28 



ELECTRICITY. 



distances from each other, when held by 

 a handle proceeding from the joint. A 

 short point projects from one of the balls 

 on the side adjacent to the other ball ; 

 and this point affords a spark at a 

 shorter distance when positively, than 

 when negatively electrified. 



CHAPTER VII. 

 Development of the Law of Induction. 



(101.) WE have next to trace the 

 consequences of that important law of 

 electricity which has been called the 

 Law of Induction. 



Active electricity existing in any sub- 

 stance tends always to induce the op- 

 posite electrical state in the bodies that 

 are near it. Now it is impossible, as 

 we have already seen, to induce one 

 electrical state in any body without at 

 the same time producing the opposite 

 state in the same body, or in the one 

 which is immediately contiguous. Ac- 

 cording to the simpler theory, the accu- 

 mulation of electricity in any one part 

 can be effected in no other way than by 

 withdrawing it from another part, nor 

 can it be abstracted from the one with- 

 out being received by another ; so that 

 there is always an equal degree of nega- 

 tive as of positive electricity, and vice 

 versa, in every case. According to the 

 more complex theory, if we decompose 

 the natural electricities residing in any 

 body, we must at the same moment ob- 

 tain equal quantities of both the vitreous 

 and resinous electricities. It follows, 

 therefore, that if the bodies subjected to 

 the inductive influence are non-conduc- 

 tors, although the tendency to produce 

 the opposite electricity still exists, yet 

 in consequence of the immobility of the 

 fluid, it can produce no visible change. 

 In proportion as the body opposes less 

 resistance to the passage of electricity, 

 the operation of the disturbing force 

 becomes sensible ; and in order to fix 

 our ideas, let us first take the case of 

 a positively charged electric, acting by 

 induction on an insulated conducting 

 body. The redundant fluid in the for- 

 mer will tend to repel all the fluid con- 

 tained in the latter: a portion of this 

 fluid will, therefore, be driven from the 

 side adjacent to the first body, towards 

 the remoter side. The adjacent side 

 will thus be rendered negative ; the 

 remote side, positive. But this will take 

 place to a certain extent only : for there 

 is a limit at which the repulsion of the 



fluid accumulated at the remote end, 

 will just balance the repulsion of the 

 fluid in the electric, added to the attrac- 

 tion of the under- saturated matter, in the 

 near end ; and when this limit has been 

 attained, the flow of electric fluid from 

 the near to the remote end of the body 

 will cease, and an equilibrium will be 

 established. 



(102.) Experiment shows the perfect 

 coincidence of theory with the actual 

 fact. Let a cylinder of metal, NP, (see 

 ftg. 25,) of some length, with rounded 

 ends, and furnished in different parts 



Fig. 25. 



with pairs of suspended pith-balls, to 

 serve as electroscopes, being previously 

 insulated, be placed in the vicinity of an 

 electrified globe of glass, E, taking care 

 that it be not sufficiently near to receive 

 any quantity of electricity by transfer- 

 ence. 



We shall find that eveiy pair of balls, 

 except those situated in a particular 

 plane Mm, about the middle of the 

 cylinder, will immediately diverge, indi- 

 cating the electrical states of the parts 

 from which they are suspended. Those 

 at either extremity of the body, n,p, 

 diverge the most ; and the divergence 

 diminishes as we approach the middle 

 plane before mentioned, at which the 

 body is in the natural or neutral state. 

 The position of this plane of neutrality, 

 Mm, varies according to the distance of 

 the electric, and the relation which that 

 distance bears to the length of the body 

 itself. If we further examine the species 

 of electricity residing in the different 

 parts, we shall find it to be negative in 

 all the parts nearer to the electric than 

 the neutral plane, and positive in all 

 those more remote. We may ascertain 

 with much greater accuracy these elec- 

 trical states by the employment of the 

 proof plane and electrometer of Cou- 

 lomb, than by the pith-balls ; and the 

 results are then found to correspond, 

 with the most rigorous precision, with 

 the deductions from the theory of elec- 

 trical action. 



(103.) These effects, it should be 

 remarked, are simply the result of the 

 action of electricity at a distance ; for 

 they depend upon no other circumstance. 



