(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? We know, by the action of C T (Fig. 42), that the top end of T B was charged while E C was in place. The charge was not conducted. You know, from previous experiments, that + and - electrifications rush together whenever possible. Why can we not suppose that a neutral body, like the box at the start, contains an equal amount of both kinds, and that these different electrifications have already rushed together? If you imagine a small army of positive soldiers struggling, "man to man," with the same number of equally strong negative soldiers, you can readily see that one-half of them can hold the other half from running away. A body remains neutral, then, according to this idea, as long as it has an equal quantity of the two opposite kinds of electrification. (See Theories, § 145, 146.) As soon as the positively charged E C was brought near T B, it destroyed the neutrality of T B, by pulling at its - electrification, and by pushing back its + electrification to the top end and into C T. We say that the charged E C produced a separation of the combined electrifications of T B by _induction_, and not by contact. As soon as the inductive action of E C was removed, T B became neutral again. [Illustration: Figs. 43-44.] =160. Note.= Figs. 43 and 44 may aid the student. In Fig. 43, T B is supposed to be neutral. The "double sign" means that the + and - electrifications are united; and, as there are an equal number of both kinds, none are left free to tell the tale. Fig. 44 shows what happens when the + E C is near. What would happen if we could cut into T B at the middle with an insulated knife while it is polarized by E C? =EXPERIMENT 76. To learn how to charge a body by induction.= _Apparatus._ Fig. 42, same as in Exp. 75. =161. Directions.= (A) Bring the charged E C within an inch of the bottom of T B, and as soon as C T is repelled, showing that T B is polarized (Exp. 75), touch T B with your finger; then remove your finger while you still hold E C in place. (B) Withdraw E C and its inductive action. Explain the motions of C T during the experiment. Is it still repelled by T B after E C is removed?