=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. [Illustration: FIG. 198.--Effect of a charged rod near an electroscope.] [Illustration: FIG. 199.--When a finger is touched to the top of the electroscope, the repelled negative charge escapes.] [Illustration: 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. Important Topics 1. Electric lines of force. Characteristics (3). 2. Electrostatic induction. Principles (2). 3. Charging by induction. Explanation. Exercises