132 LIGHT AND ELECTRICITY. 



considered as a eiirreiit round a closed circuit. It went, tliey tliouglit, 

 from one armature tlirougii tlie wire, the battery, the other wire, to the 

 other armature, and there it stopped. Maxwell, on the contrary, sup- 

 posed that in the form of a current of disi)lacement it ])asses through 

 the nonconducting plate of the condenser, and that i)recisely what 

 brings it to cessation is the opposite electrojuotive force set up by the 

 displacement of electricity in this dielectric. 



Currents become sensible in three ways — by their heating effects, l)y 

 their actions on other currents and on magnets, and by the induced 

 currents to which they give rise. We have seen why currents of con- 

 duction develop heat and why currents of displa<;ement do not. But 

 Maxwell's hypothetical currents ought at any rate to ])roduce electro- 

 magnetic and inductive effects. Why do these effects not appear? 

 The answer is, that it is because a current of displacement can not last 

 long enough. That is to say, they can not last long in one direction. 

 Consequently in a dielectric no current can long exist without alterna- 

 tion. But the effects ought to and will become observable if the current 

 is continually reversed at sutticiently short intervals. 



THE NATURE OE LItlHT. 



Such, according to Maxwell, is the origin of light. A luminiferous 

 wave is a series of alternating currents produced in dielectrics, in air, 

 or even in the interi)]anetary void, and reversed in direction a million 

 of millions of times per second. The enormous induction due to these 

 frequent alternations sets up other currents in tiie neighboring i)arts 

 of the dielectric, and so the waves are propagated. 



Calculation shows that the velocity of propagation would be equal 

 to t.lie ratio of the units, which we know is the velocity of ligiit. 



Those alternative currents are a sort of electrical oscillation. Are 

 they longitudinal, like those of sound, or are they transversal, like those 

 of Fresnel's ether? In the (-ase of sound the air undergoes alteina- 

 tive condensations and rarefactions. The ether of Fresnel, on the other 

 hand, behaves as if it were composed of incompressible layers capable 

 only of sli])piiig over one another. AVeie these currents in open patlis, 

 the electricity carried from one eud to the other would become accu- 

 mulated at one extremity. It would thus be condensed and rarefied 

 like air, and its vibrations would be longitudinal. But Maxwell only 

 admits currents in closed circuits; accumulation is im|»ossible, and 

 electricity behaves like the incompressible ether of Fresnel, with its 

 transversal vibrations. 



EXPERIMENTAL VERIFICATION. 



We thus obtain all the results of the theory of waves. Yet this 

 was not enough to decide the i)hysicists to adopt the ideas of Max- 

 well. It was a seductive hypothesis; but physicists consider hyjiotlie 

 Bes which lead to no distinct observational conse(]ueuces as beyond 



