OF ARTS AND SCIENCES. 199 



assumption, the accordance between the calculated magnetizatious and 

 the observed rotations is far from strict, the law announced 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 Kuinlt nuiNt be 

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

 experiments, of which those m 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 magnetization can In- 

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

 of metal the relation between magnetization (3), permeability (11), 

 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 /x is much greater than unity, as it is in all 



4 IT [J. 



the magnetic metals at ordinary temperature and moderate magnetiza- 

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

 rapidly than tt does. As to the rate at which ti 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 it = 20 is — x ; 



1 F 

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



4 TT 



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. II. II. 



