802 



LESSORS IN ELECTRICITY. 



I will throw out for future reflection the 

 remark, that the overcoming of the ordi- 

 nary friction produces heat then and there 

 upon the surfaces rubbed, while the force 

 expended in overcoming the electric at- 

 traction may be converted into heat 

 which sLal! appear a thousand miles away 

 from the place whore it was generated. 



Theoretic conceptions are incessantly 

 checked and corrected by the advance of 

 knowledge, and this theory of clectiic 

 fluids is doubted by many eminent scien- 

 tific men. It will, at till events, have to 

 be translated into a form which shall con- 

 nect it with heat and light, before it can 

 be accepted as complete. Nevertheless, 

 keeping ourselves unpledged to the the- 

 ory, we shall find it of exceeding service 

 both in unravelling and in connecting to- 

 gether electrical phenomena. 



13. Electric Induction, 

 the Term. 



Definition, of 



We have now to apply the theory of 

 electric fluids to the important subject of 

 electric induction. 



It was noticed by early observers that 

 contact was not necessary to electrical ex- 

 citement. Otto von Gueiickc, as we 

 have already seen ( 2), found that a 

 body brought near his sulphur globe be- 

 came electrical. By bringing his excited 

 glass tube near one end of a conductor, 

 Stephen Gray attracted light bodies at 

 the other end. lie also obtained attrac- 

 tion through the human body. From 



FIG. 15. 



the human body also Du Fay, to his 

 astonishment, obtained a spark. Can- 

 ton, in 1753, suspended pith-balls by 

 thread, and holding an excited glass 

 tube, at a considerable distance from 

 them, caused them to diverge. On re- 

 moving the tube the balls fell together, 

 no permanent charge being impaited to 

 them. Such phenomena were further 

 studied i:nd developed by Wilcke and 

 ^pinus, Coulomb and Poisson. 



These and all similar results are cm- 

 braced by the law, that when an electri- 

 fied body is brought near an unelectrified 

 conductor, the neutral fluid of the latter 

 is decomposed ; one of its constituents 

 being attracted, the other repelled. 

 When the electrified body is withdrawn, 

 the separated electricities flow again to- 

 gether and render the conductor unelec- 

 tric. 



This decomposition of the neutral fluid 

 by the mere presence of an electrified 

 body is called induction. It is also called 

 electrification by influence. 



If, whilf 't is under the influence of 

 the electrified bodv, the bodv influenced 

 be touched, the free electricity (which is 

 always of the same kind as that of the 

 influencing body) passes away, the 

 opposite electricity being held cap- 

 tive. 



On removing the electrified body the 

 captive electricity is set free, the conduc- 

 tor being charged with electricity oppo- 

 site in kind to that of the body which 



