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".

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.

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.

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.

The Regulator Tube should be two and seven-sixteenths inches long and have an inside diameter of five-sixteenths of an inch. In case the experimenter has any difficulty in securing a tube of this size he can easily make one by rolling a piece of sheet brass around a suitable sized rod and soldering the edges.

When the tube is slipped over the core and moved in and out of the bobbin, after the coil has been finished and set in operation, it will regulate the strength of the secondary or "shocking" current. When the tube is completely out of the bobbin, the current will be the strongest.

If a paper scale, divided into sixteenths of an inch and bearing numbers, is pasted around the tube it will act as an indicator of the current strength.

The base is wood and measures four and five-eighths inches long, two and three-quarters inches wide and is one-half an inch thick. The proper location of the various holes can be found by referring to the illustration in Figure 103. The corners of the base should be slightly rounded so as to improve the appearance.

Figure 104 is a top view of the coil after it has been assembled. The bobbin is mounted a little back of the centre of the base so that the core cap will be opposite the armature of the vibrator.

The Vibrator Parts are illustrated in Figures 101 and 106.

The vibrator spring is a piece of thin steel or phosphor bronze, one-quarter of an inch wide and one and three-sixteenths inches long. A soft iron armature, five-sixteenths of an inch in diameter and one-eighth of an inch thick is fastened to one end of the spring. A five-thirty-seconds inch hole is drilled in the other end and it is then mounted in the "vibrator posts". This consists of a small brass pillar, one-quarter of an inch in diameter and five-eighths of an inch long. The lower end is drilled, and tapped to receive a 4-36 screw which fastens it to the base. A saw slot one-quarter of an inch deep is made in the top to receive the vibrator spring.

The post which holds the adjustment screw is identical with that which supports the vibrator spring, except that a threaded hole is drilled through the saw slot, fifteen-thirty-seconds of an inch from the bottom, through which the screw passes. The end of the adjustment screw, which may be an ordinary knurled thumbscrew from a binding post, should be fitted with a "contact" point. The best contact point is a piece of platinum, but a small piece of silver can be made to serve the purpose very well.

The vibrator spring should also be fitted with a similar contact point.

The medical coil is very easy to connect. Run the inside primary terminal to the binding post on the rear left hand corner of the base. Connect the other terminal to the vibrator post. A wire should be led from the adjustment screw post to the binding post mounted on the forward left hand corner of the base.

One or two cells of battery should be sufficient to operate the coil. Connect the battery to the two binding posts. Adjust the thumbscrew until the contact on the end just touches the contact point on the vibrator spring. The current will then flow through the primary winding and magnetize the core. The magnetism of the core will draw the armature towards it, thus pulling the contacts apart and interrupting the current. The process will repeat itself very rapidly at a speed depending upon the adjustment of the thumbscrew and as long as the battery remains connected.

The handles for the coil are very easily made out of a piece of brass tubing about two and one-half inches long and five-eighths of an inch in diameter. They can also be made by rolling a piece of sheet brass or tin into a tube and soldering the edges together. The illustration shows two methods of making the handles and attaching the cords. The handle in the upper part of the illustration is made from a piece of tubing. A wooden plug or cork is fitted into one end. The wire passes through a hole in the centre of the plug and is turned back over the edge so that when the plug is forced into the tube, the wire will make contact with it. The handle in the lower part of the illustration is made from one of the brass cylinders in which the indelible ink for rubber stamps is supplied. A hole is bored in the bottom through which the wire passes. The end of the wire is soldered to a piece of stiff brass wire bent into the shape of a hairpin. The ends of the wire are spread apart so that they will spring against the inside of the tube. The connector is then pulled up inside of the handle.

The flexible wires or "cords" as they are sometimes called, attached to the handles, should be about eighteen inches long. The opposite ends to the handles should be led through two holes in the base, in front of the bobbin. The left hand cord should be connected to the vibrator post and the right hand cord to the outside secondary terminal.

The coil is now complete, ready for operation. Connect the battery to the binding posts. Start the interrupter and grasp one of the handles in each hand. Draw the regulator tube out of the bobbin gradually and the current will gradually grow stronger and stronger until it is impossible to hold the handles. When the regulator tube is all the way in the bobbin it is practically impossible to feel the current. Of course any intermediate strength between the two extremes can easily be secured. The shock from the coil is harmless. When the regulator is all the way out, the shock is very strong. The only result, however, is to make the person receiving it drop the handles. The current from medical coils is often prescribed by physicians for rheumatism and nervous disorders, but it must be properly applied in order to be of benefit.