﻿1126 Mr. S. J. Barnett on Electric Fields due to the 



are all in accord with Jochmann and Larmor's theory of 

 §13. That is, when the condenser remains at rest, the 

 charge is zero, because the field is polar, with external 

 charges ; when in motion, the charge is due to the intensity 



- [(o>r)B] in the conductors. The application of Maxwell's 



equations (1) and (4), as used by Larmor, to these experiments 

 was discussed in detail by Pegram in 1917. In many 

 experiments from Faraday to the present time the potential 

 difference between different circles of latitude on the 

 rotating body has been measured with a galvanometer or an 

 electrometer; recently the charge on the sphere of § 13, 

 with axis earthed, and surrounded by a concentric sphere, 

 has been measured by Swann *. All the results are in 

 agreement with the theory of Jochmann, Maxwell, and 

 Larmor. 



§16. In the case of a symmetrical electrical circuit 

 rotating about its axis of symmetry, the rotation produces no 

 rotating effect on the tubes of induction of the magnetic 

 field, as indicated in § 13 and shown very simply by Pegram. 

 Applying this result to the Ampereian vortices of a magnet, 

 it was Pegram's idea that when the magnet rotated, each 

 vortex carried its lines of induction in translation, but 

 that the lines of induction of the vortex did not share the 

 rotating motion of the magnet j\ This idea has recently heen 

 •discussed in detail by Swann (I. c. ante), but it is not new. 

 It is only in this sense that the " moving line " theory of 

 unipolar induction is true. 



We have already seen that so far as the net external result 

 is concerned, the effect of the polarization due to the trans- 

 latory motion of the magnetons may be considered neutralized 

 by that of induced charges. The intensity gat any point P 

 of any plane through the axis of rotation due to the motion 

 of an element of the magnet with magnet moment m and 

 velocity v= [cor] is 



e=-V(av) + (vV)a, 



* W. F. G. Swann, Phys. Rev. xix. p. 38 (1922). 



t This is not discussed in detail by Pegram, but was clearly in his 

 mind and is an immediate corollar}' of the case of the simple coil. After 

 Pegram's paper was read at Cleveland, but in less complete form than 

 that of the printed article, 1 stated to him that in the case of a magnet 

 I thought the fundamental way to consider the matter was to assume 

 that each electron orbit carried its lines of induction with it. He 

 immediately assented, and remarked in addition that the rotatory part of 

 the motion was not to be considered. 



