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.

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 ? 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.

Now take a brass tube of the same size internally as the center holes in the ends of the coil were made (? inch) and push as many pieces of wire into it as are required to pack it as full as it will hold. The next thing to do is to take another piece of wire and wind it as tightly as possible round the ends of the wires, pulling them gradually out of the tube as you wind, until they are entirely out, by which time a compact bundle of iron wire will have been formed. Now file the ends of the core thus formed, quite smooth, with a fine file, and drop the whole of it, wire and all, into the hottest part of a fire. Leave it there till it is bright red hot all through, and then rake it out and bury it completely in the ashes under the grate. If this can be done over night, and the coil left to get cold as the fire goes out, instead of being placed in the ashes, so much the better, as the object is to cool it as gradually, and thus make it as soft as possible.

When it has become perfectly cold take some paraffin wax and melt it in a dish. When thoroughly melted, heat the core again gently, and put it into the melted wax. Leave it there for a short time till it is thoroughly saturated with the melted wax, then take it out and hold it above the dish to let the melted paraffin run back into it. When cold you may remove the binding wire, and the wax will be found to hold all the pieces together in a solid lump. The two pieces of wood must now be fixed one at each end of the core (the holes being the same size as the bore of the brass tube, the core should fit into them quite tight), one of them (the front) being pushed a little distance over the core, so as to leave about ¼ of an inch of the core projecting from it, the other one only being pushed on sufficiently far to make the end of the coil flush with the wood (Fig. 3).

Take a sheet of thin notepaper and cut a piece exactly the width of the coil, and long enough to pass twice round it. Wind it tightly round, and fasten it, if necessary, with a little paraffin. Now the wire has to be wound on over the paper, the thickest first, to form the primary coil. Pass about three inches of one end of it through one of the holes in the disc forming the front of the coil, and then wind it evenly on the core, taking care that each coil is separate from its neighbor, and that no two coils fall one upon the other.

When the wire has reached the other end of the core, wind it back again over the first layer till it reaches the end it came in at, then pass it through the other hole and cut it off about three inches from the hole; the wire cut off will be wanted for other purposes. The secondary coil has now to be wound over the primary, first of all saturating the cotton with which the latter is covered by pouring melted paraffin over it with a spoon. All the secondary wire will be wanted; it must be wound layer above layer exactly as the primary was, first passing about three inches of the end through one of the holes in the disc at the back of the core. A thickness of notepaper should be put on between the primary and secondary coils. Everything depends on the complete insulation of one coil from another, and this is accomplished by means of the notepaper and cotton, saturated with melted wax in subsequent operations. When the whole of the secondary wire is wound on except about three inches, pass the end through the other hole in the disc.

In order to make sure that the wire has not been broken in the winding, which would entirely destroy the action of the instrument, the two ends of the coils should be joined separately with a battery and galvanometer. If the needle is deflected on joining the circuit the wire is all right. This is rather important, as it is extremely vexatious, when all the different parts have been adjusted, to find that the coil will not work owing to a fracture of the wire, which necessitates the whole coil being unwound before it can be discovered. If the galvanometer is not at hand we must take our chance; the greatest possible care must be taken in winding the secondary wire, as this thin wire is extremely brittle. The insulation must now be improved by plunging the whole coil into a deep vessel large enough to contain it, which is full of melted paraffin. This must be placed near the fire, so as to keep the wax melted, and the coils must be left in it to soak for an hour or two. When the paraffin has thoroughly permeated through it it can be taken out and held above the vessel to drain. If all the wax does not run off the ends they can be scraped afterward, taking care not to cut the wires. The appearance of the coil is vastly improved by a strip of velvet cut the right width, which can be drawn tightly and sewn in position; or the coil may be covered with a varnish made by dissolving red sealing-wax in spirits of wine by the aid of a gentle heat. The coil part of the instrument is now complete, and ready to be affixed to the base-board by means of two small screws passing through it into the discs when placed in the proper position (see Fig. 6.)

We now approach a very important and rather intricate piece of workmanship. It is necessary, in order that shocks should be obtained from the coil, that the current in the primary wire should be stopped and started again at the rate of several hundred times per minute, and the more quickly the contact between the battery wire and the primary coil is made and unmade the more powerful the shock. In order to accomplish this a “contact-breaker” becomes necessary, the method of making which is as follows:

A piece of sheet brass is taken 1½ inches long by about ? of an inch at one end, gradually tapered up till it comes to a point about ? of an inch broad at the other; it must be very thin, and must act as a spring when fastened tightly at one end. A small piece of soft iron is soldered to the small end of this to be attracted by the core when working. The next thing is to fasten a small piece of platinum foil about ¼ of an inch square on the opposite side of the brass to the soft iron, and a little below it (Fig. 4). This is rather a difficult operation, as it is such a small object to solder, and the best way is to get it done by a tinsmith, unless you are skilled in the use of the soldering bit.

A narrow strip of stout brass is now taken and bent at right angles near one end, so that when screwed down to the base-board by holes in the smallest leg the longest leg will stand upright. Stand it up on the base in front of the coil and note a point on the strip exactly opposite the core. Make a hole through this point large enough to admit a small screw used on paper fasteners. Now take the flange part of the paper-fastener and solder it to the back of the brass strip, so that the screw will work through both (Fig. 5). This is done to avoid the trouble of making a flange in the strip, but if you can do this, so much the better.

