*The Secondary* winding consists of a single layer of No. 28 B. & S. Gauge double cotton covered wire wound over a cardboard tube, twelve inches long and and three inches in diameter. The tube must be thoroughly dried before using it, by baking in an oven. A coat of shellac, both inside and out, will avoid the possibility of having to rewind the tube because of the wire becoming loose, due to drying out of the tube later. The wire should be wound on in a smooth, even layer to within about one-quarter of an inch from the ends and given a coat of hot paraffine when finished. The ends of the tube are fitted with circular wooden heads having a half inch flange. *The Base* is a rectangular piece of wood, fifteen inches long and six inches wide. The secondary is supported in position by two hard rubber uprights, four inches high, seven-eighths of an inch wide and one-half an inch thick. A round-headed brass wood screw is passed through the top part of each of the supports into the centre of each one of the wooden secondary heads. High frequency currents are very hard to insulate and wood does not possess sufficient insulating value to fit it for use as supports. Hard rubber or glass are the most satisfactory materials for the supports. [Illustration: FIG. 172.—Details of the Secondary Heads.] The secondary terminals are connected to two brass rods, five inches long and having a small brass ball at the upper end, mounted on the top of each of the hard rubber supports. The lower ends of the hard rubber supports are fastened to the base by means of screws passing upwards through the base into a threaded hole in the bottom of each support. [Illustration: FIG. 173.—Details of the Primary Head.] The secondary passes through the centre of the primary. The primary consists of eight turns of heavy copper wire wound around a wooden drum or frame. The wire should be No. 8 or No. 10 B. & S. Gauge. Flat copper or brass ribbon one-quarter of an inch wide can be used in place of the wire. The stranded copper wire, consisting of seven No. 22 B. & S. Gauge wires twisted together and commonly employed for wireless aerials can be used to good advantage in winding the primary, the idea of using a large wire, stranded wire, or ribbon being to obtain a conductor having as much surface as possible. High frequency currents travel only on the surface of wires and conductors. A hollow tube is just as good a conductor for high frequency currents as a solid rod of the same diameter. [Illustration: FIG. 174.—Primary Cross Bar.] [Illustration: FIG. 175.—Front View of the completed Tesla Coil.] [Illustration: FIG. 176—Side View of the completed Tesla Coil.]