AND MODERN PHYSICS. 197 



If the initial disturbance bo periodic, periodic 

 waves of electric force will travel out from the centre, 

 just as waves of sound travel out from a bell, or waves 

 of light from a candle flame. A wire carrying an 

 alternating current may be such a source of periodic 

 disturbance, and from the wire waves travel outwards 

 into space. 



Now, it is known that in a sound wave the dis- 

 placements of the air particles take place in the 

 direction in which the wave is travelling; they lie 

 at right angles to the wave front, and are spoken of 

 as longitudinal. In light waves, on the other hand, 

 the displacements are, as Fresnel proved, in the wave 

 front, at right angles, that is, to the direction of 

 propagation ; they are transverse. 



Theory shows that in general both these waves 

 may exist in an elastic solid body, and that they 

 travel with different velocities. Of which nature are 

 the waves of electric displacement in a dielectric ? 

 It can bo shewn to follow as a necessary consequence 

 of Maxwell's views as to the closed character of all 

 electric currents, that waves of electric displacement^ 

 are transverse. Electric vibrations, like those of light,, 

 are in the wave front and at right angles to the direction 

 of propagation ; they depend on the rigidity or quasiJ 

 rigidity of the medium through which they travel,! 

 not on its resistance to compression. 



Again, an electric current, whether due to varia-l 

 tion of displacement in a dielectric or to conduction 

 in a conductor, is accompanied by magnetic force, 

 A wave of periodic electric displacement, then, willj 

 bo also a wave of periodic magnetic force travelling at) 



