Some of my readers may have begun to wonder what a magnetic field has to do with the operation of the wireless telephone. It has already been explained that this particular type of wireless telephone depends upon magnetic induction for its operation. It is necessary to have some sort of a conception of the magnetic field before you will be able to understand magnetic induction.
[Illustration: FIG. 181.—Magnetic Phantom about a Coil of Wire carrying a current.]
In order that you may gain an idea of how the instruments work, suppose that two coils of wire, A and B, are connected as shown in Figure 182. The coil A is connected in series with a set of batteries and a telegraph key. The other coil B is connected to a telephone receiver. The coils are placed parallel to each other and only a few inches apart. When the key is pressed so that the circuit is complete and the battery current flows through the coil A, it creates a magnetic field in the vicinity of the coil and lines of force are set up in the immediate neighborhood.
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.]
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.]