Care should be exercised to leave the paraffine sheets perfect or without holes. You can make these sheets yourself by soaking them in melted paraffine wax. CONNECTING UP A CONDENSER.--When completed, one end of the condenser is connected up with one terminal of the secondary coil, and the other end of the condenser with the other secondary terminal. [Illustration: _Fig. 73._ HIGH-TENSION CIRCUIT] In Fig. 73 a high-tension circuit is shown. Two coils, side by side, are always used to show an induction coil, and a condenser is generally shown, as illustrated, by means of a pair of forks, one resting within the other. THE INTERRUPTER.--One other piece of mechanism is necessary, and that is an _Interrupter_, for the purpose of getting the effect of the pulsations given out by the secondary coil. A simple current interrupter is made as follows: Prepare a wooden base (A), one inch thick, six inches wide, and twelve inches long. Upon this mount a toothed wheel (B), six inches in diameter, of thin sheet metal, or a brass gear wheel will answer the purpose. The standard (C), which supports the wheel, may be of metal bent up to form two posts, between which the crankshaft (D) is journaled. The base of the posts has an extension plate (E), with a binding post for a wire. At the front end of the base is an L-shaped strip (F), with a binding post for a wire connection, and the upwardly projecting part of the strip contacts with the toothed wheel. When the wheel B is rotated the spring finger (F) snaps from one tooth to the next, so that, momentarily, the current is broken, and the frequency is dependent upon the speed imparted to the wheel. [Illustration: _Fig. 74._ CURRENT INTERRUPTER] USES OF HIGH-TENSION COILS.--This high-tension coil is made use of, and is the essential apparatus in wireless telegraphy, as we shall see in the chapter treating upon that subject. CHAPTER XI WIRELESS TELEGRAPHY TELEGRAPHING WITHOUT WIRES.--Wireless telegraphy is an outgrowth of the ordinary telegraph system. When Maxwell, and, later on, Hertz, discovered that electricity, magnetism, and light were transmitted through the ether, and that they differed only in their wave lengths, they laid the foundations for wireless telegraphy. Ether is a substance which is millions and millions of times lighter than air, and it pervades all space. It is so unstable that it is constantly in motion, and this phase led some one to suggest that if a proper electrical apparatus could be made, the ether would thereby be disturbed sufficiently so that its impulses would extend out a distance proportioned to the intensity of the electrical agitation thereby created.