Now, the coil having been fastened to the base by fine screws through it into the ends of the reel, nearly in the center of the base, we must find a place on the base in a straight line with the end of the core (as at C, Fig. 6), and here we fasten another piece of bent brass similar to the last. The end of the contact breaker is now soldered to this brass strip in such a way that the piece of soft iron at the other end is exactly opposite the core and about ¹/₁₆ inch distant from it. The screw of the paper fastener must now be tipped with platinum by cutting off the end and drilling a fine hole in it, in which hole a small piece of platinum wire can be soldered. The amount of wire and foil required, although very minute, will cost you about twenty-five cents, platinum being a very expensive substance. It can be bought from a chemist or electrician.

The screw having been prepared in this way, we must next fasten the brass strip to which the flange is soldered upright on the base, so that the platinum point of the screw, when inserted, will just come in contact with the square of foil on the spring. By turning the head of the screw the soft iron can thus be forced nearer the core, and the rapidity of its vibration is thus controlled. The coil is now complete, except the connections, which are made (preferably underneath the base by letting the wires through) by joining the ends of the thin wire to two “binding screws,” which are made for this purpose and can be obtained from the dealer. One end of the thick wire of the coil is fastened to the strip of brass supporting the contact-breaker, the other end is fastened to a binding-screw on one side of the base—the strip of brass supporting the screw being connected by a wire with another binding-screw on the other side. This sounds rather intricate, but will easily be understood if we consider that the current from the battery when the wires are connected with the binding-screw must pass through the brass strip to the screw, thence through the contact-breaker to the coil, and, having passed round the coil, back to the battery through the binding-screw attached to the other end of the wire. (See Fig. 6.)

It is now evident that when the contact-breaker is in contact with the screw a current will pass through the primary coil, and will cause the soft iron core to become a magnet and thus attract the soft iron. When this moves towards the magnet, contact is broken and the core is instantly demagnetized, so that the spring flies back and contact is made again. The screw is adjusted so that the contact is broken just as the soft iron touches the core. When the battery is joined on, the contact-breaker will fly backwards and forwards with tremendous speed, making a loud, buzzing noise, while brilliant sparks will appear between the platinum wire and foil.

In order to feel the effect of the shock, two handles will be required; these can be made by simply bending two pieces of tin about two inches by four inches round a ruler and neatly soldering the joins. A wire is now fastened to the end of each tube, the other ends being inserted in the binding screws connected with the thin wire of the secondary coil, which are at the opposite corners of the base to those which are joined to the ends of the primary coil. When the coil is buzzing, if these handles are tightly held, a powerful shock will be felt, in fact, a weak battery only should be used with the coil of the dimensions given, or it may be impossible to release the handles, and this is too strong to be pleasant.

The current can be regulated by means of a “regulating tube,” that is simply a brass tube which is made to slip over the core between it and the primary coil; the farther the tube is pushed over the core, the less powerful the shock. The dimensions of the coil being the same, a little ingenuity will enable any one to affix a regulating-tube. I will only say that instead of winding the coil direct on the core a tube of brown paper is formed a little larger than the core, and on this the wire is wound. Between this tube and the core the brass tube is arranged to slip in and out, the hole in the end of the reel farthest from the contact-breaker being made larger for its accommodation.

This concludes my description of the coil, but perhaps a few hints as to suitable batteries may be useful. If a strong battery which will only work the coil for a short time is required, the bottle bichromate is a good one. It can be bought from a dealer, or one can be made in a simple form by taking a jar and filling it with a strong solution of bichromate of potassium, to which a little sulphuric acid has been added. Take two pieces of gas carbon and three pieces of sheet zinc, both cut to the right size to dip in the solution to the bottom of the jar.

At the top of the zincs and carbons bore small holes, and below these place narrow strips of wood to keep them apart when in use; these must be long enough to reach across the top of the jar when the zincs and carbons are in the solution.

Arrange them thus: zinc, wood, carbon, wood, zinc, wood, carbon, wood, zinc; bind them lightly together by means of two more pieces of wood placed outside the outer zincs, and the whole tied together with string. Connect the three zincs together with one piece of wire, and the two carbons with another, taking care that the wire connecting the zincs, does not come in contact with the wire connecting the carbons. To one zinc attach a piece of covered wire, and to one carbon attach another, these two wires are connected with the binding screws of the primary coil. This battery is extremely strong, double as strong as the bottle bichromates sold, as there are more zincs and carbons employed, but it only lasts a short time before needing to be replenished.

Daniell’s battery is a weaker form, but lasts much longer, say for two or three hours in constant work. Take a deep jar and inside it place a porous jar of earthenware, which the electrician will provide. Now get a piece of sheet copper of the right size to go into the jar, and bend it round so that the porous jar will go inside it. A piece of sheet zinc will be wanted to go inside the porous jar. Both zinc and copper must be high enough to reach the level of the solutions when the jars are full. The porous jar is filled with dilute sulphuric acid, or solution of common salt; the jar outside is filled with “saturated” solution of sulphate of copper—that is, as strong as it can be made. Lumps of sulphate of copper are kept in the outer cell, which will keep the solution concentrated by slowly dissolving. Attach one wire to the zinc and another to the copper, and when these are joined to the binding screws of the primary coil the contact-breaker will begin buzzing.