﻿1124 Mr. S. J. Barnett on Electric Fields due to the 



Experiments made by the author * in 1918 with a screened 

 condenser placed symmetrically between two much larger 

 parallel magnets in motion like those above, are consistent 

 with the view that the field is polar, as required by the theory. 

 It is immaterial whether the intensity is calculated from 

 [Bu], as was done by the author, or whether it is calculated 

 from — V(Av); or we may consider that the effect of the 

 polarization is exactly neutralized by that of the equal and 

 opposite polarization due to the charges induced on the parts 

 of the conductor adjacent to the individual magnetons, and 

 that the net electric field remaining is due to the electric 



displacement produced by the motional intensity - [vB]f. 



c 



§13. Maxwell's theorem <£=— (At;) cannot in general be 



applied immediately to the case of an electromagnetic system 

 forming a solid or surface of* revolution about an axis of 

 symmetry and in steady rotation about this axis J, although 

 it may be applied to each element of which the system is 

 composed and which has its own linear velocity and vector 

 potential. 



The electric field surrounding such a body, rotating either 

 in a neutral region or in an impressed magnetic field directed 

 along the axis of rotation or symmetrical about this axis, can 

 be determined at once from equations (1) and (4). In this 



case -^— =0, so that the field is purely polar, derived from 



the potential i/r, which can be calculated from the motional 



intensity - [uB], and is due to the charges produced in and 



on the rotating electromagnetic system. 



* S. J. Barnett, Phys. Rev. xii, p. 95 (1918) ; xv. p. 527 (1920) ; xix. 

 p. 280(1922). 



f In connexion with this experiment Swann, I. c. ante, has stated that 

 Maxwell's equation (1) cannot he applied to the case of rectilinear motion 

 to show that the held is polar because (he states) in this case the vector 

 potential is not independent of the time. This is clearly an error. Several 

 examples of the contrary are given above, in addition to this particular 

 case. The theory given by Swann is quite unnecessarily complex. 



\ The theorem was derived for the general case involving rotation, but 

 "its application to the case of a symmetrical system in rotation about the 

 axis of symmetry involves the assumption that the tubes of induction 

 rotate with the system, which is inconsistent with Maxwell's general 

 theory. See also § 16. 



