INDUCTION APPARATUS — EFFECTS OF CONDUCTION IN IT. 21 



1233. I think it expedient, next in the course of these experimental researches, to 

 describe some effects due to conduction, obtained with such bodies as glass, lac, sul- 

 phur, &c., which had not been anticipated. Being understood, they will make us 

 acquainted with certain precautions necessary in investigating the great question of 

 specific inductive capacity. 



1234. One of the inductive apparatus already described (1 187, &c.) had a hemisphe- 

 rical cup of shell-lac introduced, wiiich being in the interval between the inner ball 

 and the lower hemisphere, nearly occupied the space there ; consequently when the 

 apparatus was charged, the lac was the dielectric or insulating medium through which 

 the induction took place in that part. When this apparatus was first charged with 

 electricity (1198.) up to a certain intensity, as 400°, measured by the Coulomb's elec- 

 trometer (1180.), it sank much faster from that degree than if it had been previ- 

 ously charged to a higher point, and had gradually fallen to 400° ; or than it would 

 do if the charge were, by a second application, raised up again to 400° ; all other 

 things remaining the same. Again, if after having been charged for some time, as 

 fifteen or twenty minutes, it was suddenly and perfectly discharged, even the stem 

 having all electricity removed from it (1203.), then the apparatus being left to 

 itself, would gradually recover a charge, which in nine or ten minutes would rise up 

 to 50° or 60°, and in one instance to 80°. 



1235. The electricity, which in these cases returned from an apparently latent to a 

 sensible state, was always of the same kind as that which had been given by the 

 charge. The return took place at both the inducing surfaces ; for if after the perfect 

 discharge of the apparatus the whole was insulated, as the inner ball resumed a po- 

 sitive state the outer sphere acquired a negative condition. 



1236. This effect was at once distinguished from that produced by the excited 

 stem acting in curved lines of induction (1203. 1232.), by the circumstance that all 

 the returned electricity could be perfectly and instantly discharged. It appeared to 

 depend upon the shell-lac within, and to be. in some way, due to electricity evolved 

 from it in consequence of a previous condition into which it had been brought by the 

 charge of the metallic coatings or balls. 



1237. To examine this state more accurately, the apparatus, with the hemispherical 

 cup of shell-lac in it, was charged for about forty-five minutes to above 600° with po- 

 sitive electricity at the balls A and B (fig. 1.) above and within. It was then dis- 

 charged, opened, the shell-lac taken out, and its state examined ; this was done by 

 bringing the carrier ball near the shell -lac, uninsulating it, insulating it, and then 

 observing what charge it had acquired. As it would be a charge by induction, the 

 state of the ball would indicate the opposite state of electricity in that surface of 

 the shell-lac which had produced it. At first the lac appeared quite free from any 

 charge ; but gradually its two surfaces assumed opposite states of electricity, the 

 concave surface, which had been next the inner and positive ball, assuming a posi- 

 tive state, and the convex surface, which had been in contact with the negative 



