High frequency currents do not produce a shock. If you hold a piece of metal in your hand and bring it near one of the secondary terminals, you can take the shock of a high-frequency coil, throwing a spark several inches long without feeling any sensation except that of a slight warmth.
CHAPTER XVIII. AN EXPERIMENTAL WIRELESS TELEPHONE.
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.
[Illustration: FIG. 179.—When a Bar Magnet is plunged into a Hollow Coil of Wire, a Momentary Current of Electricity is Generated.]
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.
[Illustration: FIG. 180.—Magnetic Phantom showing the Lines of Force about a Bar Magnet.]
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.