OP ARTS AND SCIENCES. 199 



assumption, the accordauce between the calculated magnetizations and 

 the observed rotations is far from strict, the law aunouuced by Du Bois 

 cannot be regarded as established beyond question. 



If the law of Du Bois is strictly true, the strict proportionality be- 

 tween Hall effect and magnetization announced by Kundt must be 

 held subject to the condition, at a given temperature. For numerous 

 experiments, of which those in the paper to which this note is ap- 

 pended are the most extensive, have shown the Hall effect in the 

 magnetic metals at ordinary temperatures to increase rapidly with rise 

 of temperature, much more rapidly than the magyietization can be 

 supposed to increase under the same conditions. In a very thin sheet 

 of metal the relation between magnetization (3), permeability (/*), 

 and intensity of magnetic field (which I will call F, and which in the 

 case considered is practically equal to 33, the magnetic induction) is 



3 = — . Now when /* is much greater than unity, as it is in all 



■i TT /i 



the magnetic metals at ordinary temperature and moderate magnetiza- 

 tions, it is evident that 3 increases with rise of temperature much less 

 rapidly than /* does. As to the rate at which /a increases with rise of 

 temperature in the metals examined by Mr. Clough and myself, there 

 is considerable doubt, but it is simply impossible for it to be great 

 enough to make the magnetization 3 keep pace with the Hall effect 



19 F 

 in the changes observed. The value of 3 when /x = 20 is -— x — 



80 TT ' 



.IF 



Its value when u, = oc is - X — >. Such considerations as this have 



4 7r 



forced me to abandon the hope, which I once entertained, of making 



out for the relation between Hall effect and magnetization a law as 



simple as that which Professor Kundt has announced. It has long 



seemed, and still seems, to me that the only satisfactory method of 



studying that whole matter will be to repeat Rowland's experiments 



with carefully prepared rings from carefully chosen material, and then 



to work down slices of the same material to the proper shape and 



dimensions for exhibiting the Hall effect. 



E. H. H. 



