Magnetic and Electric Saturation Values. 

 Super- Conductivity. 



359 



The fundamentally important experiments of Kamerlingb 

 Onnes, which show that there is a critical temperature for 

 electric conductivity in some metals and that below this 

 temperature they pass into a state of super-conductivity, can 

 be explained by an extension of the theory given above. The 

 state of super-conductivity, in which it is possible for a current 

 to continue after the removal of the applied electromotive 

 force, is analogous to residual magnetization in a ferro- 

 magnetic body which persists after the applied magnetizing 

 force is removed, and it may be explained, as for magnetism, 

 by the hypothesis that there is an intrinsic field in action, 

 which is a function of the polarization, in addition to the 

 externally applied force. Thus in equation (16) X must be 

 replaced by X + /'(Y), and then, although X may become 

 zero, the intrinsic field f(Y) may persist, under proper 

 temperature conditions, giving rise to a persistent electric 

 current. 



The problem may be treated in the same way as when the 

 gas law is made to include liquids by the introduction of an 

 intrinsic pressure. 



The extended gas law in general symbols will then be 



(X+/(Y))(i-| o )=KT, 



(17) 



K being a constant analogous to R, - 

 and if van der Waals's expression for /(Y) be adopted we have 



(X + aY*)(^-^)=KT. 



(18) 



This equation when applied to ferromagnetism yields 

 numerical results which meet with the same success as 

 those derived from the kinetic theory, and it represents 

 in the main the chief experimental facts of magnetism, 

 so that it may be applied with some confidence to electric 

 polarizations and currents the theory of which is like the 

 theory of magnetism. Equation (18) then becomes 



(E + ai>)g-i)=ST, 



and this equation implies that there are 

 for electric polarizations and currents. 



.... (19) 



critical constants 



