The base is a wooden block, three and one-half by one and three-quarters inches by one-half inch. The binding-posts are of the type commonly used on electrical instruments. One of the posts is pivoted so that it will swing from side to side. A short piece of brass rod fitted with a rubber or fiber knob passes through the wire hole in the post. A piece of No. 30 B. & S. gauge bronze wire is soldered to the end of the rod. A small brass cup contains the mineral, which may be either _galena_, or _silicon_. By twisting the post and sliding the rod back and forth, any portions of the mineral surface may be selected. *Fixed Condenser.* The construction of the condenser is illustrated in Figure 205. Take twenty-four sheets of thin typewriter paper, three by four inches, and twenty-three sheets of tinfoil, two by four inches. Pile them up, using first a sheet of paper then a sheet of tinfoil, then paper, and so on, so that every two sheets of tinfoil are separated by a sheet of paper. Each sheet of tinfoil must, however, project out beyond the edge of the paper. Connect all the tinfoil projections on one end of the condenser together and and attach a small wire. Connect all those on the opposite side in a similar manner. Then fasten a couple of rubber bands around the condenser to hold it together. [Illustration: Fig. 215.—Building up a Fixed Condenser.] [Illustration: Fig. 216.—A Fixed Condenser enclosed in a Brass Case made from a Piece of Tubing fitted with Wooden Ends.] If it is desired to give the condenser a finished appearance, it may be placed in a brass tube fitted with two wooden or fiber ends. The ends are provided with binding-posts to which the terminals of the condenser are connected. *Telephone Receivers* for use with wireless instruments must be purchased. Their construction is such that they cannot be made by the experimenter. [Illustration: Fig. 217.—A Telephone Head Set.] A seventy-five ohm, double-pole telephone receiver will do for stations not wishing to receive farther than fifty miles. In order to secure the best results from wireless instruments, it is necessary to have receivers especially made for wireless. Each receiver should have 1000 ohms resistance. Some boys may find it necessary to purchase one receiver at a time. Two receivers, a double headband, and a double cord, forming a complete head set as shown in Figure 217, should be secured as soon as possible. [Illustration: Fig. 218.—A Circuit showing how to connect a Double-Slider Tuning Coil.] *Connecting the Receiving Apparatus* Figure 218 shows how to connect a double-slide tuner, a detector, a fixed condenser and a pair of telephones to the aerial and ground. The same instruments with a loose coupler in place of the double-slide tuner are shown in Figure 219. The diagrams in Figure 220 are the same circuits as those shown in Figures 218 and 219, but show different instruments. [Illustration: Fig. 219.—Circuit showing how to connect a Loose Coupler.]