stance 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 39.3705 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.

[Illustration: 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 x 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, aroun