2,22 CLASSICS OF MODERN SCIENCE 

 currents been observed except in conductors. How was Maxwell to 

 reconcile his audacious hypothesis with a fact so well established as 

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

 rents produce manifest efifects, while 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. Max- 

 well's idea will best be understood by a comparison. 



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

 the spring is bended. So, if we can only dispose of a finite force, a 

 moment will come when the motion will cease, equilibrium being 

 reached. Finally, when the force ceases the spring will in flying back 

 restore the whole of the energy which has been expended in bend- 

 ing it. 



Suppose, on the other hand, that we wish to displace a body plunged 

 into water. Here again a resistance will be experienced, but it will 

 not go on increasing in proportion as the body advances, supposing 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 will the body in the least tend to return, nor can any por- 

 tion 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 Max- 

 well's idea by saying that dielectrics offer an elastic resistance, con- 

 ductors a viscous resistance, to the movements of electricity. Hence, 

 there are two kinds of currents ; currents of displacement which trav- 

 erse dielectrics and ordinary currents of conduction which circu- 

 late in conductors. 



Currents of the first kind, having to overcome an elastic resistance 

 which continually increases, naturally can last but a very short time, 

 since a state of equilibrium will quickly be reached. 



Currents of conduction, on the other hand, having only a viscous 

 resistance to overcome, must continue so long as there is any electro- 

 motive force. 



Let us return to the simile used by M. Cornu in his notice in the 

 Annuaire du Bureau des Longitudes for 1893. Suppose we have in 

 a reservoir water under pressure. Lead a tube plumb downward in- 



