Page:Elementary Text-book of Physics (Anthony, 1897).djvu/299

§ 253] show that an insulated conductor, electrified by induction, is electrified both positively and negatively at once; that the electrification of a dissimilar kind to that of the inducing body persists, however the insulation of the conductor be afterwards modified; and that the total positive electrification induced by a positively charged body is equal to that of the inducing body, while the negative electrification can exactly neutralize the positive electrification of the inducing body. The use of the terms positive and negative is thus justified, since they express the fact that equal electrifications of dissimilar kinds are exactly complementary, so that if they be superposed on a body that body is not electrified. These two kinds of electrification may then be spoken of as opposite.

If the glass and resin considered in the first experiment be rubbed together within the vessel, and in general if any apparatus which produces electrification be in operation within the vessel, no signs of any external electrification can be detected. It is thus shown that, whenever one kind of electrification is produced, an equal electrification of the opposite kind is also produced at the same time.

Franklin showed that, by the use of a closed conducting vessel of the kind just described, a charged conductor introduced into its interior and brought into conducting contact with its walls is always completely discharged, and the charge is transferred to the exterior of the vessel. This procedure furnishes a method of adding together the charges on any number of conductors, whether they be charged positively or negatively. It is thus theoretically possible to increase the charge of such a conductor indefinitely.

(4) If any instrument for detecting forces due to electrifications be introduced into the interior of a closed conductor charged in any manner, it is found that no signs of force due to the charge can be detected. The experiment was accurately executed by Cavendish, and afterwards tried on a large scale by Faraday. It proves that within a closed electrified conductor there is no