426 The Theory of Aether and Electrons in the 



The term D in S evidently represents the displacement- 

 current of Maxwell ; and the term curl [P . w] will be 

 recognized as a modified form of the term curl [D . w], which 

 was first introduced into the equations by Hertz.* It will 

 be remembered that Hertz supposed this term to repre- 

 sent the generation of a magnetic force within a dielectric 

 which is in motion in an electric field ; and that Heaviside,f by 

 adducing considerations relative to the energy, showed that the 

 term ought to be regarded as part of the total current, and 

 inferred from its existence that a dielectric which moves in an 

 electric field is the seat of an electric current, which produces 

 a magnetic field in the surrounding space. The modification 

 introduced by Lorentz consisted in replacing D by P in the 

 vector-product ; this implied that the moving dielectric does 

 not carry along the aethereal displacement, which is represented 

 by the term E/4?rc 2 in D, but only carries along the charges 

 which exist at opposite ends of the molecules of the ponderable 

 dielectric, and which are represented by the term P. The part 

 of the total current represented by the term curl [P . w] is 

 generally called the current of dielectric convection. 



That a magnetic field is produced when an uncharged 

 dielectric is in motion at right angles to the lines of force of a 

 constant electrostatic field had been shown experimentally in 

 1888 by Rontgen.J His experiment consisted in rotating a 

 dielectric disk between the plates of a condenser ; a magnetic 

 field was produced, equivalent to that which would be produced 

 by the rotation of the " fictitious charges " on the two faces of 

 the dielectric, i.e., charges which bear the same relation to 

 the dielectric polarization that Poisson's equivalent surface- 

 density of magnetism bears to magnetic polarization. If U 

 denote the difference of potential between the opposite coatings 

 of the condenser, and * the specific inductive capacity of the 

 dielectric, the surf ace -density of electric charge on the coatings 



* Cf. p. 366. t Cf. p. 367. 



I Ann. d. Phys. xxxv (1888), p. 264 ; xl (1890), p. 93. 

 Cf. p. 64. 



