_Apparatus._ The condenser (Fig. 52); the electrophorus (Exp. 68); hairpin discharger, H P D.
=196. Directions.= (A) Thoroughly charge the condenser.
(B) Discharge it with H P D, being sure to touch F B first, and to touch I T for an instant while H P D is against F B.
(C) After a few moments use H P D again, and see if you get a slight spark.
_=197. Residual Charge.=_ The two electrifications on the opposite sides of the dielectric have such an attraction for each other, when the condenser is charged, that they seem to penetrate, or soak into, the dielectric. These do not completely soak out again at the discharge. The small amount left is called a _residual charge_.
=EXPERIMENT 92. To study successive condensation; the chime cascade.=
_Apparatus._ Fig. 56. This really consists of two condensers, joined by a wire. The upper condenser consists of T F B (No. 41), E S (No. 27), and the insulating table, I T. (See Exp. 64.) The lower condenser consists of the cover of the tin box, C T B (No. 47), the sheet of glass G (No. 38), and B F B (No. 40). The tin box, T B (No. 47), is placed under this to raise it, simply. A wire or hairpin, H P, is hung upon the edge of T F B, its lower end being inside of C T B and not quite touching it. This acts like a pendulum, which is to swing to C T B at the proper time. The source of electrification is E C.
=Note.= You have learned that in charging the condenser with the positively charged E C, + electrification is driven from F B into the earth. Can we use this to charge a second condenser?
=198. Directions.= (A) Pass 15 or 20 good sparks from E C to the under side of I T (Fig. 56), noting the action of H P.
(B) Hold E C in the hand, and, with its insulating handle, poke H P away from the condensers. Do not discharge them.
(C) With H P D test the lower condenser for a charge, touching T B first.
(D) With H P D touch T F B first (why?), and discharge the upper condenser.
_=199. Discussion.=_ A long row of condensers may be charged in this way. There is no advantage in it, as the electrification is merely divided between them. How can two condensers be joined to get the advantages of a large surface?
_=200. Electroscopes=_ are instruments to show the presence, relative amount, or kind of electrification on a body. (See Apparatus Book, Chap. XVIII, for Home-Made Electroscopes.) The _carbon electroscope_ has been described (Exp. 58). The _pith-ball electroscope_ is made by using pith from elder, corn-stalk, or milk-weed, in place of the carbon. The _gold-leaf electroscope_ is a very delicate instrument. The gold-leaf is supported, as suggested in Fig. 57, at the lower end of a wire conductor which sticks through and hangs from the cork of a glass jar or flask. To the top end of the wire is soldered a ball or disk. The glass jar insulates the gold-leaf, and keeps it dry and free from dust.