546 Mr. Gr. H. Livens on the Electron Theory 



Of course, in many of the very simple non-ferromagnetic 

 metals the local field might be extremely small and the 

 theory in the above form would then be very exact. There 

 is, for instance, every reason to believe that this is the case 

 for such metals as copper, silver, and gold. If this exactness 

 is to be expected in any case, then an exact knowledge of 

 the constants of the various effects would, of course, provide 

 valuable information for a determination of the electron 

 constants of the metal ; not only the constant s of the force 

 exerted by the molecules on the electrons, but also of the 

 number N of the electrons per unit volume and the quantity 

 l m which determines the strength of the molecular fields of 

 force. It hardly appears, however, that the subject is in a 

 position, at present, for this point to bo pushed except, 

 perhaps, in a rough descriptive sort of way. 



A striking confirmation not only of the general theory 

 but also of the hypothetical amplification just suggested, is 

 provided in the results of an examination into the effect of 

 temperature on the various coefficients, more particularly 

 that of the Hall effect. In the simpler metals like copper 

 and silver there is practically no variation of the coefficient 

 of the Hall effect with the temperature ; a fact which proves 

 almost conclusively not only that the law of force for the 

 action of a molecule on an electron is of the simple type 

 assumed above, but also that it does not vary with the 

 temperature. The slight variation actually observed is 

 probably due to the variation of N, the number of free 

 electrons per unit volume. 



Of course, in the ferromagnetic metals or in other cases 

 Avhere the local field is at all comparable with the applied 

 field, we should expect considerable variations with the 

 temperature of the coefficients as usually defined relative to 

 the magnetic force, and this again is precisely what is 

 observed. The behaviour of the ferromagnetic metals in 

 this respect is interesting ; in these cases as the temperature 

 is increased the Hall effect coefficient increases, exactly 

 parallel with the magnetic permeability, until the critical 

 temperature is reached, when it decreases very rapidly to a 

 value more akin to that found in the simpler metals. This 

 would appear to be almost conclusive evidence of the appro- 

 priateness of the explanation of these irregularities suggested 

 above. 



There is a difficulty of another type connected with the 

 present theory. In fact it fails entirely to explain any effect 

 of the magnetic field on the aggregate motion of the electrons 

 in a direction parallel to the lines of force. This is of course 



