=4. Conductors and Non-Conductors.= Electricity can be produced upon glass and ebonite because they do not carry or conduct it away. If a piece of iron be rubbed, the electricity passes from the iron into the earth as fast as it is generated, because the iron is a _conductor_ of electricity. Glass is an _insulator_ or _non-conductor_. Frictional electricity resides upon the outside, only, of conductors. A hollow tin box will hold as great a charge as a solid piece of metal having the same outside size and shape. When frictional electricity passes from one place to another, sparks are produced. Lightning is caused by the passage of static electricity from a cloud to the earth, or from one cloud to another. In this case air forms the conductor. (For experiments, see "Study," Chapter VII.) [Illustration: Fig. 3.] =5. Electroscopes.= A piece of carbon, pith, or even a small piece of damp tissue-paper will serve as an electroscope to test the presence of static electricity. The pith is usually tied to a piece of silk thread which is a non-conductor. Fig. 3 shows the ordinary form of _pith-ball electroscope_. The _leaf electroscope_ is a very delicate apparatus. Gold-leaf is generally used, but aluminum-leaf will stand handling and will do for all ordinary purposes. Fig. 4 shows a common form, the glass being used to keep currents of air from the leaves and at the same time to insulate them from the earth. Electroscopes are used to show the presence, relative amount, or kind of static electricity on a body. (See "Study," Chapter XI.) [Illustration: Fig. 4.] =6. Two Kinds of Electrification.= It can be shown that the electrification produced on all bodies by friction is not the same; for example, that generated with glass and silk is not the same as that made with ebonite and flannel. It has been agreed to call that produced by glass and silk _positive_, and that by ebonite and flannel _negative_. The signs + and - are used for positive and negative. =7. Laws of Electrification.= (1) Charges of the same kind repel each other; (2) charges of unlike kinds attract each other; (3) either kind of a charge attracts and is attracted by a neutral body. =8. Static Electric Machines.= In order to produce static electricity in quantities for experiments, some device is necessary.