The coil is made by winding 4 oz. of No. 32 cotton-covered wire (price
6d. to 8d.) on a boxwood reel 2 inches long and 1-1/2 inches in
diameter, with a 9/16-inch central hole. Before winding, bore a hole
for the wire through one end of the reel, near the central part, and
mount the reel on a lathe or an improvised spindle provided with a
handle of some kind. The wire should be uncoiled and wound on some
circular object, to ensure its paying out regularly without kinking;
which makes neat winding almost impossible.
Draw a foot of the wire through the hole in the reel, and drive in a
tiny peg—which must not protrude inwards—to prevent it slipping. Lay
the turns on carefully, forcing them into close contact, so that the
next layer may have a level bed. On reaching the end of the layer, be
equally careful to finish it neatly before starting back again. When
the wire is all on, bore a hole as near the edge of the finishing edge
as possible, and draw the spare wire through. Then cut a strip of tough
paper of the width of the coils, coat one side with paste, and wrap it
tightly round the outside to keep the wire in place.
Note.—Insulation will be improved if every layer of wire is painted
over with shellac dissolved in alcohol before the next layer is
applied.
Flatten the reel slightly with a file at the points of contact with the
baseboard, to prevent rolling.
The plunger is a tube of thin iron, 1/16 inch less in diameter than the
hole in the reel, and 1/4 inch longer than the reel. If a ready-made
tube is not available, construct one by twisting a piece of tin round a
metal rod, and soldering the joint. 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.
Fixing the Coil.—Cut a strip of tin 1-3/4 inches wide and 4 inches
long. Punch a couple of holes near one end, and nail this to the side
of the base, with its forward end 4-1/4 inches from the crank shaft.
Pass the strip over the coil, and bend it down towards the base. Drill
a couple of screw holes, and screw the other end down so that the coil
is gripped fairly tight.
Fixing the Plunger. Two small guides, G1 G2, are made for the plunger.
The holes through which the slide rod moves should be a good fit, and
their centres at the level of the centre of the coil. Screw holes are
bored in the feet.
Pass the plunger through the coil, and place the guides on the rod.
Then draw the plunger forward till 1/2 inch projects. Bring G1 close up
to it, mark its position, and screw it to the base. The other guide,
G2, should be 1-1/2 inches away from the rear of the coil.
[Illustration: Fig. 38.—Plan of electric reciprocating engine.]
The coil and guides must be adjusted so that the plunger does not touch
the coil anywhere during a stroke, packings being placed, if necessary,
under coil or guides. When the adjustment is satisfactory, screw the
coil down tightly, and cut off any superfluous parts of the rod.
The Connecting Rod.—Bore a hole near the end of the plunger for a screw
to hold the rear end of the connecting rod. Pull the plunger out till
1-3/4 inches project, turn the crank full forward, and measure off the
distance between the centres of the plunger hole and the crank pin.
Drive a couple of wire nails into a board, and twist the ends of a
piece of 1/20-inch wire round them twice. This wire constitutes a
connecting rod amply strong enough to stand the pulls to which it will
be subjected. Fix the rod in position.
Adjusting the Wipe.—Turn the wipe, W, round until it makes contact with
B2, and, holding the crank shaft with a pair of pliers, twist the crank
on it till it just begins the return stroke. Then turn the crank to
find out how long the wipe remains in contact, and adjust the crank
relatively to the wipe so that the crank is vertical when the period of
contact is half finished. The length of this period is controlled by
the set screws at the free end of B2.