Doubtless there are many persons who would like to make an induction coil for medical use, but are deterred from so doing by the belief that the work is too difficult for any one but a skilled mechanic to undertake. This is a great mistake, however, as it is quite possible for almost anybody to make a coil that will give good results at a cost of but a few cents, and with the use of only the most ordinary tools.

For the core there may be used an iron bolt about three inches long and three-eights of an inch in diameter, as shown at A in the accompanying drawing. It is a good plan to soften the bolt by heating it red hot in a fire and allowing it to cool slowly. Make two thin wooden washers about an inch and a quarter in diameter, and glue them on the bolt to form a spool as shown at B, and cover the iron between the heads with a wrapping of two layers of paper glued on. The nut shown is not necessary, but makes a neat finish.

Handy man's workshop and laboratory 251

The first part of the winding, or primary coil, requires about half an ounce of No. 20 or No. 22 double cotton-covered magnel wire. Pass the end of the wire through a small hole in one of the heads, and wind on a smooth layer of the wire like thread on a spool. When the opposite head is reached wind a second layer of wire over the first one back to the place of beginning. Cut off the wire and pass the end through a second hole in the head near the first one, as shown at D. The excess of wire will be useful for connections.

The next part of the winding, or secondary coil, requires an ounce or two of No. 32 single cotton-covered magnet wire. Finer wire gives more powerful results because of the greater number of turns for a given weight, but it is rather delicate to handle. Before winding on any of this wire, glue on a wrapping of two or three layers of paper over the primary coil, to keep the two coils entirely separate. The secondary wire need not be wound in layers, though care is required to avoid injuring the insulation or breaking the wire by pulling it too tight. The two ends may be left projecting, as shown at H, for connection to two handles or electrodes, and the coil may be protected by a final wrapping of paper, as shown at C.

One pole of a dry battery E is connected to the tang of a large file F, and the other to one of the primary terminals D. The remaining primary terminal G is then lightly dragged along the surface of the file, thus making and breaking the circuit in rapid succession as the wire passes over the teeth. If the shocks received from the handles are too strong, use a longer piece of wire at G; if too weak, add another dry battery in series, or put more wire on the secondary.

Fig. 208 - A simple medical coil.