There are many interesting experiments which may be performed with the aid of a Tesla coil which space does not permit of describing here. The weird and strange beauty of the Tesla discharge is most evident when it takes place in the dark. 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.]