of Electricity through Metals, 199 



force due to the doublets is much greater than that due to 

 the external field. We see this from the expression 



T NM 2 F'(0)X 



w-kmi 2 ¥\Q) 

 If w is the value of w at the critical temperature, 



«> =MM 2 F'(0)," (4) 



so that I=~ W ° X 



k w — w 



or kl w 



X ~~ to — w ' 



Now kl is the part of the force on a doublet due to the other 

 doublets, and we see from this expression that when w is 

 nearly equal to w kl is very large compared with X , so 

 that the removal of X will not appreciably weaken the 

 coherence of the chains. On the other hand, at temperatures 

 considerably above the critical, kl is small compared with X , 

 so that the external force is essential for the coherence of 

 the chains. 



If the disturbing effect on the chains is entirely due to the 

 thermal energy and if this energy vanishes at the zero of 

 temperature, it will always be possible to find a value of w 

 which satisfies equation (4), and there will always be a 

 critical temperature, i. <?. the metal will be able to pass into 

 the super-conducting state. It is probable, however, that the 

 action of adjacent atoms may, independently of thermal 

 agitation, tend to make the axes of the doublet take up a 

 definite orientation, and that the doublets, when disturbed 

 from this alignment, come under the action of couples 

 tending to restore them to their original positions. We can 

 easily take this into account, all that we have to do is to 

 replace w in the preceding equation by w + D, where D is 

 proportional to the restoring couple for unit angular dis- 

 placement, due to the mutually directive action of the atoms. 

 If L is the local electric force due to the action of t\w 

 adjacent atoms, D = LM. 



The equation to the straight line (1) is now 



1_ m x k w 



We should expect the directive force either to be independent 

 of the temperature or to vary but slowly with it. In this 

 case the slope of the line will not diminish indefinitely as 



