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.]
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L Size L Length of M No. of M Size No. of Turns Wire
of Coil Secondary Sections of Hole per Section
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1/4 in. 2 3/8 in. 1 1 1/18 5,000 37
in. en.
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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.
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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.
Glue the inside and outside edges of the paper so that the tube cannot unroll and then slip the core wires into it until the tube is packed tightly and no more can be slipped in.
The core is now ready for the primary, which consists of two layers of No. 18 B. & S. Gauge cotton covered wire wound over the core for a distance of three and three-quarters inches. The winding should start five-eighths of an inch from one end of the core. The wire must be wound very smoothly and carefully. The inside end of the wire can be fastened so that it will not become loose by placing a short piece of tape lengthwise of the core and winding two or three turns over it. Then double the end back and complete the winding over it. The end of the wire can be fastened by imbedding a piece of tape under the winding with a small loop projecting and then passing the wire through the loop.
Figure 110 shows the complete core with primary winding in position.
*The Secondary* is a much more tedious job than the primary and must be very carefully made. Whenever it is possible for the experimenter to purchase a secondary already wound he is advised to do so.
The secondary consists of 5,000-6,000 turns of No. 37 B. & S. Gauge, enameled wire wound in smooth, even layers, with two layers of paper between every two layers of wire.