CHAPTER X. MEDICAL COILS AND SHOCKING COILS.
Medical coils and "shocking coils" are the same. The term "shocking coil" is usually applied to the low priced medical coils which most boys are familiar with for giving "shocks".
[Illustration: FIG. 98.—Bobbin for Medical Coil.]
A medical coil is nothing more or less than a small induction coil for raising the voltage of a battery high enough to produce a shock. The human body possesses so much resistance that the voltage of one or two cells of ordinary battery is not sufficient to pass enough current through to be felt except under exceptional conditions, as for instance when contact is made directly with a nerve through an open cut or a metal filling in a tooth.
A small lathe will be of much assistance in making some of the parts and fittings for the medical coil, although it is not absolutely essential and the ingenious young experimenter can undoubtedly find some method of doing without it. It is not necessary to follow the dimensions given here exactly, and a very good coil can be built by adapting spools, binding posts, etc., which may already be on hand, although, of course, they should be used with some sense of proportion in mind so that nothing is carried to an extreme.
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.