Many of the readers of this book are probably "wireless experimenters" who have constructed their own wireless telegraph apparatus, but not many have ever built a wireless telephone set. The arrangement described in the following chapter may be built by almost any boy and will prove a very interesting and instructive piece of apparatus. It is of no real practical value as a wireless telephone, owing to the fact that the distance over which it will transmit intelligible speech is limited to 150 to 300 feet. However if you have a chum who lives next door or across the street or within the distance named above, it is easily possible for you to construct a wireless telephone which will enable you to remain in your own rooms and hold a conversation with each other, without any connecting wires.

The instruments depend for operation upon what is known as magnetic induction. Michael Faraday, who was a famous English scientist, discovered in 1831 that if a magnet is suddenly plunged into a hollow coil of wire, a momentary current of electricity is generated in the coil. You can try this experiment for yourself by connecting a galvanometer with a hollow coil of wire and then suddenly plunging a bar magnet into the coil.

If you observe closely you will notice that the needle of the galvanometer is deflected, this indicating that a current has passed through the circuit. The galvanometer should of course be far enough away so that the needle is not affected directly by the magnetism of the bar magnet itself.

As long as the bar magnet is motionless, it will not induce any current in the coil. As soon as it is moved however, the currents are set up. The mechanical work done in moving the magnet is really the source of electrical energy. The medium which changes the mechanical energy into electricity is called the magnetic field. The real nature of a magnetic field is very hard to explain and not easily understood. It is a peculiar state or condition of the space in the immediate neighborhood of a magnet. It is possible to show its existence by placing a sheet of cardboard over a magnet and then sprinkling iron filings over the surface of the cardboard. If the cardboard is tapped slightly, the filings will settle down in curving lines, forming a magnetic "phantom." The curved lines into which the filings form themselves, represent the paths of the lines of force which make up the magnetic field. The illustration in Figure 180 shows the magnetic phantom about a bar magnet.

When the space in the neighborhood of a wire or a coil of wire through which a current of electricity is passing is examined by means of some iron filings sprinkled on a sheet of cardboard, it is found that a similar state of affairs exists there and that it also possesses a magnetic field.

This can be readily shown by punching a small hole in the centre of a sheet of cardboard or stiff paper and passing a wire which is carrying a strong current of electricity through the hole at right angles to the surface of the cardboard. Sprinkle some iron filings on the cardboard and they will arrange themselves in circles around the wire, forming a magnetic phantom giving definite proof to the eyes of the existence of the magnetic field.

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.

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.

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.

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.

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.

Coils approximately four feet in diameter and containing about seventy-five turns of wire will transmit and receive messages about one hundred and fifty feet. The range can be increased slightly by using more batteries. If too much battery is used, however, the telephone transmitter will become very hot and is liable to burn out.

It is impossible, of course, to "call up" over a set of wireless phones such as these. If you succeed in getting your set to work satisfactorily with your chum across the street and wish to talk to him it will be necessary for you to arrange some predetermined time.

In order to operate your phone place the switch lever on the contact A. Hold your finger tightly against the push button so that the circuit is completed and talk slowly and distinctly into the transmitter. The transmitter will operate the best when the mouthpiece points on a level with your mouth. The lips should be about one inch away from the mouthpiece. Speak in a low even tone. The switch at your friend's station should be resting on B. When you finish speaking throw your switch to B and listen in your telephone receivers. The operator at the other station should throw his switch to A and push the button. He may then talk to you.