﻿Motion of Constant Electromagnetic Systems. 1127 



while the intensity e' at the same point due to the corre- 

 sponding element with (equal) moment m' symmetrically 

 situated on the opposite side of the plane is evidently 



e — —\Jar — \ K vSJ)a. 



Thus at any point P the solenoidal parts of the fields' due to 

 corresponding elements cut one another out, in pairs, while 

 the polar parts of the intensities are equal and additive. The 

 polarization, due to the polar parts, being neutralized by in- 

 duction, the total gross effect of the motion of the vortices 

 vanishes. 



§ 17. From the fundamental equations of electromagnetic 

 theory as developed by Cohn *, and later hy Minkowski! , 

 and still later by Einstein and Laub J, a general expression 

 has been obtained for the polarization produced in an insulator 

 by its motion in a magnetic field. If K denotes the dielectric 

 constant of the medium, //, its permeability, I its intensity of 

 magnetization, B the magnetic induction, and v the velocity, 

 the formula for the polarization, in the approximate form 

 obtained by M. Abraham §, is 



p -£('-i)™- 



This polarization is easily shown to consist of two distinct 



parts : one, P 1? produced by the motional intensity - [uB] 



acting on the moving part of the insulator; the other, P 2 , 

 due to the motion of the magnetons. 



On the theory of Lorentz and Larmor the aether is at rest, 

 so that only the electrical fraction (K — 1)/K of the insulator 

 is in motion. Hence 



This result has been fully confirmed by experiments made on 

 air in 1901 by Blondlot||, on ebonite in 1904 by H. A. 



* E. Cohn, Ann. der Phys. vii. p. 29 (1902). 

 t H. Minkowski, Gott. Nachr. Math. Phys. El. 1908, p. 53. 

 X A. Einstein and J. Laub, Ann. der Phi,s. xxvi. p. 532 (1908). 

 § M. Abraham, 'Theorie der Elektrizitaet,' ii. §38 (1908). 

 II R. Blondlot, C. R. cxxxiii. p. 778 (1901). 



