Page:Aether and Matter, 1900.djvu/57

CHAP. II Electrodynamic view of the Aether.

15. The astronomical aberration of light is one of the small group of phenomena in which the reactions between matter and aether depend sensibly on the state of motion of the matter. Disturbances originated in the aether are equalized and smoothed out with such great speed, that the aether-field around a body, which is moving with any attainable velocity, is practically at each instant in the same equilibrium condition as if the body were at rest: it is therefore only in the case of very rapidly alternating phenomena such as radiation that there is any practical occasion to pass beyond a mere theory of covection of aethereal effect along with the molecules of the matter. It is owing to this circumstance that the electrodynamic theories of Ampere and Weber represented so well the whole range of phenomena then open to experiment, even to the extent of giving in Kirchhoff's hands the correct velocity (that of radiation) for the transmission of electric waves of very high frequency guided along a wire: and that, as regards the deeper questions of propagation of electric effect in time, theory has been, chiefly in Maxwell's hands, uniformly so far in advance of the means of verification.

The logical validity of the older electrodynamics was confined to systems of uniform currents streaming round closed paths: and all investigations purporting to deduce from experimental data expressions for the electromotive forces induced in open circuits, or for mechanical forces acting on separate portions of circuits carrying currents, were necessarily illusory from the fact that such portions were practically unknown as separate independent entities. The new departure instituted by Maxwell came, when expressed mathematically, to a statement that dynamically all electric discharges are effectively of the nature and possess the properties of systems of closed currents, being completed when necessary by so-called displacement-currents in free space and in dielectric media; in fact that the consideration of the electrodynamics of unclosed circuits never arises. That theory, as left by its author, works out by adapting the established Amperean theory of closed