Electricity for public schools and colleges . that allsolid dielectrics become strained (or distorted) to an appreciabledistance from the conducting surfaces ; and that then the straincannot at once be removed, or the condensor cannot be at oncedischarged. Some of the stress remains and must be allowed tobe relieved gradually. This gives rise to the residual chargesdiscussed below. (i.) Leyden jar with moveable coatings.—A is a Le)den jarso constructed as to be separable into three portions ; B the glass jar, D theinside coating, and C the outside coating. The jar is put together
Electricity for public schools and colleges . that allsolid dielectrics become strained (or distorted) to an appreciabledistance from the conducting surfaces ; and that then the straincannot at once be removed, or the condensor cannot be at oncedischarged. Some of the stress remains and must be allowed tobe relieved gradually. This gives rise to the residual chargesdiscussed below. (i.) Leyden jar with moveable coatings.—A is a Le)den jarso constructed as to be separable into three portions ; B the glass jar, D theinside coating, and C the outside coating. The jar is put together, charged as usual, and then placed on an insulatingstand. The inside is then removed, a small charge coming away with it, and CH. XT. ELEMENTARY DISCUSSION OF CONDENSERS 91 a small charge being set free on the outside coating. Then the glass islifted out of its outer coating and set down on the table, and finally the outercoating may be handled and removed. In all this we notice but very slightdischarges on touching D and C Fig. On carefully putting the whole together again (on the insulating stand), adischarging rod (fig. ii.) will show us that the whole charge of the condenser hasbeen (approximately) unaffected by the above process. Hence, since the coatingsB and C were separately put to earth by handling, the charge of the condensermust have resided in the glass. We could not, how-ever, discharge the glass by itself; it was necessary tohave the metal coatings as distributors. (ii.) Residual charge.—The penetration of thecharge, from the surfaces inwards, is shown by thefollowing. If a Leyden jar be discharged and then left fora time, a second small discharge can be obtained,and so on. That the interior of the glass is actually strainedor distorted is shown by the fact that its optical pro-perties, when it is under the electrical stress, undergoan alteration similar to or identical with that producedby mechanical stress. When the electrical stress is exces
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