_=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?
=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.
(B) Touch the charged T B with the finger, watching C T.
_Part 2._ (C) Bring the re-charged E C near the neutral T B, and parallel to its end surface; but keep them at least an inch apart, so that a spark cannot pass. Watch C T.
(D) Withdraw E C, and try to explain the action of C T.
_=159. Electric Polarization; Theory of Induction.=_ This experiment should remind the student of Exp. 24, in magnetism, in which a piece of soft iron was magnetized by the inductive action of a magnet. The soft iron was in a magnetic field; it became polarized. Is it possible that the box, T B, was polarized, being in the electric field of E C?