A shocking or medical coil is one of the most interesting pieces of electrical apparatus that can be made by the amateur. The one here described simply requires careful work and attention to the details of construction to operate well. Besides affording considerable entertainment to the owner and friends, it can be utilized to good advantage for medical treatment, when prescribed by a physician.

The base is of maple, walnut, or other suitable wood, 7" by 5" and 7/8" thick. The upper edge may be slightly beveled to improve the appearance. The wooden ends of the coil are 2" square and $-" thick. Holes are bored in the centers for the core and regulating tube as hereafter directed, and other small holes H, D, and F for wires, as shown in Fig. 1. These holes should be bored before putting the parts together.

The core C is made of enough lengths of No. 22 gauge, soft, annealed iron wire 35/8" long to give a diameter of 1/4". Particular care must be taken with the wire to make sure that it is all well annealed. If any doubt exists about this point, the wire when cut should be tied into a bundle with some surplus wire and heated to a bright red, and allowed to cool very slowly in hot ashes. The rough surface that results will improve the working of the core. To make the core, gather the wires into a bundle as round and compact as possible, and wind with strong twine, leaving 3/4•" exposed at each end.

A Medical Coil 84

Figure 1.

Solder the ends firmly together for 3/8", and then file smooth on the ends and around the outside of the soldering. One end is then firmly fitted to a piece of thick brass tubing B, which should, in turn, tightly fit the insulating tube I.

The regulator R consists of a piece of brass tubing 3/8" outside diameter, and as thin as can conveniently be obtained. It should slip over the core without touching. A handle K is fitted to one end ; this may be made out of anything except metal. A part of the end of an old clothes-brush will answer nicely. It should go inside the tube for about 3/8" and be fastened in place by punching two dents into the tube, one on either side, or glued. The regulator is fitted to a hole bored in E', with enough play to allow it to be readily drawn in and out, but without being loose enough to drop onto the core. A fret-saw and file will be found convenient in making these holes if the worker is not equipped with the necessary bits. The insulating tube I, 4" long, is then made.

This can be nicely done on the regulator R by fii st coating it with a layer of soap to facilitate the removal of the tube when completed. Several layers of smooth writing-paper are wrapped around the regulator tube, until the outside diameter almost equals that of the brass cap B on the end of the core. Over the writing-paper wrap five or six layers of smooth manila wrapping-paper, giving each layer, except the last, a coating of thin glue. This is then allowed to thoroughly dry, then removed from the regulator, and the writing-paper lining taken out. It should then loosely fit the regulator, so that the latter can be easily removed. When the insulating tube is quite dry it will form a firm tube on which to wind the wire. Holes are now cut in the end pieces E and E' of a size to allow this tube to be inserted with a tight fit, and well glued to prevent it from working loose. The tube is then thickly coated with melted paraffin, laid on with a brush, the joints at the ends being given an extra quantity to insure complete insulation from the core.

During the intervals while the glue is drying, a winding frame, Fig. 2, can be made. The base A is a piece of wood 15" x 10" and 7/8" thick. The posts B and B' are 2" wide, 7/8" thick, and 97/8" long, allowing 7/8" for tenons to fit mortises in base A. The mortises for the posts are 11/2" x 7/8" and are placed in the center of the base, 1" from each end. One inch from the top of each post a 1/4" hole is bored ; in post B', clear through the post; in the other, leaving 1/4" of wood to prevent the winding rod from slipping through. In the post B' saw down from the top to each side of the hole, making a slot which allows the winding rod to be put in or taken out without having to adjust the work. About 27/8" from the bottom end of each post bore 1/4" holes for the rod C, upon which the wire feed spool is run. The rod D is 21" long, with two 3" turns on one end to form a crank. A small hole is bored in the upper part of the post II to receive a wire nail N, which holds down the winding rod. Two large thread spools are beveled off to form supports for the coil-frame, as shown at E.

A Medical Coil 85

Figure 2.

An extra spool running loose is put on.the rod between the coil-frame and the post B' to prevent the rod from being displaced while winding. A hole is bored in the post B' in which may be inserted a short piece of iron rod, U-shaped on the end, to prevent the rod from turning when so desired.

Previous to commencing to wind the coil, look the frame over carefully to see that the paraffin coating is without air holes and cracks. If any are found, they should be filled up. It is then mounted on the rod D. First, put on the loose spool and then one of the tapered spools E, then the coil-frame, then the other tapered spool, the tapers being firmly centered in the holes in the end of the coil, and kept in position by paper bushing, wrapped over the rod D at the proper places. The rod and coil-frame are placed in the winder and the primary coil wound. This consists of four layers of No. 22 cotton-covered magnet wire. The end is first passed through the hole H, which ends nearest the tube, allowing about 8" extra length for connections. It is held by a temporary wedge in the outside end of the hole. By turning the crank with one hand and holding the wire between the fingers of the other, the wire can be evenly and solidly wound. After each layer is in position, a coating of thin shellac, cut in alcohol, is put on. As it dries very quickly, but little time will be required, and in addition by being well insulated, the wire will wind more evenly. The end of the fourth layer will be at the same end of the coil as the first layer, and should be carried through the other hole H, with about 8" spare wire for connections. These spare ends may, during the winding of the secondary coil, be twisted around a pencil into a small spiral.