360 Magnetic and Electric Saturation Values. 



The critical temperature will be given by 



T.=T7?' (20) 



from which it is possible to estimate a and therefore to 

 estimate the magnitude of the intrinsic field. The calcu- 

 lation can only give an upper limit to a, since the critical 

 temperatures for conductivity determined up to now are not 

 far above the absolute zero, and at such low temperatures 

 the atomic heat is a very small quantity and the kinetic 

 energy in question is no longer equal to RT. 



Taking Lead as an example in which the critical tempe- 

 rature is a little less than 4° absolute, and using the c.g.s. 

 values of i and S found above, then a must be less than 

 1*2 x 10" 4 and consequently the maximum intrinsic field (ai 2 ) 

 is less than 7*9 x 10 9 c.g.s. units or 79 volts per cm. As 

 the current densities commonly employed are only about a 

 millionth of the limiting values calculated above, it follows 

 that the intrinsic field in such a conductor as Lead, when it 

 carries even a high current density at temperatures above 

 the critical temperature, must be extremely small, indeed 

 negligible compared with the applied electromotive force. 

 This, however, is to be expected since Ohm's law is obeyed 

 with very great accuracy at ordinary temperatures, which 

 would not be the case if the intrinsic field made itself felt. 

 Below the critical temperature current densities approaching 

 the maximum should be attainable, and it is of interest to 

 find that Kamerlingh Onnes has observed a current density 

 in mercury at 2°*45 absolute, which is lower than the critical 

 temperature, of more than 10 5 amperes per sq. cm. (K. Onnes, 

 Elect, lxxi. p. 855, 1913). 



From what has been said above, it is seen that the facts 

 of electric conduction at very low temperatures as well 

 as the like facts of ferromagnetic induction are in agree- 

 ment with the ideas which underlie the fluid equation, and 

 thus both magnetic and electric experiments give to the 

 fluid law a generality wider than has commonly been accorded 

 to it ; and in the particular case in which it becomes the 

 gas law it may be said that it governs not only the free 

 translatory movements of molecules which determine the 

 behaviour of a gas, but also the free vibrations and rotations 

 of molecules which are manifested in the magnetic and 

 electric behaviour of substances in general. 



