AN electric motor has two essential parts. These are a field magnet for furnishing a strong magnetic field and a revolving armature.

It is an easy matter to make a strong electric motor suitable to operate on batteries by the exercise of a little careful workmanship.

The field frame and armature of the motor shown in Figure 133 are laminated, that is, built up of separate sheets of iron. They may be made out of sheet tin or ordinary stove pipe iron. The cheapest and simplest method of securing good flat material is to get some old scrap from a tinner's or plumbing shop.

The Details of the Field are shown in Figure 134. The exact shape and dimensions can be understood by reference to the illustration. Lay out one lamination very carefully as a pattern. Cut it out and smooth up the edges, making certain that it is perfectly true to size and shape. Then use it as a template to lay out the other laminations by placing it on the metal and scribing a line around the edges with a sharp pointed needle. Enough laminations should be cut out to make a pile five-eighths of an inch high when tightly pressed together.

The Armature is made in exactly the same manner as the field frame, that is, by cutting out a pattern according to the shape and dimensions shown in Figure 136 and using it as a template to lay out the other laminations. Enough should be cut to make a pile five-eighths of an inch high when tightly squeezed together.

The armature is one and three-sixteenths inches in diameter. The hole in the field frame which accommodates the armature is one inch and one-quarter in diameter so that there is a space in between for the armature to revolve in.

The hole through the center for the shaft should be of such diameter that the laminations will force very tightly on a shaft one-eighth of an inch in diameter. The laminations should be very carefully flattened and then forced over the steel shaft which is two and one-eighth inches long. Clean up all the rough edges with a file and smooth the outside so that it will revolve properly in the field without scraping.

Figure 137 illustrates the armature assembled on the shaft and ready to be wound.

The Armature Windings consist of four layers of No. 22 B. & S. Gauge double cotton covered magnet wire wound around each leg. The iron should be very carefully insulated with shellaced paper before the wire is put in position so that there will not be any danger of short circuit due to the sharp edges of the metal cutting through the insulation. Each leg should contain the same number of turns of wire and all should be wound in the same direction.

The Commutator is illustrated in Figure 138. It consists of a piece of brass tubing seven-sixteenths of an inch long, five-sixteenths inside and three-eighths of an inch outside. It should be forced onto a piece of fibre five-sixteenths of an inch in diameter and seven-sixteenths of an inch long. Split the tube into three equal parts by dividing it longitudinally with a hack-saw. Make a fibre ring which will force onto the tube very tightly when it is in position on the fibre core and so hold the three commutator sections firmly in position. The sections should be so arranged that there is a small space between each two and they are perfectly insulated from each other. The fibre core should have a one-eighth inch hole through the centre so that it may be forced tightly onto the shaft and up against the armature after the windings are in position. The commutator should be in such a position that the split between each two sections come directly opposite the centre of each winding. Suppose that the windings are lettered A, B, and C, the commutator section between A and B is numbered 1, that between A and C is No. 2, and the one between C and B is No. 3. Then the inside terminal of B is connected to the outside terminal of A and soldered to the end of commutator section No. 1 close to the winding. The inside end of B is connected to the outside terminal of C and to commutator section No. 2. The inside end of winding C is connected to the outside of B and to commutator section No. 3. The connection of the armature windings to the commutator are represented by the diagram in Figure 139.

The Field Winding consists of five layers of No. 18 B. & S. double cotton covered wire. A much neater job may be made of this part of the work if two fibre heads are cut to slip over the field and support the ends of the winding as shown in the illustration in Figure 133.

The Bearings are illustrated in Figure 140. They are made out of three-eighths inch brass strip one-sixteenth of an inch thick by bending and drilling as shown in the illustration. The location of the holes is best understood from the drawing. The larger bearing is assembled on the field at the side towards the commutator.

Assembling the motor is a comparatively easy matter if it is done properly and carefully. The bearings are mounted on the field frame by screws passing through the holes B and B into a nut on the outside of the bearing at the opposite side of the field.

The armature should revolve freely without binding and without any danger of scraping against the field. Slip some small fibre washers over the ends of the shaft between the armature and the bearings so as to take up all end play.

The Brushes are made of spring copper according to the shape and dimensions shown in Figure 141. They can be cut out with a pair of snips.

Each brush is mounted on a small fibre block supported on the large motor bearing. The holes marked A and C in the illustration should be threaded with a 4-36 tap. The hole B should be made one-eighth of an inch in diameter and drilled all the way through the block.

The holes A and C are used to fasten the blocks to the bearing. The brushes are fastened to the blocks by means of a 6-32 screw with a nut on the lower end.

The Base is a rectangular block, three inches wide, three and one-half inches long and three-eighths of an inch thick. The motor is fastened to the base by four small right angled brackets bent out of strip brass and secured to the field frame by two machine screws passing through the holes H and H into a nut at the opposite end.

One terminal of the field winding is connected to a binding post mounted on the base. The other terminal of the field is connected to the right hand brush. The end of the wire should be placed under the head of the screw which holds the brush to the fibre block. The brush should be on the under side of the block so that it bears against the under side of the commutator.

The left hand brush bears against the upper side of the commutator and is connected to a second binding post on the base of the motor. This makes it a "series" motor, that is, the armature and the field are connected in series.

The motor is now ready to run. Put a drop of oil on each bearing and make certain that the curved portion of the brushes bear firmly against the centre of the commutator on opposite sides. The armature having three poles, should start without assistance and run at high speed as soon as the current is applied. Two cells of dry or other battery should be sufficient. The motor may be fitted with a small pulley so that its power may be utilized for driving small models.