=213. Electrical Charges.=--The ideas gained in the study of magnetism are of assistance in the study of electricity in giving some fundamental ideas and principles that will often be referred to as a basis for comparing the actions of magnetized and electrified bodies. The process of electrifying a body is very different from that of magnetizing it. Thus if a rubber comb or rod be rubbed with a woolen cloth the object rubbed is able to attract to itself light bits of paper, thread, etc. This peculiar attraction was noticed and recorded by the ancient Greeks, 600 B.C., when it was found that amber when rubbed would attract light objects to itself. For a long time it was supposed that amber was the only substance showing this property. Dr. William Gilbert, however, discovered that the electrified condition could be produced by rubbing a great variety of substances. He named the _result_ produced, _electrification_, after the Greek name for amber (_elektron_). A body like hard rubber or amber which will attract light objects when rubbed is said to be _electrified_, or to have been given a _charge_ of electricity. =214. Law of Electric Action.=--Let a vulcanite rod be electrified by rubbing with a woolen cloth until it will attract light objects; then place it in a wire stirrup suspended by a silk thread. If a second vulcanite rod is similarly electrified and brought near the first, the two will be found to repel. (See Fig. 186.) If now a glass rod be rubbed with silk and brought near the suspended rod, the two will _attract_. This difference in behavior indicates a difference in the electrification or charge upon the rods. The two charged vulcanite rods repelling and the charged glass and vulcanite attracting indicate _the law of electric action_. _Like charges repel each other and unlike charges attract each other._ Extensive experiments with all kinds of substances indicate that there are but two kinds of electrical charges. The electrical charge upon glass when rubbed with silk or wool is called _positive_, and that upon hard rubber or vulcanite when rubbed with wool is called _negative_. [Illustration: FIG. 186.--Repulsion of like charges.] [Illustration: FIG. 187.--An aluminum foil electroscope.] [Illustration: FIG. 188.--A proof plane.]