198 Sir J. J. Thomson on Conduction 



intersect the curve again at S, showing that I retains the 

 finite value SN alter the electric force has disappeared. 

 From the point of view of this paper, however, the part 

 played by the electric force in metallic conduction is to 

 polarize the metal, i. e. to form chains : when once these are 

 formed the electricity is transmitted along them by the forces 

 exerted by the atoms on the electrons in their neighbours. 

 Thus if the polarization remains after the electric force is 

 removed the current will remain too, just as it did in Kamer- 

 lingh Onnes'' experiment with the lead ring. The argument 

 is similar to that by which Weiss explained the existence of 

 permanent magnetism below a critical temperature. 



We see that we shall have the current remaining after the 

 removal of the electric force; i. e., the metal will be in the 

 super-conducting state as soon as the slope of the line is 

 less than that of the tangent at the origin to the curve, i. e. 

 when 



^ is less than NMF'(O), 



or id less than £NM 2 F'(0). 



Thus the temperature at which the metal passes into the 

 super-conducting state is such that 



ic=NM 2 £F'(0). 



!NM& is the electrical force exerted by the doublets when 

 they all point in one direction: if we denote this force by P, 



io=MPF'(0). 



If the specific heat of the metal had not commenced to 

 diminish at this temperature, Q the temperature of transition 

 into this state would be given by the equation 



R0 O =NM 2 *F'(O). 



As, however, the transition takes place at very low tempe- 

 ratures, when the specific heats are variable and w no longer 

 equal to R# we must use a more general expression for w in 

 terms of 6 to determine the critical temperature. The per- 

 sistence of the chains after the removal of the electric force 

 is due to the disturbance due to thermal agitation being too 

 weak to break up the chains when once they are formed. 

 The chains are held together by the electric force due to the 

 doublets in the chain itself as well as by the external electric 

 force, and when we approach the critical temperature the 



