*The primary* consists of two layers of No. 18 B. & S. gauge cotton-covered wire wound over the core for a distance of five inches. One-half pound of wire is more than enough for one primary. The wire must be wound on very smoothly and carefully. In order to fasten the inside end so that it will not become loose, place a short piece of tape lengthwise of the core and wind on two or three turns over it. Then double the end back and complete the winding. After the first layer is finished, give it a coat of shellac and wind on the second layer. The end of the wire is wound with a piece of tape and fastened by slipping through a loop of tape embedded under the last few turns. The illustrations will explain more clearly just how this is accomplished. The second layer is then given a coat of shellac and allowed to dry. After it is dry, wrap about fifteen layers of paper which have been soaked in paraffin around the primary. This operation should be performed in a warm place, over a fire or lighted lamp where the paraffin may be kept soft, so that the paper will go on tightly. [Illustration: Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.] The coil is now ready to receive the secondary winding. The core and primary which have been described are suit-able for a secondary giving sparks from one-half to three-fourths of an inch long. *The secondary* winding consists of several thousand turns of very fine wire wound on in smooth even layers with paper between each two layers. The following table shows the size and amount of wire required. In addition, about two pounds of paraffin and a pad of linen paper or typewriter paper will be required. The wire may be either enamel, cotton, or silk insulated. Single silk-covered wire is preferred. ────────────────────────────────────────────────────────────────── SIZE OF COIL SIZE OF WIRE AMOUNT ────────────────────────────────────────────────────────────────── 1/2 inch 36 B. & S. 10 ounces ────────────────────────────────────────────────────────────────── 1 inch 34 B. & S. 1 lb. ────────────────────────────────────────────────────────────────── 1/2 inch 34 B. & S. 2 lbs. ──────────────────────────────────────────────────────────────────