How it works.—­The experimental engine, constructed in less than a couple of hours, which appears in Fig. 38, consists of a coil, C, strapped down by a piece of tin to a wooden bedplate; a moving plunger, P, mounted on a knitting-needle slide rod, Sr; a wire connecting rod, Sr; a wooden crank, K; and a piece of knitting-needle for crank shaft, on which are mounted a small eccentric brass wipe, W, and a copper collar, D. Against D presses a brass brush, B1 connected with the binding post, T1; while under W is a long strip of springy brass against which W presses during part of every revolution.  T2 is connected to one end of the coil winding, and T1 through a 4-volt accumulator or three dry cells, with the other end of the coil.  When W touches B2 the circuit is completed, and the coil draws in the plunger, the contact being broken before the plunger gets home.  The crank rotates at a very high speed if there is plenty of battery power, all the moving parts appearing mere blurs.

CONSTRUCTION.

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.