long that has about 1/4-in.
hole. Put ends, A, 1-1/4 in. square and cut from heavy cardboard
on this tube. Make a hole in the center of each cardboard just
large enough to allow the brass tube to fit tight. Put on two or
three layers of stout paper around the brass tube and between the
cardboard ends. Wind evenly about 2 oz. of No. 26 cotton covered
magnet wire on the paper between the ends and leave about 2 in. of
wire on each end extending from the coil. Use a board 1/2- in.
thick, 3 in. wide and 6 in. long for the base and fasten the coil
to it, as shown in Fig. 1. Bore holes for binding-posts, B, one on
each side of the board, and connect the two wires from the coil to
them. At the other end of the board and in the center drive a wire
nail and attach a small spring, C, to it. The spring should be
about 1 in. long. Take a small piece of soft iron, D, 1/2- in.
long and just large enough to slip freely through the brass
[Illustration: Battery Voltmeter Construction]
tube and solder a piece of copper wire to it; the other end of the
copper wire being hooked to the spring, C. The copper wire must be
just long enough to allow the piece of iron, D, to hang part way
in the end of the coil and still hold the spring in place.
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