Nikola Tesla Books
32 32 HIGH FREQUENCY APPARATUS may be made to produce a high frequency current through the medium of an induction coil in place of the alternating current transformer. The coil for this work should be constructed expressly for the purpose of charging condensers and its design is radically different from that of the conventional coil built to produce a long and stringy spark. While almost any coil will give some results, the greater effects will be shown with a coil having a comparatively short and thick core and a secondary winding of rather coarse wire, as secondaries go. The secondary should be bunched near the center of the core rather than spread out over the entire length. The primary should be wound, preferably, with two small wires in parallel rather than with one large wire. This method permits of a closer winding and the inside diameter of the secondary may, accordingly, be made smaller. The secondary coils should be layer wound and not pie wound. In a large coil, from four to eight sections of layerwound coils will give ample insulation as the potential is not nearly so high in this type of coil as is the case with the type built for X-Ray work. The individual sections may be impregnated with a mixture of equal parts of rosin and beeswax. Induction Coil Design.-The design of the coil for condenser charging may be summed up in a few words. We cannot calculate the different parts so nicely as we did for the transformer and our design must of necessity be a product of the "rule-of-thumb" school; that is, for the practical purposes outlined in this book. The core should take a certain fixed proportion and this may be stated as follows: The length of the core to be not greater than eight times its diameter; that is, a core eight inches long would be one inch in diameter, and so on in proportion. The number of turns in primary and secondary are dependent upon the
