The base of the instrument measures twelve by three and five-eighths inches. The primary is composed of a single layer of No. 24 B. & S. gauge single-silk-covered wire wound on a cardboard tube two and three-quarter inches in diameter and three and three-quarter inches long. The winding is laid on in a single layer and should comprise about 150 turns. After winding on tightly, it should be given a coat of clean white shellac and allowed to dry. The shellac is for the purpose of fastening the wire down tightly to the tube so that it will not loosen when the slider is moved back and forth. The primary is mounted between two heads, the details of which are shown in Figure 205. One of the heads, _B_, has a flanged hole two and three-quarter inches in diameter cut through the center so as to receive the end of the tube and permit the secondary to pass inside. [Illustration: Fig. 205.—Details of the Wooden Parts.] The secondary winding is composed of a single layer of No. 28 B. & S. gauge silk-covered wire and divided into six equal sections. The secondary is supported by two circular wooden pieces, _C_ and _F_, and slides back and forth on two guide-rods. The guide-rods should be brass. Iron or steel rods running through the center of a loose coupler will seriously weaken the signals, and such practice must by all means be avoided. [Illustration: Fig. 206.—Side View of the Loose Coupler.] [Illustration: Fig. 207.—Top View of the Loose Coupler.] The secondary sections are connected to six contacts and a switch-arm mounted on the end of the secondary so that by turning the switch either one, two, three, four, five, or six sections of the winding may be connected. [Illustration: Fig. 208.—End Views of the Loose Coupler.] [Illustration: Fig. 209.—Complete Loose Coupler.] The two binding-posts near the secondary end of the coupler are connected to the terminals of the secondary winding by means of two flexible wires. They have not been shown in several of the illustrations because they would be liable to confuse the drawing. The primary is provided with a slider moving back and forth over a narrow path scraped through the insulation so that it may make contact with each wire independently. *Detectors* Detectors are very simple devices and consist merely of an arrangement for holding a small piece of certain minerals and making a contact against the surface. The crystal detector shown in Figure 210 is a very efficient form that may be easily and quickly made. When finished, it will make a valuable addition to almost any amateur experimenter’s wireless equipment. [Illustration: Fig. 210.—A Crystal Detector.]