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 (⅜ 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.

Fig. 3.—Core and Discs.

A, front of reel.
B, back of reel.
C, core.
PP, holes for primary wire.
SS, holes for secondary wire.

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:

Fig. 4.—Hammer of Contact-breaker.

P, Platinum foil.
I, Soft iron fastened to opposite side.

A piece of sheet brass is taken 1½ inches long by about ⅜ of an inch at one end, gradually tapered up till it comes to a point about ⅛ 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.

Fig. 5.—Screw of Contact-breaker.

F, Flange of paper-fastener soldered to upright brass strip.
P, Platinum tip to screw.

Fig. 6.—Plan of Coil Complete.

A, Coil.
B B B B, Binding screws.
C, Strip of brass