The receiving set shown in Figure 316 is made up of some of the separate instruments described in Chapter XIV, and illustrates the general plan which may be followed in arranging an outfit in this manner. [Illustration: COMPLETE RECEIVING SET, CONSISTING OF DOUBLE SLIDER TUNING COIL, DETECTOR AND FIXED CONDENSER.] [Illustration: COMPLETE RECEIVING SET, CONSISTING OF A LOOSE COUPLER IN PLACE OF THE TUNING COIL, DETECTOR AND FIXED CONDENSER.] The base is of wood, and is nine inches long, seven inches wide, and one-half of an inch thick. A double-slider tuning coil, similar to that shown in Figure 203, is fastened to the back part of the base by two small wood-screws passing upwards through the base into the tuner heads. [Illustration: Fig. 316. A Complete Wireless Receiving Outfit.] The fixed condenser is enclosed in a rectangular wooden block which is hollowed out underneath to receive it and then screwed down to the base in the forward right-hand corner. The detector is mounted in the forward left-hand part of the base, and in the illustration is shown as being similar to that in Figure 210. Any type of detector may, however, be substituted. The tuning coil may be replaced by a loose coupler if desirable, but in that case the base will have to be made larger. The telephone receivers are connected to two binding-posts mounted alongside the detector. The circuit shown in Figure 218 is the one which should be followed in wiring the set. The wires which connect the various instruments should be passed through holes and along the under side of the base so that they are concealed. HOW TO BUILD A TESLA HIGH-FREQUENCY COIL A Tesla high-frequency coil or transformer opens a field of wonderful possibilities for the amateur experimenter. Innumerable weird and fascinating experiments can be performed with its aid. When a Leyden jar or a condenser discharges through a coil of wire, the spark which can be seen does not consist simply of a single spark passing in one direction, as it appears to the eye, but in reality is a number of separate sparks alternately passing in opposite directions. They move so rapidly that the eye is unable to distinguish them. The time during which the spark appears to pass may only be a fraction of a second, but during that short period the current may have oscillated back and forth several thousand times. If the discharge from such a Leyden jar or a condenser is passed through a coil of wire acting as a _primary_, and the primary is provided with a _secondary_ coil containing a larger number of turns, the secondary will produce a peculiar current known as _high-frequency_ electricity. High-frequency currents reverse their direction of flow or _alternate_ from one hundred thousand to one million times a second. [Illustration: Fig. 317.—Illustrating the Principle of the Tesla Coil. A Leyden Jar discharges through the Primary Coil and a High-Frequency Spark is produced at the Secondary.]