The term Ḋ in S evidently represents the displacementcurrent 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.[1] It will be remembered that Hertz supposed this term to represent the generation of a magnetic force within a dielectric which is in motion in an electric field, and that Heaviside[2] 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/4πc2 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 Röntgen.[3] 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 surfacedensity of magnetism[4] bears to magnetic polarization. If U denote the difference of potential between the opposite coatings of the condenser, and ε the specific inductive capacity of the dielectric, the surface-density of electric charge on the coatings
