The glass
selected for the plates must be clear white glass, free from
wrinkles, and of a uniform thickness. Two plates are necessary to
make this machine, and the glass should be of sufficient size to
cut a circular plate 16-in. in diameter. A hole must be made
exactly in the center of each plate, and this should be done
before cutting the circle. One of the best ways to make the hole
is to drill the glass with a very hard-tempered drill, the cutting
edge of which should be kept moistened with 2 parts turpentine and
1 part sweet oil while drilling. The hole is to be made 3/4 in. in
diameter. The circle is then marked on each plate and cut with a
glass cutter. The plates are trued up, after they are mounted, by
holding a piece of emery wheel to the edges while they are
turning. Water should be applied to the edges while doing the
work.
The sectors are cut from tinfoil, 1-1/2 in. wide at one end, 3/4
in. at the other, and 4 in. long. A thin coat of shellac varnish
is applied to both sides of the plates, and 16 sectors put on one
side of each plate, as shown in Fig. 1. The divisions can be
marked on the opposite side of the plate and a circle drawn as a
guide to place the sectors at proper intervals.
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