Apparatus. [Fig. 46]. Insulating table, I T (for details see [Exp. 64]); ebonite sheet, E S (No. 27); flat box complete F B (Nos. 40, 41); sheet of glass, G (No. 38); small piece of slightly moist tissue-paper, T P; charged electrophorus cover, E C. Arrange as shown.
167. Directions. (A) Hold E C, charged, near and under I T, then bring your finger, F, near T P. Explain the action of T P.
168. Successive Induction. The inductive influence of E C first polarized I T; this acted through the dielectric, E S, and polarized F B, which, in turn, polarized[65] T P through the second dielectric, G. This induction after induction is called successive induction.
169. Inductive Capacity. Dielectrics are insulators. Two substances may be equally good insulators, that is, they may equally well resist the spread of electrification over their surfaces, or the flow of the electric current through them, while one may be, nevertheless, a better dielectric than the other. The better the dielectric, the easier it is for the electric field to polarize a conductor placed beyond the dielectric. A good dielectric is said to have a high inductive power or capacity. Glass is about 3 times as good a dielectric as dry air; and as the latter (under certain conditions) is taken as the standard, or as unity, we may say that the specific inductive capacity of glass is about 3.
EXPERIMENT 80. To study the action of the electrophorus.
Apparatus. The electrophorus ([Exp. 68]); small bits of moist tissue-paper, T P.
170. Directions. (A) Thoroughly electrify E S, [Fig. 34], and place E C upon it by its handle, E R.
(B) Touch E C, as directed in [Exp. 68], and listen for a small spark which should pass from E C to your finger.
(C) Again, place a little piece of T P upon E C before lowering it upon E S. Do not touch E C, but bring your finger near T P. What does T P do? Now, touch E C and see, when you bring your finger near it, if T P acts as it did before.
(D) Again, place several pieces of T P upon E C (E S being thoroughly charged); touch E C, then lift it by its handle. Note action of T P, which should be slightly moist.