Wires of a good quality may be purchased already cut to various lengths. To buy them in this form will save a great deal of the labor required in building a core. If the wires are not quite straight they may be straightened by rolling them, one at a time, between two boards. It is best to reanneal the wires in the following manner. Place them in an iron pipe and plug the ends of the pipe with clay. Then lay it in a coal fire until the whole mass attains a red heat. The fire is then allowed to die out gradually with the pipe and wires remaining in the ashes until cool. When cool remove them from the pipe and rub each one with emery paper until bright. After this cleaning, the wires are dipped in hot water and dried. They are then dipped in a good quality of varnish and allowed to dry again.
Fig. 26. Induction Coil Core.
The varnish serves to interpose resistance to the eddy currents generated in the core and renders the losses due to this cause much less. A strong paper tube having an internal diameter equal to the diameter of the finished core is made by rolling the paper on a form and cementing with shellac. When perfectly dry. the tube is removed and the wires tightly packed in it. The following table gives the core dimensions for practical coils of different sizes.
Primary Winding.—The ratio of the number of primary turns of an induction coil to the number of secondary turns bears no relation to the ratio of the primary and the secondary currents. It has been found in practice that two layers of wire wound tightly on the core constitute the best primary. The primary should always be thoroughly shellacked or covered with insulating varnish. Since there is almost no ventilation in the primary the wire must be large enough to avoid all heating. A table containing the various sizes of primary wires is given below.
In large coils, the inductance of the primary causes a "kick back" and sparks are liable to pass between the adjacent turns. For this reason, it is always well to use double cotton covered wire and to further thoroughly insulate it by soaking the primary and core in a pan of melted paraffin and allowing them to harden therein. Afterwards the pan is slightly warmed to loosen the cake of paraffin and the excess of wax removed by scraping with a blunt instrument so as not to injure the wires. Paraffin contracts upon hardening, and the proper method to impregnate a porous substance is to allow it to soak and become set in it upon cooling.
A good method of reducing the "kick back" and also the size of the condenser shunted across the interrupter is to form the primary of a number of turns of smaller wire in parallel, the effect being to give a conductivity equal to a single wire of large diameter and at the same time to make a more compact winding of the primary on the core. This method of winding is very desirable in large coils, as it reduces the cross section of the primary and allows the secondary to be placed nearer the core, where the magnetic field is the strongest.
The primary winding ought to occupy nearly the whole length of the core, since there is no gain in carrying the end of the core very far beyond the end of the primary, for most of the magnetic lines of force bend at the end of the primary and return without passing through the extreme ends of the core.