DEPARTMENT OF TERRESTRIAL MAGNETISM. 285 



The angular momentum of the elementary magnet. S. J. Barnett. Bull. National 

 Research Council, vol. 3, 235-250 (August 1922). 



This is a general treatment of the role of angular momentum in magnetism. 

 It describes briefly Maxwell's attempt to detect by direct experiment any 

 angular momentum of electricity flowing in a coil of wire, or any angular 

 momentum associated with the Amperian currents in magentized iron; the 

 experiments of S. J. and L. J. H. Barnett on the magnetization of ferro- 

 magnetic substances by rotation, first successful with iron in 1914 (nearly a 

 quarter of a century after an attempt by John Perry) and since extended to 

 other ferromagnetic materials; the attempt of Richardson in 1907 to rotate 

 iron by magnetizing it, and the later and successful experiments in this field 

 from 1915 to 1919 by Einstein and de Hass, together and separately, on iron, 

 by J. Q. Stewart and by E. Beck on iron and nickel, and by G. Arvidsson on 

 iron. The paper also gives the general theory of all these experiments, and 

 discusses the bearing of the experimental results on the theory of the magneton 

 or elementary magnet. In the theory of magnetization by rotation special 

 attention is devoted to the cases of elastic magnetization and completely 

 inelastic magnetization. 



Note on the formula for the electric polarization of an insulator moving in a magnetic field. 

 S. J. Barnett. 

 From the fundamental equations of electromagnetic theory as developed 

 by Cohn and by Minkowski, 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, ft its permeability, I its intens- 

 ity of magnetization, B the magnetic induction, and v the velocity, the formula 

 for the polarization, in the approximate form obtained by Abraham, is 



This polarization consists of two distinct parts; one, Pi, produced by the 



motional intensity — [vB] 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 ether is at rest, so that only the 

 electrical fraction (K—l)/K of the insulator is in motion. Hence. 



ft-ti i [vB] 



47T C 



This result has been fully confirmed by experiments made on air in 1901 

 by Blondlot, on ebonite in 1904 by H. A. Wilson, and on rosin, sulphur, and 

 ebonite in the interval 1902-1908 by myself. 



The polarization P 2 is given by the expression 



This follows from a simple theorem of Maxwell's: The motion with velocity 

 v of an electromagnetic system with vector potential A produces an electric 



field the potential of whose polar part is tp = - (Av). An immediate conse- 



quence of this is that the electric moment of the distribution produced by the 



motion is equal to - [vm], where m is the magnetic moment. Applying this 



to the unit volume, we obtain the above expression for P 2 . 



Adding together the expressions for Pi and P 2 , we obtain the expression for 

 P given above as hitherto derived only on the basis of the Cohn-Minkowski 

 equations. 



