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?
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,[63] 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 ¼ 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.