About four ounces should be enough for this armature, and the ends are to be passed through small holes in the brass end (B); see Fig. 11. One end must be soldered to one commutator, the other end to the other commutator. The end-piece (B) is attached to the iron armature (A) with machine-screws; then C is to be made fast in a similar manner. When putting the parts together, it would be well to use some shellac on the wooden cylinder and driving-wheel to make them hold to the shaft. By following the plan in Fig. 10, it will be an easy matter to put the parts together; when they are assembled the complete machine will appear as shown in the drawing (Fig. 12). The driving-wheel should be of wood five-eighths of an inch thick and six inches in diameter, and held in the frame of wood and metal brackets by a bolt. A short handle can be arranged with which to turn the wheel, and a small leather belt will transmit the power to the small wheel on the armature shaft. As the armature is revolved the lines of force are cut and the current is carried out through the wire attached to the binding-posts on the blocks (G G). [Illustration: FIG. 12] Considerable current may be generated if the armature is driven at higher speed than the hand-wheel will cause it to revolve. This can be accomplished by running the belt over a larger wheel, such as the fly-wheel of a sewing-machine, or connecting it to a large pulley on a water-motor. The latter may be attached to a faucet in the wash-tub if there is pressure enough to do the work. A Small Dynamo All dynamos are constructed on the same general principle as that of the uni-direction machine just described; but they differ in their windings, the quantities of metal electrified, the sizes and lengths of wire wound on both armature and field, and in their shape and speeds. In large dynamos it is impossible to employ steel magnets of the required size. In place of them soft iron cores are used and magnetized by external electric current; or the wiring is done in “series” or “shunt,” so that the fields will be self-exciting once the machine has been properly started.