dielec 



Induction Apparatus— Effects of Condtiction in it. 365 



forces sustaining the induction are not upon the metallic sur- 

 faces only, but upon and within the dielectric also, extending 

 to a greater or smaller depth fi-om the metal linings. Let c 

 (fig. 10.) be the section of a plate of any dielectric, a and b 

 being the metallic coatings; let 6 be uninsulated, 

 and a be charged positively; after ten or fiitteen Fig. 10. 

 minutes, if a and b be discharged, insulated, and ^ 



immediately examined, no electricity will appear 

 in them ; but in a short time, upon a second ex- 

 amination, 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 po- 

 sitive force has, under the coercing influence of 

 all the forces concerned, penetrated 

 trie and taken up its place at 

 the line j), a corresponding 

 portion of the negative force 

 having also assumed its posi- 

 tion 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 will be much greater 

 one towards the other, and less in an external direction, now 

 that they are at the small distance np 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 ex- 

 ternal 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 terminates in what we call discharge, the greater 

 portion returns on its course to the surfaces of c, and conse- 

 quently to the conductors a and b, and constitutes the re- 

 charge observed. 



124:6. The following is the experiment on which I rest for 

 the truth of this view. Two plates of spermaceti, d andy 

 (fig. 11.), were put together to form the dielectric, a and b 

 being the metallic coatings of this compound plate, as before. 

 The system was charged, then discharged, insulated, ex- 

 amined, and found to give no indications of electricity to the 

 carrier ball. The plates d andy were then separated from 

 each other, and instantly a with d was found in a positive state, 

 and b with y in a negative state, nearly all the electricity 

 being in the linings a and b. Hence it is clear that, of the 

 forces sought for, the positive was in one half of the com- 

 pound plate and the negative in the other half; for when re- 

 moved bodily with the plates from each other's inductive in- 



