Effects related to the Hall Effect. 219 



but it is reasonable to suppose that it was such that — —15. 



e , ri 



Using these values in eq. (7) with — =10 7 we find 



T = 2.10- 12 . 



The specific resistance of bismuth is 



t = 1-3 10 4 , 



e = l-6 10"' i0 . 



Eq. (6) thus gives for the number of free electrons in unit 

 volume of bismuth : 



N = 5.10 20 . 



It was found that the circular current was not propor- 

 tional to the magnetic field, but increased less rapidly than 

 the magnetic force. But there are reasons (Phys. Review, 

 xxiv. p. 428, 1907) for believing that the free time of an 

 electron is dependent upon the magnetic force, and this may 

 account for the differences observed between theory and 

 experiment. 



Professor Corbino regards this effect of the magnetic field 

 upon the current in the disk as something distinct from the 

 Hall effect. It is certainly true that if the Hall effect is 

 defined as a bending of the equipotential lines by the mag- 

 netic field, then this effect is different. For in this case the 

 equipotential lines are concentric circles both in and out of 

 the magnetic field. It seems to me, however, that the effect 

 described by Professor Corbino is, essentially, the Hall effect, 

 but measured in a different way from that usually employed. 

 In fact, his method seems to be the ideally simplest way to 

 measure the Hall effect,, In the ordinary method, where a 

 uniform current is sent through a rectangular plate, and the 

 transverse current is led out at two points, complications in 

 the current distribution in the plate are introduced which 

 make it difficult to apply the electron theory of metallic 

 conduction. Experiments are now in progress here to 

 measure the effect in a number of metals in addition to 

 bismuth. 



The view that Professor Corbino's effect is really the Hall 

 effect receives support when we examine the inverse case. 

 Suppose a circular current flows through a disk in a mag- 

 netic field. A radial current should result. In order to 

 introduce a circular current into the disk the arrangement 



