130 LIGHT AND ELECTRICITY. 



DISPLACEMENT CURRENTS. 



The reader is aware that solid bodies are divided into two classes, 

 conductors tbrongli which electricity can move in the form of a galvanic 

 current, and nonconductors, or dielectrics. The electricians of former 

 days regarded dielectrics as quite inert, having no part to play but that 

 of obstinately refusing passage to electricity. Had that been so, any 

 one nonconductor might be replaced by any other without making any 

 ditlerence in the phenomena; but Faraday found that that was not the 

 case. Two condensers of the same form and dimensions put into con- 

 nection with the same source of electricity do not take the same charge, 

 though the thickness of the isolating plate be the same, unless the mat- 

 ter of tliat plate be chemically the same. Now Clerk Maxwell had too 

 deeply studied the researches of Faraday not to comi)rehend the import 

 ance of dielectrics and the imjierati ve obligation to recognize their active 

 l)art. 



r>esides, if light is but an electric phenomenon, when it traverses a 

 thickness of glass electrical events must take place in tluit glass. And 

 what can be the nature of those events? Maxwell boldly answers, 

 they are, and must be, currents. 



All the experience of his day seemed to contradict this. Never had 

 currents been observed except in conductors. How Avas Maxwell to 

 reconcile his audacious hypothesis Avith a fact so well established as 

 that? Why is it that under certain circumstances those supposed cur- 

 rents produce manifest effects, Avhile under ordinary conditions they can 

 not be observed at all. 



The answer was that dielectrics resist the passage of electricity not 

 so much more than conductors do, but in a different manner. Maxwell's 

 idea Avill best bo understood by a comi)arison. 



If we bend a spring, we meet a resistance which increases the more 

 the spring is bendeil. So, if avc can only dispose of a finite force, a 

 moment will come when the motion will cease, eciuilibrium being 

 reached. Finally, when the force ceases the spring Avill in Hying back 

 restore the whole of the energy which has been expended in bending it. 



Suppose, ou the other hand, that we wish to disjdace a body plunged 

 into water. Here again a resistance Avill be experienced, but it Avill not 

 go on increasing in proportion as the body advances, sui)X)Osing it to be 

 maintained at a constant velocity. So long as the motive force acts, 

 equilibrium will never, then, be attained ; nor when the force is removed 

 Avill the body in the least tend to return, nor can any portion of the 

 energy expended be restored. It will, in fact, have been converted into 

 heat by the viscosity of the water. 



The contrast is plain; and we ought to distinguish elastic resistance 

 from viscous resistance. Using these terms, we may express Maxwell's 

 idea by saying that dielectrics otter an elastic resistance, conductors a 



