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ECTION 13. Electric Induction.—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 excitement. Otto von Guericke, as we have already seen, found that a body brought near his sulphur globe became electrical. By bringing his excited glass tube near one end of a conductor, 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 1753, 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 Æpiuus, 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 When the electrified body is withdrawn, the separated electricities flow again together and render the body unelectric.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.