1884.] 



Explanation of Hall's Phenomenon. 



343 



The several papers abound in speculations which it is not now 

 necessary to consider ; but very great importance has been attached 

 to the phenomenon in consequence of the opinion expressed by 

 Professor Rowland and others, that it is connected with the magnetic 

 rotation of the plane of polarisation of light, and thus furnishes 

 additional evidence of an intimate relation between light and 

 electricity. 



A number of experiments, which I have from time to time made, 

 convinced me, however, that no direct action of the kind supposed 

 was ever produced ; and, after several unsuccessful attempts to frame 

 a hypothesis which would solve the mystery, I finally found that 

 Hall's phenomenon might be completely explained by the joint action 

 of mechanical strain and certain thermoelectric effects. 



The mechanical strain is produced by electromagnetic action. In 

 fig. 2, let ABCD represent the metallic strip through which a current 



Fig. 2. 



is passing from left to right. Suppose that the south pole of a 

 magnet is beneath it and the north pole above it, then the strip 

 would, if free to do so, move across the lines of force in the direction 

 indicated by the arrow EF. 



But, as arranged for Hall's experiment, the metallic strip is fixed, 

 and cannot move bodily from its position ; it will, however, be 

 strained in such a manner as to make it tend to assume a form more 

 or less similar to that indicated in fig. 3. As thus distorted, the 

 plate may be mapped out into six districts, in three of which the * 

 metal is (on the whole) subjected to longitudinal traction, while 

 in the other three, which are shaded in the figure, it undergoes 

 compression. 



