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. [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.