=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. [Illustration: FIG. 190.--The pith ball charged by contact is repelled.] [Illustration: FIG. 191.--Testing for conductivity.] Important Topics 1. Positive and negative changes. Law of electric action. 2. Electroscope and its uses. 3. Conductors and insulators. Exercises 1. Is air a conductor? Give reasons for your answer. 2. Mention two points of likeness and two points of difference between magnetism and electrification. 3. If you were testing the electrification of a body with a charged pith ball suspended by a silk thread, would attraction or repulsion be the better test? Give reasons. 4. Have you ever produced electrification by friction outside of a laboratory? Explain. 5. Are the rods upon which we produce electrification by friction, conductors or insulators? How do you explain this? 6. Are conductors or insulators of the greater importance in practical electricity? Explain. (2) ELECTRIC FIELDS AND ELECTROSTATIC INDUCTION [Illustration: FIG. 192.--An electric field about a positively charged shell.] [Illustration: FIG. 193.--A "detector."]