Nikola Tesla Books
54 HIGH FREQUENCY APPARATUS This quality is to be desired in X-Ray work in particular. For demonstration apparatus, however, the builder can do no better than use the air insulated coils. Constructional Features.-The simplest Tesla or Oudin coil to make is that in which the secondary is a cylinder of cardboard such as a large mailing tube, or a tuning coil cylinder, wound with a few hundred turns of insulated magnet wire. The primary may consist of a few turns of copper ribbon wound into a spiral with the turns separated by a strip of corrugated board such as is used for packing purposes, the whole being secured with tape at four or five points on the spiral. From this simple start, larger coils may be developed along similar lines. Cardboard cylinders may be purchased in sizes as large as 8 x 13 in. and larger ones may be made to order. Very large cylinders should be made of wooden slats pegged with wooden pegs to wooden discs of the desired diameter. Primaries may be of almost any heavy conductor available. As the tendency of the high frequency current is to travel upon the surafce of the conductor, it is highly desirable that the primary be made either of flat ribbon or strip copper, or else of copper tubing of relatively large diameter. Stranded conductor is excellent and if the precise number of turns is known and no tuning necessary, the builder may use heavy stranded cable with excellent results. The ideal primary conductor is copper strip or bar, wound edgewise into an open helix. Such a primary may be made compact and mechanically strong and it is splendid from the electrical standpoint. This conductor permits of closer coupling without danger of sparking from secondary to primary. The edgewise wound strip is difficult to make as the reader may well imagine. The mechanical problem
