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



From this it is manifest that intermediate plates of the same 

 ncn -conducting substance sometimes decreased, and sometimes 

 increased the induced electricity of the neutral sphere; and 

 further, by comparison with the former experiments, that one 

 and (he same non-conducting plate, which increased the induced 

 electricity upon a neutral disc, decreased the same when the 

 neutral body was a sphere. Hence the important theorem : — 



In experiments on induction, the effect of the action of non- 

 conducting intermediate plates depends upon the form and dimen- 

 sions, not only of those plates, hut also of the conductor used in the 

 experiment. This theorem negatives, in the simplest manner, the 

 assumption that the intermediate plates directly influence the 

 induction of the originally charged conductor. We cannot 

 directly ascertain in what manner the action of insulating plates 

 exerts itself, because the electric condition of these plates can- 

 not be examined, and reference must be had to the analogous 

 action of conducting intermediate plates, in which this examina- 

 tion is possible. The following discs, on insulating pillars, were 

 placed between the spheres. 



Here, undoubtedly, both the electricities induced upon the 

 intermediate disc acted. In order to find the general distribu- 

 tion of these electricities, the discs were placed separately at a 

 distance of a quarter of an inch from the surface of the positively 

 charged sphere, in such a manner that the line joining the 

 centres of the sphere and disc was perpendicular to the latter. 

 Several points of the disc were then touched with a pin's head, 

 insulated by shell-lac, and the electric state of the latter examined 

 in an electroscope. 



Negative electricity was present upon the whole anterior sur- 

 face (turned towards the charged sphere) of the thick brass disc, 

 and it diminished from the centre to the circumference; on the 

 cylindrical and posterior surfaces positive electricity was found, 

 which increased from the anterior to the posterior edge, and 

 decreased from the latter towards the centre of the posterior sur- 

 face. A similar distribution was detected upon the thin brass 

 disc ; on the greater discs of silver and tinfoil the negative elec- 

 tricity upon the anterior surface no longer extended to the cir- 



