As it is difficult to make a jointed tube
cylindrical, and a close fit is needed to give good results, it is worth
going to a little trouble to get a plunger of the right kind.
The ends of the plunger are plugged with wood and bored centrally for the
slide rod, which should not be cut to its final length until the parts are
assembled.
The crank shaft is 2-3/4 inches of a stout knitting needle mounted in a
sheet brass bearing. The crank, a fragment of oak or other tough wood, is
balanced, and has a throw of 5/8 inch. The crank-shaft hole should be a
trifle small, so that the crank shall get a tight hold of the shaft without
pinning. The collar, D, and wipe, W, are soldered to the shaft after this
has been passed through its bearings. The brush B1 should press firmly, but
not unnecessarily so, against the collar. For B2 one must use very springy
brass strip, a piece about 3 inches long and 1/4 inch wide being needed.
Bend it to the arc of a large circle, and screw one end down to the base by
the binding screw T2. The other end, which should not touch the base, is
confined by the heads of a couple of small screws, by means of which the
strip is adjusted relatively to the wipe.
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