INDUCTION APPARATUS — EFFECTS OF CONDUCTION IN IT. 23 



which are bodily very bad conductors of electricity, and indeed almost perfect insu- 

 lators, gave very little of this return charge. 



1243. I tried the same experiment having air only in the inductive apparatus. 

 After a continued high charge for some time I could obtain a little effect of return 

 action, but it was ultimately traced to the shell-lac of the stem. 



1244. I sought to produce something like this state with one electric power and 

 without induction; for upon the theory of an electric fluid or fluids, that did not seem 

 impossible, and then I should have obtained an absolute charge (1169. 11 77-)> or 

 something equivalent to it. In this I could not succeed. I excited the outside of a 

 cylinder of shell-lac very highly for some time, and then quickly discharging it 

 (1203.), waited and watched whether any return charge would appear, but such 

 was not the case. This is another fact in favour of the inseparability of the two elec- 

 tric forces, and another argument for the view that induction and its concomitant 

 phenomena depend upon a polarity of the particles of matter. 



1245. Although inclined at first to refer these effects to a peculiar masked con- 

 dition of a certain portion of the forces, I think I have since correctly traced them to 

 known principles of electrical action. The effects appear to be due to an actual pe- 

 netration of the charge to some distance within the electric, at each of its two sur- 

 faces, by what we call conduction ; so that, to use the ordinary phrase, the electric 

 forces sustaining the induction are not upon the metallic surfaces only, but upon and 

 within the dielectric also, extending to a greater or smaller depth from the metal 

 linings. Let c (fig. 10.) be the section of a plate of any dielectric, a and 6 being the 

 metallic coatings ; let b be uninsulated, and a be charged positively ; after ten or fif- 

 teen minutes, if a and b be discharged, insulated, and immediately examined, no elec- 

 tricity will appear in them ; but in a short time, upon a second examination, they will 

 appear charged in the same way, though not to the same degree, as they were at first. 

 Now suppose that a portion of the positive force has, under the coercing influence of 

 all the forces concerned, penetrated the dielectric and taken up its place at the line/?, 

 a corresponding portion of the negative force having also assumed its position at the 

 line n ; that in fact the electric at these two parts has become charged positive and 

 negative ; then it is clear that the induction of these two forces w^ll be much greater 

 one towards the other, and less in an external direction, now that they are at the 

 small distance n p from each other, than when they were at the larger interval a b. 

 Then let a and b be discharged ; the discharge destroys or neutralizes all external 

 induction, and the coatings are therefore found by the carrier ball unelectrified ; but 

 it also removes almost the whole of the forces by which the electric charge was driven 

 into the dielectric, and though probably a part goes forward in its passage and termi- 

 nates in what we call discharge, the greater portion returns on its course to the sur- 

 faces of c, and consequently to the conductors a and b, and constitutes the recharge 

 observed. 



1246. The following is the experiment on which I rest for the truth of this view. 



