The coil may be used just as it is for experimental purposes, but if it is intended for any amount of handling it is wise to procure a large hoop such as girls use for rolling along the sidewalk, and make the coil the same diameter as the hoop so that upon completion they may be firmly bound together with some insulating tape. Two binding-posts may then be fastened to the hoop and the terminals of the coil connected to them. Two such coils are required for a complete wireless telephone system, one to be located at each station. It is also necessary to make a double-contact strap-key. Such a key is easily built out of a few screws and some sheet-brass. The illustration shows the various parts and construction so clearly that no detailed explanation is necessary. [Illustration: Fig. 239.—A Double-Contact Strap-Key. The Dotted Lines show how the Binding-Posts are connected.] The telephone transmitter and the telephone receiver required for this experiment must be very sensitive, and it is hardly possible for the young experimenter to build one which will be satisfactory. They can be secured from a second-hand telephone or purchased at an electrical supply house. The transmitter should be of the "long distance" type. An 80-ohm receiver will serve the purpose, but if you also have a wireless station, use the same 1000-ohm receivers belonging to your wireless set and you will secure very good results. A battery capable of delivering about 10 volts and a good constant current is required. The apparatus is connected as shown in Figure 240. When the key is pressed, the coil is connected to the battery and the telephone transmitter. If words are then spoken into the transmitter they will vary the amount of current flowing and the magnetic field which is set up in the neighborhood of the coil will induce currents in the coil at the other station, provided that it is not too far away, and cause the words to be reproduced in the telephone receiver. When the key is released it will connect with the upper contact and place the telephone receiver in the circuit for receiving, so that your chum at the other station can answer your message by pressing his key and talking into his transmitter. [Illustration: Fig. 240.—The Circuit of the Wireless Telephone. When the Key is up, the Receiver is ready for Action. When the Key is pressed, the Transmitter and Battery are thrown into the Circuit.] The best plan is to mount each of the coils upon a tripod and experiment by placing them close together at first and gradually moving them apart until the maximum distance that the apparatus will work is discovered. Be very careful to keep the two coils exactly parallel. Much depends upon the battery. Be certain that it is capable of delivering a good strong current. Do not hold the key down any longer than is absolutely necessary, or the telephone transmitter will become hot.