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.