[Illustration: Fig. 255.—Details of the Field Casting.] For these reasons, I have described below a small dynamo of about ten watts output, the castings for which can be purchased from many electrical dealers with all machine work done at an extremely low price. The field magnet shown in Figure 255 is drawn to scale and represents the best proportions for a small "overtype" dynamo of ten to fifteen’ watts output. The dimensions are so clearly shown by the drawings that further comment in that respect is unnecessary. The armature is of the type known as the "Siemen’s H." It is the simplest type of armature it is possible to make, which is a feature of prime importance to the beginner at dynamo construction, although it is not the most efficient form from the electrical standpoint. The armature in this case is also a casting and therefore a pattern is required. [Illustration: Fig. 256.—Details of the Armature Casting.] The patterns for both the field and the armature are of the same size and shape as shown in Figures 255 and 256. They are made of wood, and are finished by rubbing with fine sandpaper until perfectly smooth and then given a coat of shellac. The parts are also given a slight "draft," that is, a taper toward one side, so that the pattern may be withdrawn from the mould. The patterns are turned over to a foundry, where they are carefully packed in a box, called a "flask," full of moulder’s sand. When the patterns are properly withdrawn, they will leave a perfect impression of themselves behind in the sand. The mould is then closed up and poured full of molten iron. When the iron has cooled the castings are finished except for cleaning and boring. The shaft is a piece of steel rod, three-sixteenths of an inch in diameter, and four and one-half inches in length. The portion of the field into which the armature fits is bored out to a diameter of one and five-sixteenth inches. Considerable care is necessary in performing this operation in order not to break the field magnet apart by taking too heavy a cut. [Illustration: Fig. 257.—Details of the Commutator.] The armature should be turned down to a diameter of one and one-quarter inches or one-sixteenth of an inch smaller than the tunnel in which it revolves between the field magnets. The center of the armature is bored out to fit the shaft. Figure 257 shows a two-part commutator for fitting to an armature of the "Siemen’s H" type. It consists of a short piece of brass tubing fitted on a fiber core and split length-wise on two opposite sides, so that each part is insulated from the other. The fiber is drilled with a hole to fit tightly on the shaft. It is then placed in a lathe and turned down until a suitable piece of brass tube can be driven on easily.