'2-lS Prof. E. P. Adams on some Electromagnetic 



the average since the electromagnetic force has not had 

 time to act on the electron. So B = 0. At the end of a free 

 path £=±. Thus the average tangential velocity acquired 

 by an electron during its free path is 



~d0 = -Re 2 V-a 

 tit 4-7ii 2 r 



The whole circular flowing is : 



4:111 



T r'oEN^T-W^ HeTC, r 2 



This current changes in sign with the radial current C. 

 Since e is negative, the current I is positive if C is directed 

 outwards from the centre and negative if directed inwards. 

 In the former case the circular current in the disk has the 

 same direction as the magnetizing current, and in the latter 

 case it has the opposite sign. 



This circular current was measured by Professor Corbino 

 in a bismuth disk by determining the current induced in a 

 concentric coil of wire outside the disk when the radial 

 current was made or broken. The direction of the current 

 was found to agree with that given by this viev T of the effect. 

 In order to obtain a uniform radial current a copper wire 

 which passed through a hole in one pole-piece of the electro- 

 magnet was fastened to the centre of the disk. A copper 

 ring was joined to the periphery of the disk and to this ring 

 a circular copper plate was joined parallel to the disk. A 

 copper tube enclosing and insulated from the central wire 

 was joined to the centre of the copper plate. This method 

 made it certain that the current introduced into the disk 

 was wholly radial. Nothing is said in his paper of making 

 a radial slit in the circular copper plate. Unless this w 7 ere 

 done, the effect of the circular current in the bismuth plate 

 would be paitly balanced by the effect of the opposite 

 current in the copper plate. He states that when the 

 bismuth plate w*as replaced by a copper plate no effect was 

 observed. This is just the result to be expected if no radial 

 slit to prevent a circular current were made. 



If we assume, however, that the whole effect obtained was 

 the result of the circular current in the bismuth plate, this 

 experiment gives a simple method of calculating the free 

 time of an electron in bismuth. For a radial current = 15 

 amperes in a magnetic field of 3700, the circular current I 

 was found to be 0'26 ampere. The radius r 2 of the disk 

 was 3*25 cm. The radius r 2 of the copper wire is not given, 



