Magnetic and Electric Saturation Values. 357 



coefficient of resistivity is nearly the same for all, namely, 

 about 0*0038, a number which shows that the resistivity is 

 approximately proportional to the absolute temperature. 



According to the calculation the saturation current 

 density in these metals is of the order 10 8 amperes per 

 sq. cm., and this is considerably in excess of any obser- 

 vations of high current densities which have been recorded. 

 A recent experiment by Trauenberg (Trauenberg, Phys. 

 Zeits. xviii. p. 75, 1917) shows that Ohm's law holds good 

 up to 8 x 10 6 amperes per sq. cm., presumably for silver ; 

 but this enormous current density would have to be increased 

 more than tenfold before Ohm's law would fail. There is 

 nothing then in this experimental value to make the 

 saturation current densities which have just been calculated 

 at all improbable. 



A direct proof that Ohm's law will fail for current 

 densities of the order 10 8 amperes per sq. cm. seems at 

 present beyond the reach of experimental demonstration. 



Theory of Metallic Conduction. 



In his Presidential Address to the Physical Society 

 (Thomson, Phys. Soc. Proc. vol. xxvii. part 5, p. 527 ; 

 Phil. Mag. xxix. pp. 192-202; also 'Corpuscular Theory 

 of Matter/ p. 86) Sir J. J. Thomson has outlined a theory 

 of metallic conduction based on the hypothesis that in a 

 metal there are electric doublets which under an electric 

 force can be orientated, and this is a principal function of 

 an electromotive force. These doublets, like the magnetic 

 molecules of a magnetic substance, have their alignment 

 with the direction of the force opposed by thermal agitation, 

 and according to the conditions of field-strength and tempe- 

 rature they may be free from each other's control or subject 

 to each other's influence. So far the theory would apply 

 to electric insulators as well as to conductors, but the 

 distinguishing feature of a conductor is that the doublets 

 very easily part with electrons, which pass from atom to 

 atom of a polarized chain " like a company in single file 

 passing over a series of stepping-stones." 



If the intensity of the polarization and the charge deter- 

 mined by it can be calculated, the strength of the current 

 will be given by multiplying this charge by the velocity of 

 movement of the electron. 



The problem is solved in the same way as for the deter- 

 mination of the intensity of magnetization of an assemblage 

 of magnetic molecules. 



