410 M. P. Riess on the Action of Non-conducting Bodies 



posterior surface to surpass that of the negative electricity upon 

 the anterior surface ; sometimes, too, because the former is sup- 

 ported by the action of the whole positive electricity on the cylin- 

 drical surface. Here, therefore, the influence of the intermediate 

 plate will be alike in kind to that of the charged sphere, and the 

 induction of the latter will be apparently increased by introdu- 

 cing the plate between the two spheres. It is more difficult to 

 predict the effect upon the observed iuduction produced by inter- 

 posing a disc of less extreme dimensions in comparison to the 

 distauce between the two spheres, but after observation this 

 effect may be easily explained by the distribution of the electri- 

 city upon the plate. 



In general broad thin intermediate discs apparently weaken, 

 and thick narrow discs apparently strengthen the inductive 

 effect of a charged sphere upon a neutral one. Inasmuch as no 

 doubt can exist that the action of conducting plates depends 

 solely upon the induced electricities upon these plates, we must 

 conclude that the action of non-conducting intermediate plates 

 ought to be ascribed to the same cause. It is certain that a 

 charged body induces both electricities upon the surfaces of the 

 most perfectly insulating body; and the following simple and 

 remarkable experiment will show that upon every one of the 

 interposed plates hitherto used, these electricities make their 

 appearance daring the time of an observation. The disc of 

 shell-lac 7*21 inches in diameter and 0'33 inch thick, or the disc 

 of paraffine 4*63 inches in diameter and ^ an inch thick, was 

 moved quickly to and fro between the flame of a spirit-lamp and 

 the knob of the conductor of an electric machine slightly charged 

 positively ; the anterior surface of the disc approached within one 

 foot of the knob, and the posterior surface within one inch of the 

 flame. By this means the anterior surface became so strongly 

 negatively electric as to impart a strong and durable divergence 

 to the gold leaves of the electroscope. Here, during the moment 

 of passage, the positively charged knob of the conductor imparted 

 negative electricity to the anterior surface of the plate, and posi- 

 tive to the posterior, and the negative electricity alone remained 

 behind because the flame had withdrawn the positive. Conse- 

 quently, in all the induction experiments above described, each 

 insulating plate was undoubtedly provided with both electricities 

 at the moment when it was interposed between the two discs or 

 spheres ; and to explain the different actions of these plates, we 

 have only to assume that these electricities occupied different 

 positions in space upon the thick and upon the thin plates. If, 

 however, the action of the insulating plates depends upon the 

 induced electricities on their surface, it follows that their inter- 

 position between the charged and neutral conductors is unessen- 



