=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_.
=EXPERIMENT 79. To find whether a polarized conductor can act inductively upon another conductor.=