50 ADAMS— HALL AND CORBINO EFFECTS. [Ap"i 22, 



This increase may be readily calculated. The rate of heat pro- 

 duction by the radial current is 



P log - 



ri 



27rkt 

 By the circular current it is 



„„ cumnog- 



2TrC^ n 



, , ^2 2Tkt 



kt log — 

 the total rate of heat production is thus 



P log - 



If k' is the conductivity of the disk in the magnetic fleld we may 

 write the total rate of heat production when a radial current / is 

 sent through it 



P log - 



n 



2Trk't 

 Thus 



k a' d<x 



-7 = - = I + cW2 or — = cm\ 



In this expression a is the specific resistance of the metal and o-' its 

 effective specific resistance in the transverse magnetic field. Now 

 according to this view the resistance of a conductor should always 

 be increased by a magnetic field. It is known, however, that with 

 some metals notably iron and nickel, the resistance is decreased in a 

 transverse magnetic field. Furthermore, the increase of resistance 

 calculated from this formula is very much less than the increase 

 actually observed. In the thought that the change of resistance 

 might be dependent on the geometrical form of the metal Mr. 

 Lester has measured the effect of a transverse magnetic field on the 

 resistance of a number of metals, using disks with a radial current. 



