Large coils can be made by drawing a circle about four feet in diameter on the floor of your shop. Then drive a number of small nails around the circle about three inches apart. This arrangement will serve as a form to wind the coil. After the winding is completed you can pull a few of the nails out and lift it off and you will have a circular coil four feet in diameter. Two coils will be required. They should be made of No. 20 B. & S. Gauge cotton covered wire. Each coil should contain about seventy-five turns of wire. About three pounds of wire will be required for each coil. After winding the coils should be tied with string or tape about every six inches so that they will not come apart. The nails can then be pulled out of the floor and the coil is ready for use. If it is intended to subject the coils to any amount of handling, it is a good idea to secure two large hoops such as girls use for rolling along the sidewalk, about four feet in diameter. Then make the coils exactly the same diameter as the hoops and when they are finished bind them to the hoops with tape. Two binding posts may be mounted on each of the hoops and the terminals of the coil connected to them. Two coils are required in order to establish a complete wireless telephone system, one at each station. Each coil may be used for both sending and receiving messages. Each station should be arranged as in Figure 184. The transmitter should be of the type known as "long distance." The switch is a two point switch. [Illustration: FIG. 184.—Circuit Diagram showing how the Coil is connected so as to serve for either transmitting or receiving.] It is a good plan to use two telephone receivers mounted on a head band so that both ears can be used and the hands left free to manipulate the switch. If a set of telephone receivers wound to a high resistance for wireless telegraph purposes is used it will greatly increase the range of the outfit. Six to eight first class dry cells will be required to form the battery. A push button is placed in series with the transmitter so that the battery current will be consumed only when the button is pressed. This makes it necessary to push the button when sending a message. When the lever of the two point switch rests on the contact A the circuit is completed for transmitting. When the lever rests on B the telephone receivers are connected to the coil and the apparatus is ready for receiving a message. If the apparatus is used only for demonstrating purposes, a camera tripod will be found very convenient for supporting the coils. The transmitter and switch can be fastened to the top of the tripod and the batteries placed in a box on the ground. It is absolutely necessary that the coils be perfectly parallel to each other when messages are being transmitted back and forth. It is well to start with the coils comparatively close to each other, say ten or twelve feet at first, and then gradually increase the distance.