Nikola Tesla Books
525 52 HIGH FREQUENCY APPARATUS to be practically the only solid, or nearly solid, insulating substance that will stand up under the strain. Last of all comes air. We say last because the air insulation must be greater in extent than either of the predecessors; but notwithstanding this fact, the air insulation is the one cheap, light-weight, and absolutely reliable insulation that the high frequency worker has at his command. A separation of a few inches more, perhaps, between primary and secondary, but a mechanically and electrically good construction, and, what is more, accessibility of all of the parts at an instant's notice, is the significant list of advantages possessed by this insulating medium. On air-insulated coils, the secondaries should be wound with a space between each turn and its neighbors. This space will depend upon the potential to be set up at the top of the coil and ordinarily the separation should be about the width of the wire itself. Bare wire may be used but it is not recommended. Double cotton covered wire has been the choice of the author after many experiments to determine the relative merits of all known coverings. The double cotton covering forms an excellent base to soak up shellac which, when applied in a half-a-dozen coats, and thoroughly dried out, has been found to seal up the turns on the open cylinder, preventing in large measure, the leakage between the turns when the coil is operating at full power. Theoretically the winding of the secondary should start with turns close together at the base, the separation gradually widening out as the top or high potential end of the coil is reached. This method of winding is impracticable for the average amateur constructor, however, and he must needs resort to the next best expedient of making all turns sufficiently separated to take care of the potential. Oil and Wax Insulations.-The oil insulated coil may be very closely coupled; that is, the primary may be but slightly
