Arrange these laminations on the armature shaft so that when the shaft is in position the mass of iron will be within the lugs of the field-magnets. The holes through the iron plate should be so snug as to call for some driving to put them in place. Each disk of iron should be given a coat of shellac to insulate it, and between each piece there should be a thin cardboard or stout paper separator to keep the disks apart. These paper washers should be dipped in hot paraffine, or thick shellac may be used to obtain a good sticking effect and so solidify the laminations into a compact mass. When this operation is completed the armature core should appear as shown in Fig. 25. [Illustration: FIG. 22] [Illustration: FIG. 23] [Illustration: FIG. 24] [Illustration: FIG. 25] [Illustration: FIG. 26] [Illustration: FIG. 27] [Illustration: FIG. 29] [Illustration: FIG. 30] From maple, or other hard-wood with a close grain, make a cylinder three-quarters of an inch long and one inch in diameter to fit the shaft. Over this drive a piece of copper or brass tubing, and at four equal distances, near the rear or inner edge, make holes and drive small, round-headed screws into the wood. Then, with a hack-saw, cut the tube into four equal parts between the screws. This is the commutator. In order to hold the quarter circular plates fast to the cylinder, remove one screw at a time, and place thick shellac on the cylinder. Then press the plate firmly into place and reset the screw. Repeat this with the other three, and the armature will be ready for the winding. The voltage and amperage of a dynamo is reckoned by its windings, the size of wire, the number of turns, and the direction. This is a matter of figuring, and need not now concern the young electrician, since it is a technical and theoretical subject that may be studied later on in more advanced text-books. For this dynamo use No. 22 cotton-insulated copper wire for the armature, and No. 16 double cotton-insulated copper wire for the field. The armature, when properly wound and ready for assembling with the brushes and wiring, will appear as shown in Fig. 26. A small driving-wheel two inches in diameter and half an inch thick must now be turned from brass and provided with a V-shaped groove on its face. The hub, at one side, is fitted with a set-screw, so that it can be bound tightly on the shaft. This pulley is made fast to the shaft at the rear of the dynamo, and on the opposite end to where the commutator hub is attached.