The details of the wooden base are shown in Figure 128. It is a rectangular shaped piece of wood, five inches long, four inches wide and five-eighths of an inch thick. The corners are slightly rounded. *The Bearings* are small brass castings. They are both alike. The details are illustrated in Figure 129. It will be necessary to make a wooden pattern and send it to a brass foundry for castings. The castings should be smoothed up with a file and then drilled. The shaft hole should be three-sixteenths of an inch in diameter and the screw holes just large enough to pass an 8-32 screw. The bearings are fastened to the projecting arms on the field casting by means of round headed 8-32 brass machine screws. The armature should revolve exactly in the centre of the tunnel in the field and should be free in the bearings so that it runs easily and without binding. *The Brushes* are illustrated in Figure 131. They are cut out of spring copper and bent according to the shape and dimensions shown. The field is fastened to the base by means of two large machine screws passing upwards through the base into threaded holes in the bottom of the casting. The brushes are bent at right angles and mounted on the base on either side of the commutator with small round headed wood screws. They should bear firmly against the commutator. The commutator should be in such a position on the shaft, in relation to the armature, that the dividing lines between the two sections are directly opposite the centre of the iron faces of the armature as shown in Figure 127. [Illustration: FIG. 129.—Details of the Bearings.] [Illustration: FIG. 130.—The Pulley.] [Illustration: FIG. 131.—The Brushes.] The shaft should be fitted with a small grooved pulley to accommodate a small round belt. The completed dynamo is shown in Figure 132. The dynamo is connected in what is known as "shunt." One terminal of the field magnet is connected to one brush and the other terminal to the other brush. A wire is then led from each one of the brushes to a binding post. Before the dynamo will generate current it will be necessary to magnetize the field by connecting the terminals to several strong batteries and allowing the current to flow through for several seconds. A shunt wound dynamo will only generate when run in a certain direction. In order to make it generate when run in the opposite direction, it is necessary to reverse the field connections to the brushes. [Illustration: FIG. 132.—The Completed Dynamo.] The dynamo will be found to operate as a very powerful little motor, but on account of having only a two pole armature, it must be started when the current is turned on by giving the shaft a twist. The dynamo may be driven by a small water motor or from the driving wheel of a sewing machine. It may be used as a generator for lighting lamps, ringing bells, electroplating, etc. CHAPTER XIII. AN ELECTRIC BATTERY MOTOR.