_=171. Discussion.=_ The electrification upon the ebonite is negative (Exp. 60). Although E S and E C (Fig. 34) seem quite smooth, there are many little hills, valleys, and air-spaces between them, which keep them from touching each other perfectly. The ebonite has the electric field at the start, and it really acts across these minute air-spaces _by induction_ (Exp. 75), and polarizes E C. The air-spaces form the dielectric (Exp. 78). The - electrification of E C being repelled by the - of E S, it is driven to the top of E C, while the + is drawn to the bottom. This + is kept from rushing to the - of E S by the air dielectric, and because E S is a non-conductor. By touching E C the free - escapes to the earth, leaving E C _positively_ charged when it is lifted. [Illustration: Fig. 47.] [Illustration: Fig. 48.] [Illustration: Fig. 49.] [Illustration: Fig. 50.] [Illustration: Fig. 51.] =172. Details of Action.= The different steps in the action of the electrophorus are shown graphically in Figs. 47 to 51. Fig. 47 shows E S negatively charged. E C is neutral at first, Fig. 48; that is, it is supposed to contain both + and -, as shown by the "double sign" (§ 160). Fig. 49 shows that E C has been polarized by the inductive action of E S. The repelled - escapes to the finger (this escaping is what gave the small spark to the finger and charged the T P in the last experiment), leaving the top uncharged, while the + is _bound_ (Fig. 50). As soon as E C is lifted (Fig. 51) the + spreads all over E C, which is then charged. The +, upon going to the top, charged the pieces of T P (Exp. 80, D), causing them to be repelled. The charge of - upon E S has not been removed, so the operation may be repeated many times before E S must be again electrified. The - electrification on the ebonite acts inductively through E S, drawing up + electrification from the earth. To make this action easier a "sole," or metal conductor, is often placed under the ebonite. =EXPERIMENT 81. To see, hear, and feel the results of inductive influence and polarization.= _Apparatus._ Ebonite sheet, E S (No. 26); insulating table, I T; flannel cloth, F C. =173. Directions.= (A) Thoroughly charge E S with F C. With the right hand bring E S near and parallel to the top surface of I T, but do not let them touch each other. (B) Remove E S, then touch I T to see if it is charged. (C) Repeat (A), and while you hold E S about 1/2 inch from I T, their flat surfaces being parallel, touch I T. Watch for any sparks, and note any peculiar actions of E S. (D) Remove your finger from I T, then withdraw E S; finally touch I T with your knuckle.