A small shocking coil, suitable for medical purposes, may be constructed of materials found in nearly every amateur mechanic’s collection of odds and ends.  The core, A, Fig. 1, is a piece of round soft iron rod about 1/4 in. in diameter and about 4 in. long.  A strip of stiff paper about 3/4 in. wide is covered with glue and wrapped around one end of the core, as shown at B, until the diameter is about 3/8 in.  The portion of the core remaining uncovered is then wrapped with a piece of paper about 4 in. wide.  No glue is used on this piece, as it is removed later to form the space, C, after the paper shell, D, has been wound upon it.  This paper shell is made of stiff paper and glue the same as B and is made about 3/64 in. thick.  Two pieces of hardwood, Ee, 1-3/4 in. square and about 5/16 in. thick, are drilled in the center and glued on the ends of the paper shell as shown.

The primary winding consists of 4 or 5 layers of No. 18 or 20 single cotton-covered magnet wire, the ends of which may be passed through small holes in the wooden ends.  If a drill small enough is not available, the holes may be made with a hot knitting needle or a piece of wire heated to redness.  After the primary coil is wound it should be thoroughly insulated before winding the secondary.  This may be done by wrapping with 4 or 5 thicknesses of paper.

The secondary coil should be wound with single covered wire, preferably silk-covered, although cotton will do.  The more turns there are on the secondary the higher the voltage will be, so the wire used must be fine.  Number 32 to 36 will give good results, the latter giving more voltage but less amperage.  Each layer of the secondary winding should be insulated from the others by a piece of thin paraffined paper wrapped over each layer as it is finished.  It is well not to wind to the extreme ends of the paper insulations, but to leave a space of about 1/8-in. at each end of the winding to prevent the wires of one layer slipping over the ends of the paraffin

[Illustration:  Induction Coil]

paper and coming in contact with the layer beneath, thus causing a short circuit.  The secondary winding should have at least a dozen layers and should be carefully wound to prevent short circuiting.

In order to reduce the strength of the current a piece of brass tubing, F, is pushed into the space, C, surrounding the core, or if no brass tubing of the required size is on hand, roll a paper tube, cover with 4 or 5 thicknesses of tinfoil and then wrap with more paper, using glue to hold the tinfoil in place and to keep the tube from unwinding.  When the tube is pushed all the way in, the current produced

[Illustration:  Induction Coil]

will be almost unnoticeable, but when it is withdrawn the current will be so strong that a person cannot let go the handles until the coil is shut off.  After the secondary coil is wound it should be covered with stiff paper, and the whole coil, including the wood ends, should then be enameled black.