The parts of an intensity coil are: reel, primary coil, secondary coil, iron bundle or core, contact - breaker, condenser, pedestal or base, and commutator. The dimensions given may be considerably varied without impairing the efficiency of the apparatus.
The reel consists of a hollow cylinder or tube, with a square or circular plate firmly fixed on each end. The cylinder is formed of paper, and the plates or reel - ends of guttapercha or ebonite. The reel - ends are flat, and not less than 3/8 in. thick; if circular, a facet is made on the edge of each, so that when the reel is complete it may stand steadily on the pedestal. The hole through the centre of the reel - ends is turned perfectly true, so as to fit the outside of the cylinder; and a shoulder is left on the outer face sufficient to prevent the paper cylinder from being pushed through the ends when being fastened on. The reel - ends may be 4 1/2 in. in diameter if circular, or 4 in. by 4 in. if square. They can be glued to the paper cylinder. The cylinder is formed of cartridge - paper cut into a long strip, and when gammed or pasted on one side, wound round a rod \ in. diameter. When properly done, a firm tube, 7 in. long, 1 in. diameter, and about 1/16 in. thick, is obtained. This is allowed to dry thoroughly, and the ends are cut at right angles to the axis. The ends are firmly fastened to the cylinder; in order to effect this, the holes through the discs are slightly tapered, the larger dimensions being towards the shoulder.
Before fixing the paper cylinder, a slightly conical - plug is provided, fitting the inside of the cylinder. When the parts are ready and carefully coated with the glue, they are put together, and the conical plug is gradually pressed into the end of the cylinder, which will expand it a little, and force it into close contact with the sides of the hole in the disc; it remains in this position until the glue is thoroughly set, when the plug can be removed. The reel is provided with a hollow groove in the edges of the discs, if circular, to receive the pieces of catgut cord that are to fasten it to the base; if square, they can be fastened by screws; 2 holes are drilled through one end of the reel, to allow the primary wire to be passed through; these should be about 1/8 in. diameter, and somewhat oblique in direction, so that the wire, where passed through the reel - end, may not be at right angles with the axis of the reel.
The primary coil consists of No. 16 cotton - covered copper wire, averaging about 18 yd. to the lb. One end of the wire is passed from the inside through one hole in the reel - end, so as to project 6 or 8 in., and the wire is then carefully wound over the cylinder up to the other end, and back again, so as to form 2 layers, one over the other. When completed, the remaining end of the wire is passed through the second hole in the reel - end. Before putting on the wire, fit a wooden or metallic rod inside the paper cylinder of the reel, or the cylinder is likely to be damaged by the force required to wind the wire round it. When the primary wire is on, it is varnished with 2 or 3 successive coats of shellac dissolved in spirits of wine, care being taken that one coat is thoroughly dry before another is put on. The first coat should be thin, so as to be readily absorbed by the covering of the wire and conveyed to the nether layer. When the varnish is dry and hard, the primary wire is covered with a strip of cartridge - paper passed 2 or 3 times over the wire, and fastened by gum or glue.
This paper must be cut exactly to the width between the inside faces of the ends of the reel, and drawn tightly when put on, but not so tightly as to show on its surface the interstices between the rows of wire. This paper covering, when dry, is varnished to present a smooth cylindrical surface, having no space between it and the inside face of the reel ends. Shellac varnish forms a good insulator, but is not so effective as ordinary black rosin and beeswax. This preparation is rather more difficult to apply, but greatly superior to the varnish when done. The rosin is melted in an earthen vessel, and a small quantity of beeswax is added to it, the proportion to be determined by experiment, the use of the wax being to diminish the friability of the rosin without interfering with its hardness: usually about 1/5 by weight will be found suitable. The rosin and wax, fully melted and heated almost to boiling, are poured over the wire from a ladle, turning the coil round, and repeating the application until the mixture has completely permeated the strands and filled up all the interstices between the wires. If this be done neatly, the paper covering may not be required.
When the rosin mixture is employed as the insulating material, it is convenient to wind the wires on the cylinder, and insulate before the reel - ends are fixed on.
This is formed of No. 38 copper wire, covered with silk, and averaging 180 yd. to the oz.; the quantity required is about 6 oz. In winding on the reel, scrupulous care is needed, to avoid any break in the wire, and any kink or bend in it. The diameter of this wire is 0067 in. The layers of the secondary wire should not be carried close up to the end of the primary coil, thus avoiding the possibility of the wire of one layer sinking down to the level of that below it. When a layer of wire is finished and insulated, it is next to impossible to take it off again; consequently every care must be taken to prevent any failure in winding it an. One layer of the secondary wire is wound on at a time, and then coated with the shellac Tarnish or rosin mixture. When done, the layer is further insulated by wrapping round several thicknesses of guttapercha tissue, or thin white demy paper soaked in the rosin mixture, and allowed to become, hard. Whichever material be than the length of the layer of wire it is to cover, and wound on tightly but smoothly. The length of the atrip should be such as to wrap 3 or 4 times round the coil; it is fattened with the varnish or rosin mixture.
When the requisite quantity of wire is put on, 8 or 10 folds of the insulating paper or tissue are wrapped round the coil before the ornamental covering of silk velvet is applied. If the wires forming the coil have been put on before the reel - ends have been fixed to the inner paper cylinder, the reel - ends must now be put in place, and, when firmly set, (he spaces between the ends of the layers of wire and the inside of the reel - ends are tilled up with the rosin mixture so that the insulation may be perfect. The winding of the secondary wire begins at the opposite end of the coil to that at which the winding of the primary coil commenced, and finishes at the end where it began. The 2 ends of the wire are wound into helices, and these can be passed through 2 holes in the reel - ends, in order that they may be connected with the other part of the apparatus.
Before winding the secondary wire, it is tested in the following way: - Attach one end of the wire on the bobbin (la it cornea from the covers) to one electrode of a battery, and the other end to one of the binding strews of a galvanometer. The circuit is completed by uniting the other electrode of the battery with the other binding screw of the galvanometer, and if there be no break in the wire, a deflection of the needle will ensue. Should no deflection take place, the wire must be unwound from the bobbin, carefully era mined, and the break detected and soldered. When the continuity of the wire has been effected, winding it on to the reel can be commenced. Each should be tested by the galvanometer. For this operation a different course is adopted. The beginning of the secondary wire on the reel is connected with one of the binding - screws of the galvanometer, and the end of the wire that still remains on the bobbin Is attached to the other binding screw, as in Fig. 62.
The 2 ends of the primary coil are connected with the battery, and if there be no fault, the needle of the galvanometer will be deflected; the layer can then be insulated, and the same course adopted with each layer. To solder 2 ends of the secondary wire together, the silk coating must be removed from each end - the ends brightened by rubbing with fine glass paper, put aide by aide in contact with each other, a small piece of tinfoil wrapped round both wires, moistened with a solution of zinc chloride, and moved over the flame of a very small spirit - lamp; in a few seconds, the tinfoil melts and unites the wires. Should it be necessary to apply the wires again to the lamp, they are first moistened with a fresh portion of zinc chloride. The wires should overlap each other about 1/2 in.; when the soldering is complete, the silk covering is carefully replaced.
This is a bundle of un - covered iron wires, about No. 18 gauge, quite straight, of exactly equal lengths, and about 1/2 in. longer than the outside measurement of the coil. The centre of the coil is filled with these wires; and then a short piece of larger wire, carrying on one end an iron disc about 1/4 in. thick and $ in. diameter, is pushed into the centre of the coil at each end, so as to secure the bundle in place.