It is very difficult to construct a first class dynamo without resort to the materials and methods employed in the manufacture of commercial machines. The necessity for careful workmanship in every detail, can hardly be overestimated. Poor workmanship and poor materials will always result in an inefficient machine. Telegraph instruments, toy motors, etc., may be constructed from all sorts of odds and ends of materials, and they will work fairly well, but in order to make a real dynamo it will be necessary to use certain materials for which nothing can be substituted. [Illustration: FIG. 123—Front view of the Field Casting.] The field casting must be soft gray cast iron and the magnet wire should be of good quality. Both the field and the armature of the small dynamo described in the following pages are cast iron and patterns will be necessary in order to form the moulds for the castings. It may be possible for several experimenters to club together and make the patterns, or have them made, for building this dynamo. The dynamo described has an output of about ten watts. It may also be used as a motor and as such will develop considerable power. The castings for this machine are already on the market and may be obtained from the publishers of this book. [Illustration: FIG. 124.—Side elevation of the Field Casting.] *The Field* is shown in Figures 123 and 124. The details in both illustrations are fully dimensioned and probably no comment in that direction will be necessary. If the experimenter decides to make his own patterns he should use every care to make certain that they are carefully and accurately made. They should be made of wood and finished by rubbing with fine sandpaper until perfectly smooth and then given a coat of shellac. The parts should also be given a slight "draft" or taper toward one side so that the pattern may be easily withdrawn from the mould. [Illustration: FIG. 125.—Details of the Armature.] The easiest way to bore out the "tunnel" of the field is to perform the work on a lathe. If no lathe is handy, the work can be accomplished with nothing more than the aid of a file and a little patience. It should be cleaned out until it is perfectly round and measures one and five-sixteenths of an inch in diameter. Any rough spots on the casting should be smoothed up with a file. *The Armature* is illustrated in Figure 125. The centre of the armature should be bored out to fit a three-sixteenths inch shaft. The shaft is a piece of steel rod four inches long. The outside of the armature should be turned down to a diameter of one and one-quarter inches, making it one-sixteenth of an inch smaller in diameter than the tunnel in the field. *The Commutator* is illustrated in Figure 126. It has two sections and consists of a short piece of brass tubing fitted on a fibre core and split lengthwise on two opposite sides so that each section is insulated from the other.