Possible plans of wireless telegraphy were shown by experiments of John Trowbridge of Harvard University, W. H. Preece of the British Postal Telegraph and others between 1880 and 1890. These earlier attempts used the earth as a conductor or depended upon inducing currents by electromagnetic induction (see Electricity) between two systems of wires. In 1892 Preece for the British Lighthouse Board used a combination of the induction and conduction methods to reach islands six miles and more from the mainland. He concluded that, though communication across space was thus practical under certain conditions, the apparatus was too cumbrous and costly except for special cases.
The only wireless telegraphy in use now is that based on the electrical researches of James Clerk-Maxwell and Heinrich Hertz, and large credit for developing this is due to Guglielmo Marconi (q. v.). Marconi's early experiments in wireless telegraphy, made in 1896, were supported by the British postal authorities and reached about ten miles; by 1899 he had established wireless telegraphy 30 miles across the English Channel; and on Dec. 12, 1901, a signal was received by wireless telegraphy in Newfoundland from Cornwall, England, a distance of 1,800 miles. On Jan. 19, 1903, a wireless message was sent from President Roosevelt to King Edward across the Atlantic between Cape Cod and Cornwall.
Marconi's system is in all principles the carrying out on a large scale of Hertz's electric wave experiment of 1887. Marconi' s transmitter consists (Fig. I) of an induction coil I which produces electric sparks across an air or oil gap G, between two metal balls, when the battery key K is closed. When the balls, wires and other parts, EGA, are of proper size and shape, the electric discharge produces a train of electric waves which go out in all directions with the velocity of light. These electric waves pass through air, wood, dry earth etc., that is, through poor electric conductors, as light passes through glass. They are reflected from metals and other good conductors.
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| COHERER |
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Fig. II. (T) exhausted glass tube; (PP) silver terminals separated by metal filings and connected with (A) vertical wire and (E) ground. |
The receiver mostly used at the present time for detecting these electric waves is the coherer invented by E. Branly of Paris in 1890. It consists of (Fig. II) a glass-tube T with two silver plugs PP, the plugs being separated by a small quantity of metal filings mostly nickel and silver. The loose filings offer a high resistance to the electric current, but become a fairly good conductor under the action of an electric wave; that is, they cohere. The silver plugs are in a circuit with the usual telegraphic receiving instruments, (Fig. III) a relay R, and a sounder M (or a Morse ink recorder), and a tapper t, for shaking the filings so that the coherer may be ready for the next signal. Each train of electric waves thus closes the circuit through the telegraphic instruments. The ordinary Morse alphabet, made by combinations of long and short signals, is used. (See Telegraph.)
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| RECEIVER |
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Fig. III. (T) coherer; (A) vertical wire; (E) ground; (R) relay; (M) sounder or recorder; (t) tapper. |
Marconi's great advances have been (1) in showing, as part both of transmitter and of receiver, a high vertical wire or series of wires using towers often 100 feet or more high; (2) in increasing the sensitiveness of the coherer far beyond anything known before; (3) in devising and using very powerful discharge-currents.
A defect of wireless telegraphy has been that all receiving-stations within the radius of action of the transmitter receive the electric signals. To overcome this, attempts have been made to tune or make syntonic the transmitting and receiving apparatus, so that receivers will respond only to signals from corresponding transmitters. The method in syntonic telegraphy is to use combinations of coils and condensers or Leyden jars with the discharge-balls and the wave-detector, instead of using the
