The Core is made of soft iron "core wire" about No. 20 or 22 B. & S. Gauge. Each piece should be four and three-quarters inches long. Iron wire may be bought already cut to length from various houses dealing in experimenter’s supplies. Considering the amount of labor that is required to cut each piece to length arid then straighten it out, it is cheaper to purchase the wire already cut. [Illustration: FIG. 110.—The Primary and Core.] ────────────────────────────────────────────────────────────────── Size of Coil. O Length of P Diameter of Primary Wire Core Core ────────────────────────────────────────────────────────────────── 1/4 inch 4 3/4 inches 1/2 inch 18 B. & S. ────────────────────────────────────────────────────────────────── 1/2 inch 5 inches 5/8 inch 18 B. & S. ────────────────────────────────────────────────────────────────── 1 inch 7 inches 5/8 inch 18 B. & S. ────────────────────────────────────────────────────────────────── "Core wire" is a special grade of wire which is very soft and has been annealed. If ordinary iron wire is used in making a core, it must first be annealed. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire until it is red hot. After the wire is hot, cover it with ashes and allow the fire to die away. When the fire has gone out and the wire has cooled the annealing process is complete. Enough wires each four and three-quarters inches long to form a compact bundle one-half inch in diameter will be required. [Illustration: FIG. 111.—The Secondary Winding.] ─────────────────────────────────────────────────────────────────── L Size L Length of M No. of M Size No. of Turns Wire of Coil Secondary Sections of Hole per Section ─────────────────────────────────────────────────────────────────── 1/4 in. 2 3/8 in. 1 1 1/18 5,000 37 in. en. ─────────────────────────────────────────────────────────────────── 1/2 in. 2 3/8 in. 1 1 1/8 10,000 37 in. en. ─────────────────────────────────────────────────────────────────── 1 in.¹ 2 2/8 in. 1 1 1/8 10,000 37 in. en. ─────────────────────────────────────────────────────────────────── ¹ There are two sections of secondary used on the 1-inch coils. Cut a piece of tough wrapping paper about six inches long and four and one-half inches wide. Roll it up in the form of a tube so that it is six inches long and one-half inch in diameter inside.