(D) Touch E C; raise it by E R; count 10 as before; then test with your knuckle to see if E C is still charged. _=155. Electric Density; Electric Wind.=_ A charge resides upon the outside of a conductor (Exp. 73), and it continually tries to escape. It seems to pile up at points and corners, and we say that it is denser at such places than at well-rounded parts of a charged conductor. All points and sharp places should be removed from a conductor, if it is desired to keep a charge for any length of time. Electrification may escape from a point so rapidly that currents are produced in the surrounding air. As the particles of air become charged, they repel each other. The movement of the air particles may be so great that a lighted candle will be affected when placed near the point. This current of air is called _electric wind_. Electrification easily passes from points, and the electrophorus may be easily and silently discharged by holding a pointed pin near it (Exp. 71, C). Thorns, leaves with sharp edges, etc., have a great effect upon atmospheric electricity. They allow a silent escape of electrification from the earth to neutralize that in the clouds which is opposite in nature. (See Atmospheric Electricity.) CHAPTER IX. INDUCED ELECTRIFICATION. _=156. Electric Field; Lines of Force.=_ In our study of magnetism you learned that a magnet can act through the air, and induce a piece of iron to become a magnet. You saw how the iron filings arranged themselves around the magnet, showing that the lines of force reached out from the poles in a very peculiar manner. There is an _electric field_ all around a charged conductor, just as there is a magnetic field about a magnet. The lines of force in the electric field pass from the positively charged body to the negatively charged one, or to some neutral one, which, you will soon see, is practically the same thing. When the positively charged electrophorus cover is held above the negatively charged ebonite sheet, a very strong electric field exists between them. =157. Note.= You have seen that we can _charge_ an insulated conductor by _touching_ it with the charged cover, or by allowing a spark to pass to the conductor. What effect, if any, has a charged body upon an insulated conductor _before_ they touch each other, and before any spark passes to the conductor? [Illustration: Fig. 42.] =EXPERIMENT 75. To study electric induction.= _Apparatus._ Fig. 42. The insulating table, I T (for details see Exp. 64); tin box, T B (No. 47, Fig. 42); moist cotton thread, C T; the electrophorus (Exp. 68); tie C T around one end of the closed T B, and leave the ends of C T long enough to hang down over the end. Place a match on each side of T B to keep it from rolling. =158. Directions.= _Part 1._--(A) Pass a spark from the charged E C to T B, and note the action of the thread, which will be our electroscope. Remove E C.