The bearings are small brass castings of the dimensions shown in Figure 260. It is necessary first to make a wooden pattern and send it to the foundry for the castings. The bearings are fastened to the projecting arms on the field casting by means of machine screws eight-thirty-seconds of an inch in thickness. The field magnet should not be screwed down on to the base until the armature runs easily and truly in the tunnel. The brushes are made from thin gauge sheet-copper according to the shape and dimensions shown in Figure 261. They are bent at right angles and mounted on the base on either side of the commutator with small round-headed wood screws. The completed dynamo is shown in Figure 262. One end of the shaft is provided with a small pulley to accommodate a small leather belt. [Illustration: Fig. 261.—The Brushes.] The dynamo is connected as a "shunt" machine, that is, one terminal of the field magnet is connected to one of the brushes, and the other terminal to the other brush. A wire is then led from each of the brushes to a binding-post. A shunt 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. The dynamo just described should have an output of from 10 to 15 watts and deliver about 6 volts and 1 3/4 to 2 1/2 amperes. In order to secure current from the dynamo it will first be necessary to magnetize the field by connecting it to several batteries. [Illustration: THE JUNIOR DYNAMO MOUNTED ON A LONG WOODEN BASE AND BELTED TO A GROOVED WHEEL FITTED WITH A CRANK SO THAT THE DYNAMO CAN BE RUN AT HIGH SPEED BY HAND POWER. THE ILLUSTRATION ALSO SHOWS A SMALL INCANDESCENT LAMP CONNECTED TO THE DYNAMO SO THAT WHEN THE CRANK IS TURNED THE LAMP WILL LIGHT.] It will be found that the dynamo will also operate as a very efficient little motor, but that on account of having a two-pole armature it must be started by giving the shaft a twist. [Illustration: Fig. 262.—Complete Dynamo.] It can be used as a generator for lighting small lamps, electro-plating, etc., but cannot be used for recharging storage cells on account of having a two-pole armature. The dynamo may be driven with a small water motor or from the driving-wheel of a sewing-machine. Before the machine will generate as a dynamo, it must be connected to a battery and run as a motor. This will give the field the "residual magnetism" which is necessary before it can produce current itself. CHAPTER XVIII AN ELECTRIC RAILWAY No toys loom up before the mind of the average boy with more appeal to his love of adventure than do railway cars and trains. In England, the construction and operation of miniature railways is the hobby not only of boys but of grown men, and on a scale that is hardly appreciated in this country.