The first part which will be required is the bobbin. This is shown in Fig. 98 and consists of a wooden spool, two and seven-sixteenth inches long and one inch in diameter. It has a three-eighths inch hole running all the way through the center. The dimensions of the flanges, etc., can probably be best understood by referring directly to the illustration. The flanges are flattened slightly on one side so that when the bobbin is mounted the flat portion will rest on the base and give a better support. [Illustration: FIG. 99.—Bobbin with Winding.] *The Primary Winding* consists of two layers of No. 24 B. & S. Gauge single cotton covered wire. The terminals of the winding should be led out through two small holes in the right hand head of the bobbin. Cover the winding with two or three layers of thin shellaced paper and then wind on the secondary. The secondary is composed of ten layers of No. 36 B. & S. Gauge single cotton covered wire. The inside end of the secondary should be connected to the outside end of the primary. The outside terminal is led out through a small hole in the head at the opposite end from the primary leads. The wire on both the primary and secondary should be wound on in smooth even layers. The illustration in Figure 99 shows the bobbin after the windings are finished. *The Core* is made out of thin sheet iron rolled up into a cylinder two and three-sixteenths inches long and nine-thirty-seconds inches in diameter. The core should be rolled up just as tightly as possibly, because the more iron there is in the core, within of course reasonable limit, the better the coil will work. Thin tin plated iron cut from a cracker box may be used for the core. The thinner it is the better. A core of this sort, rolled up out of sheet iron will give much better results than a core composed of a solid bar of iron. [Illustration: FIG. 100.—Construction of the Core.] The end of the core is slipped into an iron "core" cap. This is shown in Figure 101. It consists of a small cylinder, seven-sixteenths of an inch in length, cut from a piece of iron rod, three-eighths of an inch in diameter. A hole nine-thirty-seconds of an inch in diameter and five-sixteenths of an inch deep should be drilled in one end. This cap is slipped over one end of the core. It should fit very tightly so that it will not pull out. The core is then slipped into the bobbin with the outside end of the core cap projecting about one-sixteenth of an inch out of the end at which the primary terminals lead out. The cap should fit into the bobbin very snugly so that the core is held firmly in place. Since the hole through the bobbin is three-eighths of an inch in diameter and the core is only nine-thirty-seconds of an inch in diameter there should be a space all around the core, except at the end where the cap is, so that a thin brass "regulator" tube can be slipped in and out over the tube. [Illustration: FIG. 101.—Vibrator Parts and Core Cap.]