The lines of force will travel through and about coil B and *induce in it a current of electricity*. The electric current will cause a sound like a click in the telephone receiver. When a telephone transmitter is substituted for the key and words are spoken into the mouthpiece, the current passing through the coil from the battery will be varied in accordance with the vibrations of the voice. The changes in the current will produce changes in the magnetic field set up by the coil and the changes in the magnetic field will produce a varying current in the coil B so that the words will be distinctly repeated by the receiver connected to B. [Illustration: FIG. 182.—Illustrating the Principle of the Induction Wireless Telephone.] This experiment is very easy to perform and may be carried out by almost any boy with the materials he probably already has around his shop. Twenty-five to thirty turns of magnet wire of almost any size wound around one end of a cardboard tube three or four inches in diameter will serve as the coil A. A similar coil wound around the tube five or six inches away, an ordinary telephone transmitter and receiver and two or three dry cells form all the rest of the equipment desired. Words spoken into the transmitter connected to the coil A will be distinctly heard in the telephone receiver connected to the coil B, although there is no connection between the two coils other than the *magnetic field*. Such an outfit as this is of course only experimental and intended solely for the purpose of demonstrating the principles of operation of the larger outfit. If it is desired to make a practical set for talking over a considerable distance it will be necessary to make coils much larger in diameter and to build them so they contain a larger number of turns. [Illustration: FIG. 183.—Showing how the Coils may be formed by winding around nails set in a circle in the Floor.]