219. Electrostatic Induction.—If a charged body is brought near an aluminum-foil electroscope, the leaves separate. (See Fig. 198.) The nearer the charge is brought the wider the leaves spread, but when the charge is removed, the leaves collapse showing that nothing was given to the electroscope. It was simply affected by the charge in its vicinity. This production of an electrified condition in a body by the influence of a charge near it is called electrostatic induction. Placing insulators, such as a sheet of glass, between the charge and the electroscope does not affect the result, which is apparently brought about by the action of the electric lines of force. These lines of force extend without difficulty through uncharged insulators and terminate often at the surface of a conductor, where their influence causes a charge to accumulate. Charged insulators, however, do affect inductive action. This may be noticed by using a sensitive electroscope.
Fig. 196.—Production of two charges by the influence of a third charge.
Fig. 197.—The two charges separated.
220. Electrical Separation by Induction.—The action just described may be illustrated further by taking two insulated, uncharged brass shells, A and B. (See Fig. 196.) Bring a charged vulcanite rod near shell "A" while the shells are touching each other. Then remove shell B (Fig. 197) while the rod remains near A. On testing the shells for electrification, A is found to possess a positive charge. This action is in some respects similar to magnetic induction, for if one places a north-seeking pole near a piece of iron, the iron develops by induction a south-seeking pole at the end nearest the magnet and a north-seeking at the other end. There is, however, one striking difference. If the magnetized iron be separated into two parts, each part is a complete magnet possessing two unlike poles; while if the object affected by electrostatic induction is separated into two parts one part has a positive charge and the other a negative charge.
Fig. 198.—Effect of a charged rod near an electroscope.
Fig. 199.—When a finger is touched to the top of the electroscope, the repelled negative charge escapes.
Fig. 200.—The electroscope is now positively charged.
221. Charging a body by induction is easily accomplished. To charge an aluminum-foil electroscope by induction bring near (say 10 cm.) from the top of the electroscope a charged rubber rod. (See Fig. 198.) The separated leaves show the presence of the repelled or negative charge, the positive charge being on the disc at the top. If while the charged rod is held near, the metal top of the electroscope is touched by the finger the leaves at once fall together showing that the repelled negative charge has escaped from the electroscope (Fig. 199). On removing first the finger and next the charged rod, the positive charge spreads over the metal parts of the electroscope, as is shown by the separation of the leaves (Fig. 200). The electroscope is now charged positively by induction. If the charged rubber rod is brought to about 30 cm. from the electroscope, its leaves tend to move together. If a body charged similarly to the electroscope or positively, is moved toward the electroscope the leaves separate further. This behavior of the electroscope enables one to determine the kind of charge upon a body.
Two principles of electrostatic induction may now be stated: (1) Two equal, unlike charges are always produced by electrostatic induction.
(2) If the body affected by induction is connected to the earth by a conductor, the repelled or "free" charge is conducted away from the body while the "bound" charge is held by the inducing charge.
These principles apply in every case of induction.