Researches in TLlectriciti). (Series XL) 35 



lars ; the middle one, A, was a fixture, but the outer plates 

 B and C were moveable on slides, so that all three could be 

 brought with their sides almost into contact, or separated to 

 any required distance. Two gold-leaves were suspended in 

 a glass jar from insulated wires; one of the outer plates B was 

 connected with one of the gold-leaves, and the other outer plate 

 with the other leaf. The outer plates B and C were adjusted 

 at the distance of an inch and a quarter from the middle 

 plate A, and the gold-leaves were fixed at two inches apart ; 

 A was then slightly charged with electricity, and the plates 

 B and C, with their gold-leaves, thrown out of insulation 

 at the same time, and then left insulated. In this state of 

 things A was charged positive inductrically, and B with C 

 negative inducteously ; the same dielectric, air, being in the 

 two intervals, and the gold-leaves hanging, of course, parallel 

 to each other in a relatively unelectrified state. 



1308. A plate of shell-lac three quarters of an inch in 

 thickness, and four inches square, suspended by clean white 

 silk thread, was very carefully deprived of all charge (1203.), 

 so that it produced no efl^'ect on the gold-leaves if A were un- 

 charged, and then introduced between plates A and B ; the 

 electric relation of the three plates was immediately altered, 

 and the gold-leaves attracted each other. On removing the 

 shell-lac this attraction ceased ; on introducing it between A 

 and C it was renewed ; on removing it the attraction again 

 ceased ; and the shell-lac when examined by a delicate Cou- 

 lomb electrometer was still without charge. 



1309. As A was positive, B and C were of course nega- 

 tive; but as the specific inductive capacity of shell-lac is about 

 twice that of air (1270.), it was expected that when the lac 

 was introduced between A and B, A would induce more to- 

 wards B than towards C ; that therefore B would become 

 more negative than before towards A, and consequently, be- 

 cause of its insulated condition, be positive externally, as at 

 its back or at the gold-leaves ; whilst C would be less nega- 

 tive towards A, and therefore negative outwards or at the 

 gold -leaves. This was found to be the case ; for on which- 

 ever side of A the shell-lac was introduced the external plate 

 at that side was positive, and the external plate on the other 

 side negative towards each other, and also to uninsulated ex- 

 tei'ual bodies. 



1310. On employing a plate of sulphur instead of shell- 

 lac, the same results were obtained ; consistent with the con- 

 clusions drawn regarding the high specific inductive capacity 

 of that body already given (1276.). 



D2 



