An Electric engine is really a form of electric motor but differs from the most common form of the latter in that the armature, instead of revolving, oscillates back and forth, like the piston of a steam or gasoline engine. Electric engines are not as efficient as electric motors from the standpoint of the amount of power delivered in proportion to the current used, but they make very interesting models and the young experimenter will derive fully as much pleasure in constructing one as from the construction of an electric motor. Various forms of electric engines were made before the first practical electric motor was invented. They amounted to little more than curiosities, however, and could only be used where the expense of electric current was not to be regarded.

The engine illustrated in Figure 143 is of the double action type. It is provided with two electromagnets arranged so that one pulls the armature forward and the other pulls it back. The motion of the armature is transmitted to the shaft by means of a connecting rod and crank. It is very simple to build and the design is such that it will operate equally well whether it is made large or small. If you do not happen to have all the necessary materials to build an engine according to the dimensions shown in the drawings you can make it just one-half that size and it will work equally well although it will of course not give as much power.

The complete engine is shown in Figure 143. All the various parts have been marked so that you can easily identify them in the other drawings. It is well to study this illustration carefully first so that you will understand just how all the parts are arranged.

The Base is illustrated in Figure 144. It is made of a piece of hardwood, seven inches long, three and one-half inches wide and one-half an inch thick.

The Magnets. The size of the electromagnets will largely determine the dimensions of the rest of the engine. The magnets shown in Figure 145 are made of three-eighths inch round iron, two and one-half inches long, provided with two fibre washers one and one-eighth inches in diameter. One end of each of the steel cores is drilled and tapped to receive an 8-32 iron screw. The experimenter may possibly be able to secure some old magnet cores fitted with fibre heads from an old telephone bell or "ringer" as they are sometimes called. A suitable bolt can be made to serve the purpose by cutting it off to the right dimensions with a hack saw. If a drill and a tap are not available for drilling and tapping the end so that the core can be properly mounted in the frame of the engine it is possible by the exercise of a little ingenuity to use the threaded portion of a bolt to good advantage. The hole in the frame should then be made larger so that the end of the bolt will slip through instead of an 8-32 screw and the core clamped in position by a nut on each side.

The fibre washers are spaced two and one-sixteenth inches apart. The space in between should be wound full of No. 18 B. & S. Gauge cotton covered magnet wire. Before winding in the wire, cover the core with a layer of paper so that the wire does not touch the metal. The ends of the wire should be led out through small holes in the fibre heads.

It is not absolutely necessary to use No. 18 B. & S. Gauge wire in winding the magnets but this is the size which will give the best results on the average battery.

If you use larger wire, the engine will require more current from the battery. If you use finer wire, a battery of a higher voltage will be necessary. The current consumption will, however, be less.

The Frame.—The electromagnets are mounted in the frame of the engine by means of two screws passing through the holes E and D. The details of the frame are illustrated in Figure 146. It is made of a strip of wrought iron or cold rolled steel, nine and one-quarter inches long, an inch or an inch and one-eighth wide and one-eighth of an inch thick. The material for making this part of the engine and also the bearings can probably best be obtained at some blacksmith shop or hardware store. Heavy galvanized iron can be used but it is not usually thick enough and it may be necessary use two thicknesses. The ends of the strip are rounded and bent at right angles so as to form a U-shaped piece with sides one and three-quarters inches high. The holes D and E should be large enough to pass an 8-32 screw. The holes A, B and C should be about one-eighth of an inch in diameter. They are used to pass the screw which hold the frame of the engine to the wooden base.

The Bearings are shown in Figure 147. They are U-shaped and are made out of a strip of iron or steel in the same manner as the frame of the engine but are three-quarters of an inch wide instead of an inch and one-eighth. The dimensions will be understood best by referring to the drawing. The 3/32 inch holes near the top of each side are the bearing holes for each end of the shaft. The one-eighth inch holes below are used to fasten the brush holder in position. The holes in the bottom serve to fasten the bearings to the base.

The Shaft will probably prove the most difficult part of the engine to make properly. The details are given in Figure 148. It is made of a piece of one-eighth inch steel rod bent so that it has a "throw" of one-half an inch, that is, is offset one-quarter of an inch so that the connecting rod moves back and forth a distance of one half an inch. The finished shaft should be three inches long. The piece of steel used should be longer than this, so that it can be cut off to exact dimensions after the shaft is finished. A second crank should be bent in one end of the shaft so as to form an offset contact for the brushes. This second crank will have to be at right angles to the first one and should be much smaller. The ends of the shaft are turned or filed down to a diameter of three-thirty second of an inch for a distance of about the same amount so that they will fit in the bearing holes and turn freely but not allow the whole shaft to slip through. The work of making the shaft will require a small vice, a light hammer, files and a couple of pairs of pliers. One pair of pliers should be of the round nosed type and the other a pair of ordinary square jawed side cutters. It may require two or three attempts before a perfect shaft is secured. When finished it should be perfectly true and turn freely in the holes. The bearings can be adjusted slightly by bending so that the shaft will fit in the holes and be free but yet not be loose enough to slip out.

