FIRST RESEARCHES IN PHYSICS 49 



resistance of conductors can be thought of as a viscous 

 resistance. 



What now of that of non-conductors ? This term is also 

 relative, since these differ among themselves ; and hence at 

 first they were thought of as but extremely bad conductors. 

 But here Maxwell had a fresh idea, that of their non-con- 

 ductivity as by no means comparable to an exaggerated 

 viscosity, but of a contrasted nature, like the resistance 

 offered by elastic springs, which do not waste the kinetic 

 energy expended on them into friction and heat, but store 

 it as potential, in their coils, as far as the structure of these 

 allows ; and then give it out anew, as the pressure upon 

 them is reduced or withdrawn. Thus while the familiar 

 current of conduction along a wire goes on as Jong as its 

 electro-motive force continues, the currents of displacement, 

 which Maxwell's speculative eye discovered in the non- 

 conducting body (answering to the metal springs of his 

 mechanical image above), can but have a short duration, 

 for their distortion soon comes to an equilibrium, of electro- 

 static energy. Now imagine the coiled springs to break, 

 or burst free ; there is a sudden and complete discharge of 

 their energy a process obviously sharply contrasted with 

 that dissipation into heat which we find in conductors 

 carrying a current. 



Thus Maxwell escaped from the old and merely negative 

 view of the non-conductor as a passive obstacle ; and saw 

 it thrilling with its own internal currents of displacement, 

 like the rapid oscillations of a mass of springs. But ordinary 

 currents manifest themselves (i) by being wasted into heat 

 by the resistance due to the imperfections of the conductor, 



(2) by their action on the magnet, so conveniently shown 

 by introducing a galvanometer into the circuit, and also 



(3) by their induction of currents in conductors in their 

 neighbourhood. So if Maxwell's hypothetical currents in 

 non-conductors really exist, they must have these pro- 

 perties ; but so rapid are their oscillations, and so brief 

 is their duration,, that no ordinary experiment can detect 



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