[Illustration: Fig. 220.—A Diagram showing how to connect some of the Instruments described in this Chapter.] After the instruments are connected, place a piece of galena or silicon in the cup of the detector and bring the wire down on it. Then move the sliders on the tuning coil or loose coupler and adjust the detector until you can hear a message buzzing in the telephones. It may require a little patience and practice, but if you persist you will soon learn how to adjust the apparatus so as to receive the signals loudly and clearly with very little trouble. *The Transmitting Apparatus* Spark coils have already been described in Chapter XII. They may be used to transmit wireless messages simply by connecting to a spark-gap and a key. Spark coils which are especially made for wireless telegraphy will usually send farther than an ordinary spark coil used for experimental purposes. [Illustration: Fig. 221.—A Wireless Spark Coil.] A good one-inch coil costs from $4.50 to $5.00 and will send from three to five miles if used with a fair aerial. A spark coil requires considerable current for its successful operation and will give the best results if operated on storage cells, dry cells, or bichromate cells. If dry cells are used, it is a good plan to connect them in series multiple as shown in Figure 69. Spark-gaps may be made by mounting two double binding-posts on a wooden base as shown in Figure 222. Zinc possesses some peculiar property which makes it very efficient for a spark-gap, and for this reason the electrodes of a spark-gap are usually zinc. [Illustration: Fig. 222.—Small Spark Gaps.] The figure shows two different forms of electrodes. In one, they are made of zinc rods and provided with “electrose” handles. In the other gap, the zinc electrodes are in the shape of "tips" fitted on the ends of two short brass rods. A one-inch spark coil will give very good results by connecting the spark-gap directly across the secondary of the coils. The aerial is connected to one side of the gap and the ground to the other. The transmitter may be "tuned" and the range sometimes increased by using a condenser and a helix. A condenser is most easily made by coating the inside and outside of a test-tube with tinfoil so as to form a miniature Leyden jar. The end of the tube is closed with a cork through which passes a brass rod connecting to the inner coating of tinfoil. [Illustration: Fig. 223.—Diagram showing how to connect a Simple Transmitter.] If such a condenser is connected directly across the spark-gap, the spark will become very white and crackling. Several tubes may be arranged in a rack as shown in Figure 225. A helix consists of a spiral of brass ribbon set in a wooden frame. The two strips composing the frame are each nine inches long. The spiral consists of eight turns of brass ribbon, three-eighths of an inch wide, set in saw-cuts made in the frame. A binding-post is connected to the outside end of the ribbon.