The secondary section windings, condenser, and other parts being completed, the important operation of assembling the coil is next in order. But little has been said regarding the core, it being assumed that the wire for the same will be purchased cut to length ready for use. This is extremely desirable, as two important requirements make the preparation of core wire by the amateur without special tools a quite difficult matter.

In the first place, core wire should be very thoroughly annealed, and this process, as carried out in kitchen stove or furnace, is not at all satisfactory, both because this kind of annealing is most imperfect, and the scale and dirt collecting on the wire prevents the subsequent gathering of the wire into a compact bundle. Wire, as annealed by wire manufacturers, is kept in the furnace for several days, and is entirely free from scale. The cutting is done by machine, and each piece is straight, and all are of the same length, making the work of forming the core much easier and better than it otherwise would be. A core made of such wire will also more strongly energize the coil than will one made of poorer and rougher wire. The importance of using the best possible core wire cannot be too strongly emphasized, especially with coils of large size, where the value of a heavy outlay for magnet wire may be to quite an extent lost because of a poor core.

The operation of forming the core should be very carefully done. The writer has found the following method gives good results, and is as convenient as any which he has seen described. A cylindrical bundle of loose wire slightly larger than the ultimate size of the coil, is gathered in one hand, and several small rubber bands slipped over it. The bundle is then rolled on a smooth, flat surface, to bring the wires as closely together as possible, adding additional rubber bands as the rolling proceeds. When the bundle is made as compact as possible, the size should be that desired for the core. If too large remove a few wires, one at a time, and continue the rolling until the correct size is reached. It is easier to reduce than to increase the size, so the bundle at first should be fully large enough.

The next process is to wind the bundle with strong, black linen thread, in close even layers, beginning at one end and winding to the other, removing the rubber bands as the winding proceeds, and using the greatest care that the bundle is not forced out of round by the winding. Only one layer of thread is necessary, allowance being made for same, as well as the primary winding in determining the size of the core. The layer of thread being in place, the core should form a round, solid bundle, which will be free from movement when the finger is applied to any part. A coating of shellac, or mixture of paraffine and beeswax, should then be given the core, the latter being preferable except in hot climates. Two or three layers of strong ma-nila paper are then wound upon the core, the number being dependent upon the thickness of the paper and the purpose to give a smooth surface upon which to wind the primary. The primary is then wound in even layers upon the paper surface just mentioned. Finally, the core is given a complete boiling in the parafnne-beeswax mixture until all bubbles cease to rise therefrom. The mixture must be heated in a water bath, and owing to the shape of the core, utensils for this purpose will not be easily obtained by many. If a tin can, used for shipping photographic carbon paper, can be obtained from a photographic studio, it will be found just the thing, after cutting off to the desired length. Any deep kitchen utensil can be used to contain the water and tin holding the mixture.

After a thorough boiling in the mixture, the core is removed, but will have to be basted on the outside ends with the mixture, as the iron retains the heat longer than the mixture, keeping the mixture fluid in the center for some time after that on the surface has hardened. As it cools it contracts, and the basting is necessary to avoid cracks. Just before the mixture becomes cold and set, the core is placed inside of the ebonite or mica tube, which separates it from the secondary, and any space between the primary and ebonite tube, filled with the fluid mixture. The ebonite tube should be a fairly close fit over the primary, when the latter has been coated liberally with the mixture; too much space reducing the output of the coil.

The bobbin, or coil end, is then placed in position at one end, and the secondary sections put on, taking the greatest care to see that joints between double sections are well made by soldering and then fully insulated with thread. The paraffine mixture is kept at hand, and the space between the inside of the sections and the ebonite tube completely filled with the mixture, which is applied to the joint between sections. Care must also be taken to see that the sections are put on with the windings in the right direction, so that the current through them will be in a uniform direction around the core. Extra care must be taken at the ends of the secondary to see that the insulation is ample, as the tendency to break down and short circuit is greatest at the ends.

The leads from the secondary to the terminal posts should be taken from the tops of the windings, so that they will be as short and direct as possible. It may be necessary to unwind the ends of the two end sections to do this, but this is easily done. In connecting up the sections it is also advisable to test frequently for breaks, and if any are suspected because of poor sparks, careful inspection should be made and the trouble located before proceeding with the work. By testing for breaks is meant the use of a battery of four or five dry cells and a galvanometer, leads with detachable clips being used for connections.