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. [Illustration: FIG. 144.—The Engine Base.] *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. [Illustration: FIG. 145.—Details of the Electromagnet Bobbin.] 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. [Illustration: FIG. 146.—Details of the Engine Frame.]