239. Pith-Ball Electroscope. Fig. 107. The pith from elder, corn-stalk, milk-weed, etc., is very light and porous. When this is tied to the end of a silk thread, we get the pith-ball electroscope, so much talked about in nearly every text-book on physics. The upper end of the thread may be tied to any suitable support. Fig. 117 shows a book, lead pencil, and a small weight to hold the pencil steady. The thread is tied to one end of the pencil.
240. Support for Electroscopes, etc. Fig. 108. Glue or nail a spool, S, to a wooden base, B, measuring about 4 × 5 in. Wrap some paper around a 7 in. length of ¼ in. dowel, D, to make it fit the hole in S. Wind one end of a wire, W, around the top end of D. To the outer end of W tie a silk thread, S T, on the lower end of which may be tied a piece of pith or material to serve as an electroscope.
Fig. 108.
241. Carbon Electroscope. Carbon will be found to make a most excellent electroscope, as it is light and a good conductor of electricity. Light an ordinary match and let it burn until it is charred through and through. The black substance remaining is carbon. Tie a small piece of the carbon, about ¼ in. long, to one end of a silk thread, and support the thread as in Fig. 107 or 108.
242. Pivoted Electroscope. Fig. 109 and 110. Fold a piece of stiff paper double, then cut it into the shape shown. It should be about 3 in. long and 1 in. wide when opened out. A hole, B, about ½ in. in diameter should be cut in it while folded. A piece of paper, C, should be pasted to A, so that its top, where it is creased, will be about ⅛ in. above the top of A. The support consists of a pin, E, stuck through a cork, D. Balance the paper on the pin, which passes up through the hole, B. An electrified body brought near this apparatus will make it whirl around very decidedly.