1890.] on Electromagnetic Badiation. 81 



frame was turned so that tlie wires were parallel to the magnetic 

 force. It behaved just like a tourmaline to polarised light. It is of 

 great interest to verify experimentally Maxwell's theory that the 

 plane of polarisation of light is the plane of the magnetic force. This 

 has been done by Mr. Trouton, who has shown that these radiations 

 are not reflected at the polarising angle by the surface of a non- 

 conductor, when the plane of the magnetic force in the incident 

 vibration is perpendicular to the plane of incidence, but the radiations 

 are reflected at all angles of incidence when the plane of the mag- 

 netic force coincides with the plane of incidence. Thus the long- 

 standing dispute as to the direction of vibration of light in a polarised 

 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 electrifica- 

 tion 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 oscil- 

 lator, 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 ' Annalen ' for January 1889, 

 and in 'Nature 'for March 7th, 1889, 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 bo 

 likened to a diverging whirl ring, was generated, not at the oscillator, 

 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 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 electromagnetic 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 oscillator 

 which is wholly radiated away during each vibration. It follows 

 that in ordinary electromagnetic alternating currents at from 100 to 

 200 alternations per second, it is only the energy which is some 3000 

 miles away which is lost. If an electromagnetic wave, having mag- 

 netic force comparable to that near an ordinary electro-magnet, were 

 producible, the power of the radiation would be stupendous. If we 

 consider the possible radiating power of an atom by calculating it upon 



Vol. XIIL (No. 84.) g 



