158 THE POPULAR SCIENCE MONTHLY. 



LESSONS IN ELECTKICITY. 1 



HOLIDAY LECTUEES AT THE ROYAL INSTITUTION. 

 By Pkof. TYNDALL, F. E. S. 



III. 



SECTION 13. Electric Induction. "We have now to apply the the- 

 ory of electric fluids to the important subject of electric induction. 



It was noticed by early observers that contact was not necessary 

 to electrical excitement. Otto von Guericke, as we have already 

 seen, found that a body brought near his sulphur globe became elec- 

 trical. By bringing his excited glass tube near one end of a conduct- 

 or, Stephen Gray attracted light bodies at the other end. He also 

 obtained attraction through the human body. From the human body, 

 also, Du Fay, to his astonishment, obtained a spark. Canton, in 1*753, 

 suspended pith-balls by thread, and, holding an excited glass tube at 

 a considerable distance, caused them to diverge. On removing the 

 tube the balls fell together, no permanent charge being imparted to 

 them. Such phenomena were further studied and developed by 

 Wilcke and iEpiuus, Coulomb and Poisson. 



These and all similar results are embraced by the law that, when an 

 electrified body is brought near an unelectrified one, the neutral fluid 

 of the latter is decomposed, one. of its constituents being attracted, 

 the other repelled. When the electrified body is withdrawn, the sep- 

 arated electricities flow again together and render the body unelectric. 



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, while it is under the influence of the electrified body, the body 

 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 captive. 



On removing the electrified body the captive electricity is set free, 

 the conductor being charged with electricity opposite in kind to that 

 of the body which electrified it. 



You cannot do better here than repeat Stephen Gray's experiment. 

 Support a small plank upon a warm tumbler, and bring under one of 

 its ends and near it scraps of light paper or of gold-leaf. Excite your 

 glass tube vigorously, and bring it over the other end of the plank, 

 without touching it. The ends may be six or eight feet apart ; the light 

 bodies will be attracted. The experiment is easily made, and you are 

 not to rest satisfied till you can make it with ease and certainty. 



1 A course of six lectures, with simple experiments in frictional electricity, before 

 juvenile audiences during the Christmas holidays. 



