174 



NATURE 



[June 19, 1890 



solid cannot be analogous to the ether because the latter 

 may have a constant magnetic force existing in it for any 

 length of time, while an elastic solid cannot have con- 

 tinuous rotation of its elements in one direction existing 

 within it. The most satisfactory model, with properties 

 quite analogous to those of the ether, is one consisting of 

 wheels geared with elastic bands. The wheels can rotate 

 continuously in one directionn, and their rotation is the 

 analogue of magnetic force. The elastic bands are stretched ' 

 by a difference of rotation of the wheels, and introduce 

 stresses quite analogous to electric forces. By making the 

 elastic bands of lines of governor balls, the whole model 

 may have only kinetic energy, and so represent a funda- ; 

 mental theory. Such a model can represent media differ- 

 ing in electric and magnetic inductive capacity. If the 

 elasticity of the bands be less in one region than another, 

 such a region represents a body of higher electric in- 

 ductive capacity, and waves would be propagated more 

 slowly in it. A region in which the masses of the wheels 

 was large would be one of high magnetic inductive 

 capacity. A region where the bands slipped would be a 

 conducting region. Such a model, unlike most others 

 proposed, illustrates both electric and magnetic forces and | 

 their inter-relations, and consequently light propagation. 



In the neighbourhood of an electric generator the 

 general distribution of the electric and magnetic forces is 

 easily seen. The electric lines of force must lie in planes 

 passing through the axis of the generator, while the lines ^ 

 of magnetic force lie in circles round this axis and perpen- 

 dicular to the lines of electric force. It is thus evident 

 that the wave is, at least originally, polarized. To show 

 this, the small-sized oscillators with parabolic mirrors 

 were used, and a light square frame, on which wires 

 parallel to one direction were strung, was interposed \ 

 between the mirrors. It was shown that such a system of 

 wires was opaque to the radiation when the wires were 

 parallel to the electric force, but was quite transparent 

 when the frame was turned so that the wires were parallel 

 to the magnetic force. It behaved just like a tourmaline 

 to polarized light. It is of great interest to verify experi- 

 mentally Maxwell's theory that the plane of polarization 

 of light is the plane of the magnetic force. This has been 

 done by Mr. Trouton, who has shown that these radia- 

 tions are not reflected at the polarizing angle by the 

 surface of a non-conductor, when the plane of the 

 magnetic force in the incident vibration is perpen- 

 dicular to the plane of incidence, but the radiations 

 are reflected at all angles of incidence when the plane 

 of the magnetic force coincides with the plane of 

 incidence. Thus the long-standing dispute as to the 

 direction of vibration of light in a polarized ray has 

 been at last experimentally determined. The electric and 

 magnetic forces are not simultaneous near the oscillator. 

 The electric force is greatest when the electrification is 

 greatest, and the magnetic force when the current is 

 greatest, which occurs when the electrification is zero : 

 thus the two, when near the oscillator, differ in phase by 

 a quarter of a period. In the waves, as existing far from 

 the oscillator, they are always in the same phase. It is 

 interesting to see how one gains on the other. It maybe 

 worth observing, again, that though what follows deals 

 with electric oscillators, the theory of magnetic oscillators 

 is just the same, only that the distribution of magnetic 

 and electric forces must be interchanged. Diagrams 

 drawn from Hertz's figures published in Wiedemann's 

 A finalen ior January 1889, and in Nature, vol. xxxix. 

 p. 451, and in the Philosophical Magazine for March 

 1890, were thrown on the screen in succession, and it was 

 pointed out how the electric wave, which might be likened 

 to a diverging whirl ring, was generated, not at the oscil- 

 lator, but at a point about a quarter of a wave-length on 

 each side of the oscillator, while it was explained that the 

 magnetic force wave starts from the oscillator. It thus 



NO. 1077, VOL. 42] 



appears how one gains the quarter-period on the other. 

 The outflow of the waves was exhibited by causing the 

 images to succeed one another rapidly by means of a 

 zoetrope, in which all the light is used and the succession 

 of images formed by having a separate lens for each 

 picture and rotating the beam of light so as to illuminate 

 the pictures in rapid succession. 



As the direction of flow of energy in an electro-magnetic 

 field depends on the directions of electric and magnetic 

 force, being reversed when either of these is reversed, it 

 follows that in the neighbourhood of the oscillator the 

 energy of the field alternates between the electric and 

 magnetic forms, and that it is only the energy beyond 

 about a quarter of the wave-length from the oscilUator 

 which is wholly radiated away during each vibration. It 

 follows that in ordinary electro-magnetic alternating cur- 

 rents at from 100 to 200 alternations per second, it is only 

 the energy which is some 3000 miles away which is lost. 

 If an electro-magnetic wave, having magnetic force com- 

 parable to that near an ordinary electro-magnet, were 

 producible, the power of the radiation would be stu- 

 pendous. If we consider the possible radiating power of 

 an atom by calculating it upon the hypothesis that the 

 atomic charge oscillates across the diaineter of the atom,, 

 we find that it may be millions of millions of times as great 

 as Prof Wiedemann has found to be the radiating power 

 of a sodium atom in a Bunsen burner, so that, if there is 

 reason to think that any greater oscillation might disin- 

 tegrate the atom, it is evident that we are still a long way 

 from doing so. It is to be observed that ordinary light- 

 waves are very much longer than the period of the vibra- 

 tion above referred to. l3r. Lodge has pointed out that 

 quite large oscillators in comparison to molecules — namely, 

 i about the size of the rods and cones in the retina — are of 

 the size to resound to light-waves of the length we see, 

 and so might be used to generate such waves. This 

 seems to show that the electro-magnetic structure of an 

 atom must be more compHcated than a small sphere or 

 other simple shape with an oscillating charge on it, for 

 the period of vibration of a small system can be made 

 i long by making the system complex, e.g. a small Leyden 

 jar of large capacity with a long wire wound many times- 

 round connecting its coats, could easily be constructed to 

 i produce electro-magnetic waves whose length would bear 

 1 the same proportion to the size of the jar as ordinary 

 light-waves do to an atom. The rate at which the energy 

 of a Hertzian vibrator is transferred to the ether is so 

 great that we would expect an atom to possess the great 

 radiating power it has. This shows, on the other hand, 

 how completely the vibrations of an atom must be forced 

 by the vibrations of the ether in its neighbourhood, so 

 I that atoms, being close compared with a wave-length, are, 

 in any given small space, probably in similar phases of 

 vibration. It is interesting to consider this in connection 

 j with the action of molecules in collision as to how far the 

 j forces between molecules after collision is the same as 

 I before. In the same connection the existence of intra- 

 . atomic electro-magnetic oscillations is interesting in the 

 theories of anomalous dispersion. An electro-magnetic 

 model of a prism with anomalous dispersion might be 

 constructed out of pitch, through which conductors, each 

 with the same rate of electro-magnetic oscillation, were 

 j dispersed. In theories of dispersion a dissipation of 

 1 energy is assumed, and it may be the radiation of the 

 j induced electro-magnetic vibrations. These can evidently 

 ; never be greater than the incident electro-magnetic vibra- 

 ! tion, on account of this radiation of their own energy. 

 In some theories a vibration of something much less than 

 the whole molecule is assumed, and the possibility of 

 intra-atomic electro-magnetic oscillations would acqjaunt 

 for this. Some such assumption seems also required, in 

 order to explain such secondary, if not tertiary, actions 

 as the Hall effect and the rotation of the plane of polariza- 



