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?