The Shaft is formed by a piece of knitting-needle, one and seven-eighths of an inch long. Assemble the three pieces, forming the armature, on the shaft as shown in Figure 21. Bind them temporarily together with a piece of iron wire and then solder them along the edges. The iron wire should be removed after they are soldered.
The Commutator Core is formed by cutting a strip of paper, three-eighths of an inch wide and about five inches long. It should be given a coat of shellac on one side and allowed to dry until it becomes sticky.
FIG. 21.—The Three-pole Armature assembled on the shaft.
The strip is then wrapped around the shaft until its diameter is three-sixteenths of an inch. The sticky shellac should be sufficient to hold the paper tightly in position when dry and to form a hard, firm core.
The illustration in Figure 22 shows the position of the core on the shaft in relation to the rest of the armature.
The Winding of the Armature may seem somewhat more difficult at first than was the case with the two-pole armature, but it is really very easy. The wire used for this purpose should be No. 25 or No. 26 B. & S. Gauge, double cotton-covered. Single cotton-covered wire for this purpose is liable to give trouble on account of short circuits.
In order to wind the armature, cut three pieces of wire about three and one-half feet long. Wrap a strip of paper around each section of the armature so that the sharp edges of the tin will not cut through the insulation on the wire and then wind four layers of wire on each section of the armature.
FIG. 22.—Showing the Armature and Shaft with the Commutator Core in position.