24 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



Two plates of spermaceti, d andy (fig. 11 .), were put together to form the dielectric, 

 a and h being the metallic coatings of this compound plate, as before. The system 

 was charged, then discharged, insulated, examined, and found to give no indications 

 of electricity to the carrier ball. The plates d and f were then separated from each 

 other, and instantly a with d Wtis found in a positive state, and b with/* in a negative 

 state, nearly all the electricity being in the linings a and h. Hence it is clear that, 

 of the forces sought for, the positive was in one half of the compound plate and the 

 negative in the other half; for when removed bodily with the plates from each other's 

 inductive influence, they appeared in separate places, and resumed of necessity their 

 power of acting by induction on the electricity of surrounding bodies. Had the effect 

 depended upon a peculiar relation of the contiguous particles of matter only, then 

 each half plate, d and/, should have shown positive force on one surface and nega- 

 tive on the other. 



1247. Thus it would appear that the best solid insulators, such as shell-lac, glass, 

 and sulphur, have conductive properties to such an extent, that electricity can pene- 

 trate them bodily, though always subject to the overruling condition of induction 

 (1178.). As to the depth to which the forces penetrate in this form of charge 

 of the particles, theoretically, it should be throughout the mass, for what the charge 

 of the metal does for the portion of dielectric next to it, should be done by the 

 charged dielectric for the portion next beyond it again ; but probably in the best in- 

 sulators the sensible charge is to a very small depth only in the dielectric, for other- 

 wise more would disappear in the first instance whilst the original charge is sustained, 

 less time would be required for the assumption of the particular state, and more elec- 

 tricity would re-appear as return charge. 



1248. The condition of time required for this penetration of the charge is important, 

 both as respects the general relation of the cases to conduction, and also the removal 

 of an objection that might otherwise properly be raised to certain results respecting 

 specific inductive capacities, hereafter to be given (1269. 1277.)- 



1249. It is the assumption for a time of this charged state of the glass between the 

 coatings in the Leyden jar, which gives origin to a well-known phenomenon, usually 

 referred to the diffusion of electricity over the uncoated portion of the glass, namely, 

 the residual charge. The extent of charge which can spontaneously be recovered by 

 a large battery, after perfect uninsulation of both surfaces, is very considerable, and 

 by far the largest portion of this is due to the return of electricity in the manner de- 

 scribed. A plate of shell-lac six inches square, and half an inch thick, or a similar 

 plate of spermaceti an inch thick, being coated on the sides with tin foil as a Leyden 

 arrangement, will show this effect exceedingly well. 



1250. The peculiar condition of dielectrics which has now been described, is evi- 

 dently capable of producing an effect interfering with the results and conclusions 

 drawn from the use of the two inductive apparatus, when shell-lac, glass, &c. is used 

 in one or both of them (1192. 1207.) ; for upon dividing the charge in such cases ac- 



