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 ½ 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.
174. Discussion. This apparatus is really the electrophorus upside down. It shows very clearly (1) the escape of the - electrification from I T, by the spark; (2) that the attraction between I T and E S is much greater than before, when this - is removed; and (3) it shows the different steps of the inducing and charging process, as described in [Exp. 75], and as shown in Figs. 43 and 44.