of Electric Induction and Conduction. 357 



The slippery buttons 3, 5, and 6 will again gradually slide 

 back almost to their mean position, the tension which their elas- 

 tics exerted towards the right will be relieved, and we shall pre- 

 sently have an unbalanced tension in the same direction as at 

 first: this is the " residual charge;" and the state of things is re- 

 presented in IIII, fig. 2. On unscrewing S a second motion of 

 the cord will take place, and the buttons will take up a fresh posi- 

 sition of temporary equilibrium. The numbers annexed to the 

 four cases I, II, III, and IIII indicate the electromotive force, or 

 the stress on the cord, in each case. Thus when first charged it 

 is 24 ; after standing some time it falls (say) to 17 ; immediately 

 after the first discharge it is of course ; and after again waiting- 

 some time it rises to 3 in the same direction as before, again to 

 fall to zero when the second discharge occurs. Still, however, 

 the jar is not wholly discharged, because the slippery buttons 

 will again have been displaced beyond their mean position and 

 will slide back again ; so it will take an infinite number of mo- 

 mentary contacts completely to discharge the jar, unless indeed 

 every stratum allows a little slipping to go on. 



This is Professor Clerk Maxwell's theory of a composite 

 dielectric, which he explains in art. 328 of his ' Electricity and 

 Magnetism,' and which he also illustrates mechanically in 

 art. 334. 



§ 5. If any button is perfectly smooth, it does not get dis- 

 placed at all, and it therefore takes no part in the action. It 

 would represent a film of gold leaf lying inside the glass of 

 the jar parallel with the coatings ; and it would become oppo- 

 sitely charged with electricity on its two sides — negative on 

 the side facing the positively charged coating, and positive on 

 the other ; for the other buttons would evidently move closer 

 up to it on one side, and away from it on the other. 



This, together with the preceding, really illustrates the whole 

 of the phenomena ordinarily spoken of as charge in conductors 

 according to the views of Faraday and Maxwell ; for to elec- 

 trify any conductor by induction, you place it inside a polar-, 

 ized dielectric (usually the air),' and the opposite electricities 

 appear on its surface exactly as in the above piece of gold leaf; 

 while to " charge a conductor " with one kind of electricity 

 you use the conductor to coat one face of a polarized dielectric, 

 which is done by judiciously relieving its state of strain on 

 one side only of the conductor you wish to charge. Conduc- 

 tors then are to be looked upon statically as mere interruptors 

 in the continuity of a dielectric medium ; and if the medium 

 is polarized, charge appears at the surface of discontinuity. 

 Regarded in this way, the analogy between electrification and 

 magnetization is very close ; and we can no more get positive 



