APPARATUS 132.

[Ill.u.s.tration: Fig. 106.]

_233. Resistance Coils._ Fig. 106. The construction of one coil is given in App. 131. To have the set of coils so that they can be easily used, place the spools upon a base which, in the model, is 8-1/2 4 7/8 in.

The spools are 1-3/4 in. apart, center to center, and should be glued to the base. Fig. 106 is a plan of the apparatus. U, V, etc., are binding-posts like App. 46. The figures between them show how many Ohms resistance there are in the coil above. The coils A, B, C, D, and E are wound respectively for 1, 2, 2, 5 and 10 Ohms.

234. Connections. If you join a Wheatstone's bridge, for example, with U and V (Fig. 106), the resistance added will be but 1 Ohm; if you join with U and W, the coils A and B will be in the circuit and make 3 Ohms resistance; if V and X, 4 Ohms; if V and Y, 9 Ohms; if U and Z, the whole, or 20 Ohms.

APPARATUS 133.

_235. Resistance Coils._ For use in some experiments in comparing the resistance, diameter, lengths, etc., of wires (See text-book), it is very handy to have coils made a certain number of meters long. (The meter is a French unit of measure and represents 393705 of our inches).

German-silver wire has a much greater resistance than copper wire of the same size and length.

(a) Make a coil (See App. 131 for method) containing 1 meter of No. 30 German-silver wire.

(b) Make a coil with 2 meters No. 30 German-silver wire.

(c) Make one with 2 meters of No. 28 German-silver wire.

(d) Make one with 20 meters of No. 30 copper wire.

The above wire must be insulated if it is to be wound upon spools. Bare wire may be arranged on boards or racks so that the current may not be short circuited.

CHAPTER XVIII.

APPARATUS FOR STATIC ELECTRICITY.

_236. Static or Frictional Electricity._ There are many interesting and instructive experiments in this branch of electricity. All that can be done here is to explain a few pieces of simple apparatus to show the presence of static electricity, it being taken for granted that you know how to produce it, and that you have some book of simple experiments.

_237. Electroscopes_ are instruments for showing the presence of static electricity.

APPARATUS 134.

_238. Thread Electroscope._ A piece of ordinary thread may be used for this purpose. Tie one end of it to the back of a chair or other support.

APPARATUS 135.

_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.

[Ill.u.s.tration: Fig. 107.]

APPARATUS 136.

_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 1/4 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.

[Ill.u.s.tration: Fig. 108.]

APPARATUS 137.

_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 1/4 in. long, to one end of a silk thread, and support the thread as in Fig. 107 or 108.

APPARATUS 138.

_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 1/2 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 1/8 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 pa.s.ses up through the hole, B. An electrified body brought near this apparatus will make it whirl around very decidedly.

[Ill.u.s.tration: Fig. 109.]

[Ill.u.s.tration: Fig. 110.]

APPARATUS 139.

_243. Fancy Electroscope._ Fig. 111. Fold a piece of stiff paper double, then cut out some fancy-shaped figure, as suggested, and draw the face, clothes, etc., to suit. This being folded through the center for cutting, it can be balanced upon a pin-point as explained in App. 138.

[Ill.u.s.tration: Fig. 111.]

[Ill.u.s.tration: Fig. 112.]

APPARATUS 140.

_244. Box-Cover Electroscope._ Fig. 112. A pasteboard box-cover, balanced upon a pin, makes a fairly good electroscope, although it is not nearly so sensitive as App. 138. The pin may be stuck in the upper end of the dowel, D, shown in Fig. 108.

APPARATUS 141.

_245. Leaf Electroscope._ Fig. 113. This is a very sensitive instrument, and can be used to tell the kind of static electricity on a body, as well as the mere presence of it. (See experiments in text-book.) The lamp chimney acts as a support for the leaves, L, and it protects them from currents of air. A tin box-cover, C, has a small hole punched through its center. Through this is pushed one end of a wire, W. This may be a hairpin, straightened. The upper end is bent over at right angles, after pa.s.sing it through the hole. The lower end is bent as shown. On this horizontal part is fastened the leaf. These should be made of aluminum leaf, or of Dutch metal. The former will stand more rough handling than the latter. Goldleaf is used for very sensitive instruments. It is a little too delicate for unskilled hands.

[Ill.u.s.tration: Fig. 113.]

[Ill.u.s.tration: Fig. 114.]

246. To cut the aluminum leaf, place it between two pieces of paper, then cut paper and all into the desired shape. The piece should be about 3 in. long and 1 in. wide. Fold this across the middle, and stick it to the underside of the wire (Fig. 113). Saliva will make it adhere to the wire, if you have nothing better.