TRANSACTIONS OF THE SECTIONS. 23 



ill the condition of a neutral insulated body of small capacity, placed 

 within an extremely small distance of a charged body ; and subject 

 to the same laws as subsist between two such bodies under ordinary 

 circumstances at more considerable distances, but at which a com- 

 munication of electricity can take place, a rigorous examination of 

 this question would probably elucidate many phenomena of elec- 

 trical action, at present involved in some obscurity. In the mean- 

 time he thinks it not unimportant to review such facts connected 

 with this point as are already known. It has been found, for ex- 

 ample, that the attractive force between charged and neutral bodies 

 is less when the neutral bodies are insulated, that very perfect in- 

 sulators are not sensibly attracted by electrified substances, and that 

 in every case of electrical attraction the force is only in proportion 

 to the previous induction of which the bodies are susceptible ; in ac- 

 cordance with tliese facts, a perfectly insulating disc reposes on a 

 charged surface without becoming sensibly electrified, an insulated 

 neutral conducting disc more or less so in proportion to its thick- 

 ness, whilst a similar disc, whose inductive susceptibility is rendered 

 nearly perfect by artificial methods, becomes charged with an in- 

 tensity nearly equal to that of the point to which it is applied. 

 Should therefore the inductive susceptibility of the tangent disc be 

 so influenced by position, in respect of the electrical molecules of the 

 charged body, as to become at any time nothing ; it would be as in- 

 efficient in abstracting electricity, as a similar disc of any noncon- 

 ducting substance. Now it is not improbable that an insulated con- 

 ducting body of small dimensions, plunged within a spherical charged 

 shell, is thus circumstanced ; and hence it fails to become in any de- 

 gree charged, notwithstanding that electricity may, experimentally, 

 be clearly proved to exist there. 



From these and a variety of other considerations, the author is 

 disposed to believe, that the force communicated to a proof plane can- 

 not always be considered as a faithful indication of the electrical 

 state of a charged surface, since it forms no integral part of that sur- 

 face, being really placed under the conditions of an insulated neutral 

 body of small and variable capacity, arising from the circumstances 

 of position, thickness, and the like, and which is about to receive 

 electricity from a charged body. He thinks that we really know 

 little about the actual distribution of electricity upon a charged sur- 

 face, except through the medium of insulated discs, in some way 

 applied to it. Now he has already shown* that any charged body 

 only gives oflF its electricity under the influence of an attractive force ; 

 so that an electrified sphere, when completely insulated, will retain 

 its charge in the best vacuum which can be obtained by ordinary 

 means, provided it be fi-ee from any sensible source of attraction. It 

 is not therefore until we employ some substance susceptible of in- 

 duction, that we begin to disturb the electrical distribution in 

 charged bodies, which may be previously uniform or nearly so. 



In conclusion, the author observed, that our present theories of 

 electricity may probably be found to require some considerable qio- 



* Phil. Tram, foi 1834, p. 242. 



