INDUCTION IN CURVED LINES— IN AIR. 17 



at a . . above 1000° 

 b it was. . . 149 



c 270 



d 512 



h 130 



1219. To comprehend the full force of these results, it must first be understood, 

 that all the charges of the ball B and the carrier are charges by induction, from the 

 action of the excited surface of the shell-lac cylinder; for whatever electricity the ball 

 B received by communication from the shell-lac, either in the first instance or after- 

 wards, was removed by the uninsulating contacts, only that due to induction remain- 

 ing; and this is shown by the charges taken from the ball in this its uninsulated state 

 being always positive, or of the contrary character to the electricity of the shell-lac. 

 In the next place the charges at a, c, and d were of such a nature as might be ex- 

 pected from an inductive action in straight lines, but that obtained at h is not so : it 

 is clearly a charge by induction, but induction in a curved line ; for the carrier ball 

 whilst applied to b, and after its removal to a distance of six inches or more from B, 

 could not, in consequence of the size of B, be connected by a straight line with any 

 part of the excited and inducing shell-lac. 



1220. To suppose that the upper part of the uninsulated ball B, should in some way 

 be retained in an electrified state by that portion of the surface which is in sight of 

 the shell-lac, would be in opposition to what we know already of the subject. Elec- 

 tricity is retained upon the surface of conductors only by induction (1178.); and 

 though some persons may not be prepared as yet to admit this with respect to in- 

 sulated conductors, all will as regards uninsulated conductors like the ball B ; and 

 to decide the matter we have only to place the carrier ball at e (fig. 4,), so that it shall 

 not come in contact with B, uninsulate it by a metallic rod descending perpendicu- 

 larly, insulate it, remove it, and examine its state ; it will be found charged with the 

 same kind of electricity as, and even to a higher degree (1224.) than, if it had been in 

 contact with the summit of B. 



1221. To suppose, again, that induction acts in some way through or across the 

 metal of the ball, is negatived by the simplest considerations ; but a fact in proof 

 will be better. If instead of the ball B a small disc of metal be used, the carrier may 

 be charged at, or above the middle of its upper surface : but if the plate be enlarged 

 to about 1 J or 2 inches in diameter, C (fig. 5.), then no charge will be given to the 

 carrier at/, though when applied nearer to the edge at g, or even above the middle at 

 h, a charge will be obtained ; and this is true though the plate may be a mere thin 

 film of gold leaf. Hence it is clear that the induction is not through the metal, but 

 through the air or dielectric, and that in curved lines. 



1222. I had another arrangement, in which a wire passing downwards through the 

 middle of the shell-lac cylinder to the earth, was connected with the ball B (fig. 6.) 



MDCCCXXXVIII. D 



