_=162. Free and Bound Electrifications.=_ As explained in Exp. 75, and as shown in Fig. 44, T B became polarized. The - electrification was drawn towards E C; it was held or _bound_ there as long as E C was near. The + was actually repelled by E C, and it was _free_ to escape through your arm as soon as T B was touched, leaving the top end of T B neutral. As soon as E C was removed, the - electrification, no longer held by E C, spread all over T B and on to C T. T B was _charged by induction_. It was charged negatively by driving out + electrification. =EXPERIMENT 77. To show that a neutral body is polarized before it is attracted by a charged one.= _Apparatus._ The electrophorus (Exp. 68); dry tissue-paper, T P. Cut out 2 pieces of T P, each about 1/4 inch square. =163. Directions.= (A) Place the bits of dry T P upon a board or table, and convince yourself that they are attracted equally by the charged E C. (B) Slightly moisten one piece of T P only. See if one is attracted by E C more readily than the other. _=164. Polarization Precedes Attraction.=_ Dry tissue-paper is not a good conductor; you have seen (Exp. 52) that it can be electrified, which indicates that it is at least a partial insulator. Insulators are not easily polarized. (Why?) Even if the pieces of T P were polarized, the opposite electrifications were so near each other that the attraction of E C for the - was nearly overcome by the repulsion for the +; the result being that T P was not strongly attracted by E C until the + had a chance to escape. The moist tissue-paper allowed its + to escape more quickly than the dry piece. A conductor is attracted by a charged body more strongly than an insulator, because the latter is not easily polarized. A neutral body, then, is really no longer neutral when it is in the electric field. _Polarization precedes attraction._ =EXPERIMENT 78. To find whether electric induction will act through an insulator.= _Apparatus._ Small bits of carbon (Exp. 58); bits of moist tissue-paper, T P; one-half of the flat box, T F B (No. 41); sheet of glass, G (No. 38); electrophorus (Exp. 68). Place the carbon and T P into T F B (Fig. 45), and cover with the glass. =165. Directions.= (A) Charge the electrophorus cover, E C (Exp. 68), move it about a little above the glass, and see if the carbon, etc., are attracted. _=166. Dielectrics.=_ The carbon must have been polarized and attracted _through_ the glass. You saw, Exp. 7, that the lines of magnetic force could penetrate and act through paper, glass, etc.; it is now evident that the electric field is not easily fenced in, even by an insulator. Substances, like the glass, which allow this inductive influence to act through them, are called _dielectrics_. [Illustration: Fig. 45.] [Illustration: Fig. 46.] =EXPERIMENT 79. To find whether a polarized conductor can act inductively upon another conductor.=