 are calculated from re- 

 sults given in a paper by K. Noll * on " The Thermo-electricity 

 of Chemically Pure Metals." They show that thermo-couples 

 composed of copper, iron, gold, and mercury follow Tait's 

 law very closely over the somewhat limited range of tem- 

 peratures for which data are given, and other couples not 

 plotted also show approximated straight lines for the values of 



-77m. A couple composed of cobalt and mercury, and another 



composed of platinum and copper the results of which are not 

 plotted, show both lines curved in opposite directions, a fact 

 which suggests that thermo-electrically there may be two classes 



of metals: — (1) the ordinary metals, for which the —^ curves 

 are straight ; and (2) the platinum metals, together with a 



few other metals such as nickel and cobalt, for which the T— - 



(/I 



curves are straight. A couple composed of one metal from 



each class would of course give both lines curved. On the 



other hand, it is unlikely that there would be any sharp division 



of this kind, and both formulse may be particular cases of 



a more general one. r™ 



In view of the curvature exhibited by the —^ curves of 



J dT 



thermo-couples composed of platinum and platinum alloyed 



with iridium or rhodium, there can be no object in applying 



the parabolic formula to their indications, though this has 



been done by several experimenters. 



In conclusion, the author wishes to express his thanks to 

 Prof. Roberts-Austen for the helpful interest he has taken in 

 the research. 



* AVied. Ann. 1894, p. 874. 



