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Aviation History
1914
1914 - 1119.PDF
NOVEMBER 13, 1914. fJOGHf] WIRELESS TELEGRAPHY AND AIRCRAFT. BEARING in mind the great part which scouting by aero planes has played in the present war, and the necessity for means of communication to and from the reconnoitring machine, a description of one of the systems of wireless installations fitted to aircraft should be of interest to our readers. It may be useful, first of all, to briefly sketch the elementary principles of wireless telegraphy for the benefit of those who may be unacquainted with this important method of communication. The fundamental basis of wireless telegraphy consists of the following " Flight" Copyright. Diagrams illustrating simple wireless telegraphy sets. Fig. 1.—The simplest form of transmitting and receiving set. Fig. 2.—The transmitting set of a more modern system. Fig. 3.—The receiving set for the same system. electric phenomenon. If an electric current flowing through a circuit consisting, say, of a coil of wire, is intermittently interrupted, or if the direction of flow of the current alternates, a varying magnetic field will be set up round the circuit which will induce secondary currents in an entirely independent circuit placed some distance away from the first or primary circuit. Thus, a telephone receiver inserted in the secondary circuit will respond to these interruptions or reversals of current in the primary circuit. The distance at which the secondary circuit can be so affected is somewhat limited, but similar forms of induction -over great distances can be caused by means of electro-magnetic waves produced by the surging backwards and forwards of an electric current in a length of wire termed an " aerial." These waves, which have the velocity of light, are known as Hertzian waves, and are employed in the Marconi system of wire less telegraphy. How these waves are brought about and caused to transmit messages, may briefly be ex plained with the help of the accompanying diagrams. Fig. 1 shows the simplest form of transmitting and receiving apparatus, in which A represents the aerial, a long wire carried by a mast and insulated from the earth. The lower end of the aerial is connected to one of a pair of spark balls, S, of an induction coil, I. The other spark ball is connected by a wire to the earth, E. When a primary current from the battery, B, is made to flow through the induction coil, the aerial is charged with a high tension current until the latter breaks through the air space between the spark balls, S, in the form of a spark. The aerial then discharges itself, the current oscillating backwards and forwards from the elevated aerial and the earth MM the spark gap, each oscillation diminishing in strength until they die out altogether, when the aerial is charged again as before. Each of these trains of oscillations in the aerial produces a similar train of Hertzian waves, which radiate from the aerial into space, like ripples caused by a stone dropped into water. So long as a primary current flows through the induction coil, so as to produce a regular series of sparks at the spark gap, a constant train of waves is emitted from the transmitting aerial, but any interruptions in the primary current cause similar interruptions in the waves. By inserting a Morse key, K., in the primary circuit, therefore, the waves can l>e broken up into long or short trains instead of continuous ones, thus produc ing " dash " and " dot" signals. The apparatus for receiving these signals consists of an aerial, A\ with its lower end connected to one terminal of a " detector," D, the other terminal of which is connected to the earth, B. Also connected to the terminals of the detecttn is a telephone receiver, R, in series with a battery, B\ the voltage of which can be varied. When the waves from the transmitting aerial strike the receiving aerial, they induce oscillations in the latter corresponding to those originating from the former. These induced oscillating currents flow from the aerial to the earth through the detector, and thereby modify the direct current flowing from the battery through the telephone receiver and the detector, in response to the "dash" and "dot" signals. Although the type of apparatus just described is not employed in modern wireless installations, owing to its limited range of action, and other disadvantages, the general principles are the same—in fact, it might be O^""^. Counterpoise /•'£r7/\.l 1 i^TN ^^^ ^T^V^**^^ A»rt»|.N»j ^v " Flight" Copyright Fig. 4.—Sketch showing the fitting of a "counterpoise," which ukes the place of "earth" on a biplane; also showing the transmitting and receiving aerial. said that the modern system is only the old one elaborated. This is illustrated by Figs. 2 and 3, which show diagrammatically a modern transmitting and receiving set, in its simplest form. There are, of course, various modifications in detail in the systems now in use, such as tuning devices, wave detectors, 1119
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