Page:EB1911 - Volume 01.djvu/334

 where, when magnetic quality is inoperative, the magnetic induction (a,b,c) is identical with the magnetic force.

These equations determine all the phenomena. They take this simple form, however, only when the movement of the matter is one of translation. If v varies with respect to locality, or if there is a velocity of convection (p,q,r) variable with respect to direction and position, and analytical expression of the relation (ii) assumes a more complex form; we thus derive the most general equations of electrodynamic propagation for matter treated as continuous, anyhow distributed and moving in any manner.

For the simplest case of polarized waves travelling parallel to the axis of x, with the magnetic oscillation along z and the electric oscillation Q along y, all the quantities are functions of x and t alone; the total current is along y and given with respect to our moving axes by

v＝$d⁄dt$ − v$d⁄dx$$Q+v⁄4^{2}$ + $d⁄dt$$K−1⁄4^{2}$Q;

also the circuital relations here reduce to

− $d⁄dx$＝4v,&emsp; $dQ⁄dx$＝−$d⁄dt$;

thus

giving, on substitution for v,

(2−v2)$d&#8202;^{2}Q⁄dx^{2}$＝K$dv⁄dt$ − 2v$d&#8202;^{2}Q⁄dx^{2}$.

For a simple wave-train, Q varies as sin m(x−Vt), leading on substitution to the velocity of propagation V relative to the moving material, by means of the equation KV2 + 2 vV＝2−v2; this gives, to the first order of v/, V＝/K$d&#8202;^{2}Q⁄dt&#8202;^{2}$ − v/K, which is in accordance with Fresnel’s law. Trains of waves nearly but not quite homogeneous as regards wave-length will as usual be propagated as wave-groups travelling with the slightly different velocity d(V-1)/d−1, the value of K occurring in V being a function of determined by the law of optical dispersion of the medium.

For purposes of theoretical discussions relating to moving radiators and reflectors, it is important to remember that the dynamics of all this theory of electrons involves the neglect of terms of the order (v/)2, not merely in the value of K but throughout.

Recent Experimental Developments.—The modification of the spectrum of a radiating gas by a magnetic field, such as would result from the hypothesis that the radiators are the system of revolving or oscillating electrons in the molecule, was detected by P. Zeeman in 1896, and worked up, in conjunction with H. A. Lorentz, on the general lines suggested by the electron-theory of molecular constitution. While it cannot be said that the full significance of this very definite phenomenon, consisting of the splitting of the spectral line into a number of polarized components, has yet been made out, a wide field of correlation with optical theory, especially in the neighbourhood of absorption bands, has been developed by Zeeman himself, by A. H. Becquerel, by D. Macaluso and O. M. Corbino, and by other workers.

The most fundamental experimental confirmation that the theory of the aether has received on the optical side in recent years has been the verification of Maxwell’s proposition that radiation exerts mechanical force on a material system, on which it falls, which may be represented in all cases as the resultant of pressures operating along the rays, and of intensity equal at each point of free space to the density of radiant energy. A high vacuum is needed for the detection of the minute forces here concerned; but just in that case the indirect radiometer-effect of the heating of the residual gas masks the effect. P. N. Lebedew in 1900 succeeded, by operating on metallic vanes so thin that the exposed and averted faces were practically at the same temperature, in satisfactorily verifying the relation for metals; and very soon after, E. F. Nichols and G. F. Hull published accounts of an exact and extensive research, in which the principle had been fully and precisely confirmed as regards both transparent and opaque bodies. The experiment of J. H. Poynting may also be mentioned, in which the tangential component of the thrust of obliquely incident radiation is separately put in evidence, by the torsion produced in an arrangement which is not sensitive to the normal component or to the radiometer-pressure of the residual gas. (See .)

Next to these researches on the pressure of radiation, which, by forming the mechanical link between radiation and matter, are fundamental for the thermodynamics of radiant energy, the most striking recent result has been the discovery of H. Rubens and E. Hagen that for dark heat rays of only about ten times the wave-length of luminous radiation, the properties of metals are determined by their electric resistance alone, which then masks all resonance due to periods of free vibration of the molecules; and, moreover, that the resistance for such alternations is practically the same as the ohmic resistance for ordinary steady currents. They found that the absorbing powers of the metals, and therefore, by the principle of exchanges, their radiating powers also, are proportional to the square roots of their electric conductivities. Maxwell had himself, at an early stage of his theory, tested the absorbing power of gold-leaf for light, and found that the effective conductivity for luminous vibrations must be very much greater than its steady ohmic value; it is, in fact, there a case of incipient conductivity, which is continually being undone on account of the rapid alternation of force before it is fully established. That, however, complete conduction should arrive with alternations only ten times slower than light was an unexpected and remarkable fact, which verifies the presumption that the process of conduction is one in which the dynamic activities of the molecules do not come into play. The corollary, that the electric resistance of a metal can be determined in absolute units by experiments on the reflexion of heat-rays from its surface, is a striking illustration of the unification of the various branches of physical science, which has come in the train of the development of the theory of the aether. (See .)

Finally, reference should be made to the phenomena of radioactivity, whether excited by the electric discharge in vacuum tubes, foreshadowed in part by Sir Wm. Crookes and G. G. Stokes, and later by A. Schuster and others, but first fully developed with astonishing results including the experimental discovery of the free electron by J. J. Thomson, or the correlated phenomena occurring spontaneously in radio-active bodies as discovered by H. Becquerel and by M. and Mme Curie, and investigated by them and by E. Rutherford and others. These results constitute a far-reaching development of the modern or electrodynamic theory of the aether, of which the issue can hardly yet be foreseen.

AETHICUS (＝) ISTER, “the philosopher of Istria,” the supposed but unknown author of a description of the world written in Greek. An abridgment, under the title of Cosmographia Ethici, written in barbarous Latin, and wrongly described as the work of St Jerome, probably belongs to the 7th century. After a discussion of the creation of the world and a description of the earth, an account of the wonderful journeys of Aethicus is given, with digressions on various subjects, such as Alexander the Great and the kings of Rome, full of obscure and fabulous details.

The name Aethicus is also attached to another geographical treatise probably dating from the 6th Century, a reproduction,