Fig. 186.—Repulsion of like charges.
Fig. 187.—An aluminum foil electroscope.
Fig. 188.—A proof plane.
215. The Electroscope and its Uses.—An electroscope is a device employed to test the presence of an electrical charge. The aluminum foil electroscope consists of a flask closed by a rubber stopper through which passes a rod which ends at the top in a ball or plate and below is attached two narrow leaves of thin aluminum-foil. Ordinarily the two leaves hang close together and parallel but if a charged body is brought near the electroscope the leaves spread apart at the bottom. (See Fig. 187.) The kind of charge upon a body may be determined with an electroscope as follows: Make a proof-plane by sealing a small metal disc on the end of a hard rubber rod. (See Fig. 188.) Touch the disc of the proof-plane first to a charged rubber rod and then to the top of the electroscope. The leaves of the latter will separate showing that the electroscope is charged. This charge remains after the proof-plane is removed. If the charged vulcanite rod is brought near the electroscope, the leaves separate further That is, a charge like that on the electroscope makes the leaves separate further. But if an unlike charge, as that on a positively charged glass rod, is cautiously brought near, the leaves will be seen to move together.
Fig. 189.—Rod with woolen cap.
216. Two Charges are Produced at the Same Time.—A closely fitting woolen cover or cap some 3 in. long is made for the end of a vulcanite rod. A silk thread attached to the cap enables one to hold the latter while the rod is turned within it. (See Fig. 189.) If the rod bearing the cap is held near a charged electroscope little or no effect is noticed. If now the cap is removed by the silk thread and held near the electroscope, it will be found to be positively charged while the rod is negatively charged. The fact that no result is seen when the cap and rod are together, indicates that one charge neutralizes the other. In other words, the charges must be equal. This illustrates the truth that when electrification is produced by friction, the two objects rubbed together acquire equal and opposite charges.
217. Charging by Contact and Conduction.—If a small pith ball is suspended by a silk thread, a charged rod brought near is at first attracted, but after contact is repelled (see Fig. 190) showing that the ball has become charged with the same kind of electrification that is upon the rod. That is, a charge given to an object by contact with a charged body is of the same kind as that upon the charged one. The proof-plane in Art. 215 carries the same kind of charge that is upon the rod it is charged from. Some substances have the ability to transfer charges of electrification. These are called conductors, those that do not conduct electrification are insulators. The conducting power of a body is readily tested by placing one end of a rod of the material upon the top of an electroscope and the other end upon an insulated support, as in Fig. 191. If now a charge be put in contact with the body of a, the electroscope will show by its leaves whether the rod tested conducts or not. The leaves separate instantly when conducting substances are tested, while no action results with insulators. In testing some materials for conductivity the leaves are found to diverge gradually. Such bodies are said to be poor conductors. All degrees of conductivity are found. The metals are the best conductors. The best insulators are rubber, mica, shellac, glass, silk, porcelain, paraffin, and oils.
Fig. 190.—The pith ball charged by contact is repelled.
Fig. 191.—Testing for conductivity.