in Electric Induction. 403 



quantities of electricity, present whenever an electrified body acts 

 upon a neutral one, two quantities being completely occupied 

 with one another have no further action ; I have intimated that 

 the experiments which ought to support this theory may be ex- 

 plained by the simultaneous action of all three electricities. Such 

 an intimation appeared to me sufficient, inasmuch as these ex- 

 periments belong to those complicated cases which were men- 

 tioned at the commencement of this paper. There still exists, 

 however, an uncertainty as to the signification of a part of those 

 experiments, which are the more particularly important because 

 they appear to establish an essential difference between the in- 

 ductive actions of conducting and non-conducting bodies, and 

 on that account it will not be deemed superfluous to remove that 

 uncertainty in an experimental manner. 



In order to determine what he calls the specific inductive 

 capacities of insulators, Faraday employed an instrument called 

 the differential inductometer*, which consists of three, or simpler 

 of two, insulated metallic discs placed at a short distance from 

 one another with their faces parallel ; one disc was charged with 

 electricity directly, and the electric state of the other disc, which 

 was charged by the inductive action of the first, examined. 

 When between both plates, but without touching them, a non- 

 conducting plate of shell-lac or sulphur was introduced, the 

 amount of induced electricity was found to be greater than before, 

 i. e. when both discs were separated from one another by air 

 only. It follows from other experiments made by Faraday f, 

 that the induction would have been diminished had a conduct- 

 ing plate been introduced between the two ; for, according to 

 Faraday's opinion, the introduction of the conducting plate 

 would have caused the induction to take place in curved lines 

 around the edges of the plate, instead of in right lines through 

 the intervening stratum of air. 



The experiments with insulating intermediate plates have been 

 repeated with various results. In accordance with Faraday, 

 Knockenhauer f found, by introducing a plate of shell-lac be- 

 tween two discs, an augmentation of induced electricity upon 

 the induced plate, but could not conceive the cause of this result. 

 M tiller §, who considered this result to be incompatible with the 

 cause of the phamomenon, found on frequent repetition of the 

 experiment a diminution of the induced electricity, and ascribed 

 the results of both his predecessors to errors in the testing of 

 the kind of electricity here excited. 



* Experimental Researches, 130/. t Ibid. 1218. 



X Pogg. Ann. vol. li. p. 120". 



§ Bericht iiber die neuaten Fortschritte der Phys., Braunschweig, 1849, 

 p. Gl. 



2D2 



