High-frequency currents possess many curious properties. They 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. High-frequency currents do not produce a shock. If you hold a piece of metal in your hand you can take the shock from a high-frequency coil throwing a spark two or three feet long with scarcely any sensation save that of a slight warmth. The Tesla coil described below is of a size best adapted for use with a two-inch or three-inch spark coil, or a small high-potential wireless transformer. The purpose of the spark coil or the transformer is to charge the Leyden jars or condenser which discharge through the primary of the Tesla coil. [Illustration: Fig. 318.—Details of the Wooden Rings used as the Primary Heads.] If the young experimenter wishes to make a Tesla coil which will be suited to a smaller spark coil, for instance, one capable of giving a one-inch spark, the dimensions of the Tesla coil herein described can be cut exactly in half. Instead of making the secondary twelve inches long and three inches in diameter, make it six inches long and one and one-half inches in diameter, etc. *The Primary* consists of eight turns of No. 10 B. & S. gauge copper wire wound around a drum. The heads of the drum are wooden rings, seven inches in diameter and one-half inch thick. A circular hole four and one-half inches in diameter is cut in the center of each of the heads. [Illustration: Fig. 319.—Details of the Cross Bars which support the Primary Winding.] The cross bars are two and one-half inches long, three-quarters of an inch thick and one-half of an inch wide. Six cross bars are required. They are spaced at equal distances around the rings and fastened by means of a _brass_ screw passing through the ring. When the drum is completed it should resemble a "squirrel cage." Small grooves are cut in the cross bars to accommodate the wire. The wires should pass around the drum in the form of a spiral and be spaced about five-sixteenths of an inch apart. The ends of the wire should be fastened to binding-posts mounted on the heads. *The Secondary* is a single layer of No. 26 B. & S. silk- or cotton-covered wire wound over a cardboard tube, twelve inches long and three inches in diameter. The tube should be dried in an oven and then given a thick coat of shellac, both inside and out, before it is used. This treatment will prevent it from shrinkage and avoid the possibility of having to rewind the tube in case the wire should become loose. [Illustration: Fig. 320.—The Secondary Head.] The secondary is fitted with two circular wooden heads just large enough to fit tightly into the tube, having a half-inch flange, and an outside diameter of three and seven-eighths inches. *The Base* of the coil is fifteen inches long and six inches wide and is made of wood.