=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. [Illustration: 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_.