Part 15 (1/2)

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.