The shafts should be of hard brass or of steel. The one at the front should be one inch and a half in length, and that at the rear six inches long, measuring from the outer face of the brass end to the end of the shaft. From boxwood or maple turn a cylinder three-quarters of an inch in diameter and an inch long, with a quarter-inch hole through it. Over this slip a piece of three-quarter-inch brass or copper tubing that fits snugly, and at opposite sides drill holes and drive in short screws that will hold the tube fast to the hub. They must not be so long as to reach the hole through the centre. Place this hub in a vise, and with a hack-saw cut the tube across in two opposite places, so that you will have the cylinder with two half-circular shells or commutators screwed fast to it, as shown at Fig. 7. This hub will fit over the shaft at the front end of the armature, and will occupy the position shown at F in Fig. 10. Cut two small blocks of wood for the brushes and binding-posts, and bore a hole through them, so that the foot-screw of a binding-post may pass through the block and into the post, as shown at Fig. 8. From thin spring copper cut a narrow strip and bend it over the block, catching it at the top with a screw and lapping it under the binding-post at the outside. From boxwood or maple have a small wooden pulley turned, with a groove in it and a quarter-inch hole through the centre. This pulley should be half an inch wide and one inch and a half in diameter, as shown at Fig. 9. This is to be attached at the end of the long shaft, where it will occupy the position shown at E in Fig. 10. All the parts are now ready for assembling except the armature, which must be wound. Before laying on the turns of wire the channel in the iron must be lined with silk, held in place with glue or shellac. A band of silk ribbon is given two turns about the centre of the iron, and the sides are so completely covered with silk that not a single strand of wire will come into direct contact with the iron. Great care must be taken, when winding on the wire, not to kink, chafe, or part the strands. The channel should be filled but not overcrowded, and when full several wraps of insulating tape should be made fast about the armature to hold the wire firmly in place and prevent it from working out at the centre when the armature is driven at high speed. The armature, when properly wound and wrapped, will appear as shown at A in Fig. 11, and it is then ready to have the ends screwed on. Several sizes of wire may be used to wind the armature, according to the current desired, but for general use it would be well to use No. 30 silk-insulated copper wire. [Illustration: FIG. 10 FIG. 11 PLAN OF THE UNI-DIRECTION DYNAMO]