Wireless Telegraphy and Telephony

 30 WIRELESS TELEGRAPHY done in moving the magnet. The medium which changes the mechanical energy into electricity is called the magnetic field. The magnetic field is a peculiar state or condition of the space in the immediate neighborhood of a magnet. Its real nature is very hard to explain and not easily un- derstood. Suffice it to say, however, that the current is induced in the coil of wire only when the magnetic field is changing, either decreasing or increasing. The change is produced by moving the magnet because its influence on Fig. 31.—Magnetic phantom formed by bar magnet. the coil will be great or small accordingly as it is near or far. It is possible to show the existence of the magnetic field by placing a sheet of glass over a bar magnet and then sprinkling iron filings on the glass. They will settle down in curving lines as in Fig. 31, forming a magnetic phantom. The curved lines formed by the filings represent the direc- tion of the lines of force which make up the magnetic field. If we should examine the space in the immediate neigh- borhood of a coil of wire carrying a current of electricity it would be found that a similar state of affairs existed there and that the, coil also possessed a magnetic field com- posed of lines of force flowing around it. This is readily shown by punching a small hole in a piece