The Armature is a strip of soft iron, two and one-eighth inches long, seven-sixteenths of an inch wide and three-sixteenths of an inch thick. A one-sixteenth inch slot, three-eighths of an inch long is cut in one end. A one-sixteenth inch hole is drilled through from one side to the other, one-eighth of an inch from each end. The hole which passes through the slot is used to pass the pin which pivots the armature to the connecting rod. The other hole is used to mount the armature in its bearing. The armature bearing is a small edition of the one which is used to support the engine shaft. The details and the dimensions are given in the lower left hand side of Figure 149. The armature is shown in the centre of the same illustration. The connecting rod is illustrated at the right. This is made from a strip of three-sixty-fourths inch brass, three-sixteenths of an inch wide and one and five-eighths inches long. A one-eighth inch hole should be drilled close to one end and a one-sixteenth inch hole close to the other.

The Brushes are two strips of thin phosphor bronze sheet, two and three-sixteenths inches long and nine-thirty-seconds of an inch wide. They are illustrated in Figure 150. The block upon which they are mounted is hard fibre. It is one and five-eighths inches long and three-eighths of an inch square.

The Flywheel. It may be possible to secure a flywheel for the engine from some old toy. It should be about three and one-half inches in diameter. A flywheel can be made out of sheet iron or steel by following the suggestion in Figure 151, which shows a wheel cut out of one-eighth inch sheet steel. It is given the appearance of having spokes by boring six three-quarter inch holes through the face as shown. The hole in the centre of the wheel should be one-eighth of an inch in diameter. The wheel is slipped over the shaft and fastened in position by soldering.

The parts are now all ready to assemble into the complete engine. Mount the electromagnets in the frame and fasten the frame down to the wooden base so that one end of the frame comes practically flush with the left hand edge of the base. Fasten the bearing across the frame at right angles by a screw passing through the centre hole in the bottom of the bearing, through the hole A and into the base. The bottom of the bearing should be bent slightly so as to straddle the frame. The bearings should be secured and prevented from turning or twisting by two screws passed through the other two holes in the bottom. Use round-headed wood screws in mounting the bearing and the frame. The armature bearing should be mounted on the frame directly between the two electromagnets. Then place the armature in position by slipping a piece of one-sixteenth inch brass rod through the bearing holes and the hole in the lower part of the armature.

Solder the flywheel in position on the shaft and snap the latter into the bearings. Adjust the bearings so that the shaft will turn freely. The connecting rod should be slipped over the shaft before it is placed in the bearings. Fasten the other end of the connecting rod to the armature by means of a piece of one-sixteenth inch brass rod which passes through the small holes bored for that purpose. When the flywheel is spun with the fingers, the armature should move back and forth between the two electromagnets and almost but not quite touch the two magnet poles. All the moving parts should be fitted firmly together but be free enough so that there is no unnecessary friction and so that the engine will continue to run for a few seconds when the flywheel is spun with the fingers.

The brushes, supported on their fibre block should be mounted on the bearing by means of two screws passing through the holes in the bearing into the block. The position of the brushes should be such that the shaft passes between the two upper ends but does not touch them unless the small contact crank mentioned above is in proper position to do so. The proper adjustment of the brushes, so that they make contact with the shaft at the proper moment, will largely determine the speed and power which the finished engine will develop.

Two binding posts should be mounted on the right hand end of the base so that the engine can be easily connected to a battery. Connect one terminal of the right hand electromagnet to one of the binding posts. Run the other terminal of the electromagnet to the brush on the opposite side of the shaft. Connect one terminal of the left hand electromagnet to the other binding post and run the other terminal to the brush on the opposite side of the shaft. Save for a few minor adjustments, the engine is now ready to run. Connect two or three cells of dry battery to the two binding posts and turn the flywheel so that it moves from right to left across the top. Just as the crank passes "dead centre" and the armature starts to move back away from the left hand magnet, the small contact crank should touch the left hand brush and send the current through the right hand magnet. This will draw the armature over to the right. Just before the armature gets all the way over to the right, the contact should break connection with the left hand brush and interrupt the current so that the inertia of the flywheel will cause it to keep moving and the armature to start to move over towards the left hand magnet at which point the contact on the shaft should commence to bear against the right hand brush, thus throwing the left hand magnet into circuit and drawing the armature over to that side. If the brushes and the cranks are in proper relation to each other the engine will continue to repeat this operation and gradually gain speed until it is running at a good rate.

The appearance of the engine can be improved by painting the metal parts black and the flywheel red. The magnets can be wrapped with a piece of bright red cloth to protect the wire against injury and brighten the appearance of the engine.