250 Prof. E. P. Adams on some Electromagnetic 



shown in the accompanying figure might be adopted. A 

 sector of small angle is cut out of the disk and two wires of 



Fk-. 1. 



very low resistance soldered to the edges. If the current is 

 introduced through these wires, then, if their resistance can 

 be neglected, it may be shown that the circular current 

 density in the disk is inversely proportional to the radius ; 

 there is no radial current. The equipotential lines are the 

 radii. Now apply a magnetic field perpendicular to the disk. 

 Obviously it is impossible to lead out the radial current so as 

 to make this experiment the exact inverse of the former one. 

 The best that can be done is to connect the extremities, say 

 A, B, of a radius to a galvanometer which will show that 

 these points are no longer at the same potential. But this 

 effect is certainly the Hall effect. The rectangular plate 

 ordinarily employed has only to be thought of as bent and 

 stretched into the form shown. 



For the theory of the two other electromagnetic effects, 

 we shall follow Professor Corbino's argument, but expressed 

 in terms of the characteristics of the electrons. Suppose 

 the disk carrying the radial current C is suspended in a 

 uniform magnetic field so that its normal makes an angle cj> 

 with the lines of force. The energy of the disk in the 

 magnetic field is given by 



Vi = - 9 J ^ 4m2? , — wr- dr H cos* </>, 



assuming the magnetic permeability to be unity. 



