To most boys electricity offers many attractions, and as I have recently constructed an induction coil out of materials which are cheap and easily obtained, I think I shall confer a benefit on many readers if I give them a short description of how this was accomplished, so that if like-minded they can proceed in the same way. Induction coils may be used for medical and scientific purposes as well as for amusement, so that a good deal of work comes within their scope. An “induction coil” is composed principally of two portions—one is the “primary” coil, the other the “secondary.” It is the secondary coil that gives the spark, and on the length of this depends the power of the coil; in some instruments for scientific purposes it is composed of a wire nearly 300 miles long—but we are not going to soar to such heights as that!
To make the coil itself you want an ounce of “No. 24” cotton-covered wire, and two or three ounces of “No. 36.” This can be bought from an electrical supply dealer. If you are very ambitious, silk-covered wire can be used; this gives better effect, the insulation being more complete.
Fig. 1.—Front Disc.
AA, holes for primary wire. C, hole for core.
Fig. 2.—Back Disc.
BB, holes for secondary wire. C, hole for core.
To form the groundwork of the apparatus take a piece of mahogany about half an inch in thickness and polish it up to look ornamental; it should be about 4 inches by 6 inches for the sized coil I am describing. We now take another piece of mahogany about ¼ inch thick, and from it cut two circular pieces about 1½ inch in circumference; these are to form the ends of the coil; they must each have a hole ⅜ inch in diameter drilled in the center for the ends of the core to pass through. In one of them, which is to form the coil, two much smaller holes are drilled with a small bradawl to allow the ends of the primary coil to pass through (Fig. 1); in the other two similar holes are drilled further from the center for the ends of the secondary coil (Fig. 2). This having been done, we proceed to form the core, and this being the most important part of the instrument, it must be made with great care. Take a length of fine iron wire (annealed) and cut it into pieces 2½ inches long.