180 Dr. E. H. Kennard on 



radical difference to exist between induced and permanent 

 magnetism, for which there is no other evidence ; it even 

 requires that the remanent magnetism shall not acquire the 

 inducing properties of permanent magnetism until the mag- 

 netizing force is removed, for the remanence in the bar 

 employed amounted to over 25 per cent. This experiment 

 serves, therefore, to throw grave doubt upon the moving-line 

 theory; but beyond that its theoretical significance is limited 

 by the fact that the rotating magnet was necessarily enclosed 

 in a second metallic screen of small diameter, and this might 

 conceivably act so as to cut off the effect. 



Fisr. l. 



--^--i 



A more instructive but more difficult arrangement consists 

 of a cylindrical condenser inside a coaxial solenoid (fig. 1), 

 both capable of rotation about their longitudinal axis ; the 

 condenser is connected to an electrometer as shown. Let 

 the condenser be short-circuited and set in rotation with the 

 solenoid energized. Then according to either Lorentz's or 

 the moving-line theory (but not according to Hertz's) the 

 two cylinders of the condenser should come to different 

 potentials, and the inner one at least should be charged. 

 According to Lorentz's theory rotation of the solenoid should 

 have no effect upon this charge. According to the moving- 

 line theory rotation of the solenoid alone should charge the 

 condenser in the same manner, while the condenser should 

 remain uncharged when condenser and solenoid rotate 

 together with no relative motion between them. 



Barnett constructed an apparatus of this sort*, and 

 showed that rotation of the solenoid alone developed no 

 charge upon the condenser. Later, Fehrle f reported con- 

 firmatory observations with a rather different arrangement, 

 but his work is marred by several results which must cer- 

 tainly be wrong : one of his results even contradicts Faraday's 

 law for closed circuits ! 



Under these circumstances, while there appeared to be little 

 doubt of the correctness of Lorentz's theory, yet the matter 



* S. J. Barnett, Phys. Rev. Nov. 1912, p. 323. 

 t Fehrle, Ann. xlii. p. 1109 (1913). 



