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.

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.

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 The parts should also be given a slight "draft" or taper toward one side so that the pattern may be easily withdrawn from the mould.

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.

It is not very difficult to make such a commutator. A hole is drilled through the fibre, which fits very tightly on the shaft. The shaft is then placed in a lathe with the fibre in position and the latter turned down until a piece of seven-sixteenths inch brass tubing can be driven on. The tube should be five-eighths of an inch long. Then mark two lines along the tube at points diametrically opposite. Bore two small holes to receive two small screws, a short distance away from each side of these lines and on each side of them. Make certain that the screws do not go into the fibre far enough to touch the shaft. The commutator may then be split along each side of the lines with a hacksaw, continuing the cut right through the brass and slightly into the insulating core. The heads of the screws should be filed off flush with the surface of the commutator and the latter trued up and made perfectly smooth.

If each section of the commutator is provided with a small brass machine screw near the back edge as shown in Figure 126, it will greatly facilitate connection with the ends of the armature windings.

The armature, shaft and commutator, as they should appear when assembled are shown in Figure 127.

Those portions of the armature and shaft which will come into contact with the armature wire should be insulated with shellaced paper. Soak the paper in the shellac until it is soft and it can be very easily pressed into proper shape to fit the armature. Allow the shellac to dry and harden before winding on the wire.

The armature will not need to be fastened on the shaft if it is a tight fit and cannot be twisted. If it is loose, it may be fastened by means of a small set screw or pin. The commutator should fit the shaft very tightly so that it will not slip or twist.

The Armature Winding is No. 20 B. & S. Gauge single-cotton covered magnet wire. Sufficient wire should be put on to fill up the winding space completely. Do not, however, put on too much wire or it will interfere with the field magnets and prevent the armature from revolving. Test the winding after it is finished to see that the wire is not "grounded" or connected to the armature at any point. If the insulation is perfect, give the winding a good coat of shellac and allow it to dry. The ends of the winding are each connected to one of the commutator sections as shown in Figure 127.

The Field Winding is No. 20 B. & S. Gauge single cotton covered wire. The wire should be wound on in smooth, even layers, and the winding space between the flanges completely filled up. The winding space in the field frame should be insulated with shellaced paper by covering the core and the flanges. The flanges are best insulated with paper disks cut in two halves so that they will slip around the core.

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.

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.

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.