Three different methods have been made use of in wireless telegraphy, which may be classed as conduction, induction and wave methods. In the first method currents are sent through the earth from an electrode to another at the sending station. In induction, use is made of the property which alternating currents possess of exciting similar currents in neighboring conductors, the aim being to get as intense a current as possible in the secondary circuit. Mr. W. H. Preece, of England, by combining the two, signaled in this way as far as forty miles. The third and the only method which has proved practically available is by the use of electro-magnetic waves.
Guglielmo Marconi, an Italian, after long experiment, patented in 1897 a method entirely independent of wires, the electric waves being sent, presumably, through the ether, by the aid of a transmitting apparatus, and being detected by a coherer, a glass tube filled with metallic filings, into the end of which the terminals of a relay circuit enter. The wave falls on conducting material and, the spark gap being replaced by a coherer, the metallic filings magnetically cling together, closing the relay circuit, so that a signal is made. On breaking the current, a slight tap on the coherer or other means breaks the cohesion of the filings and the relay circuit is broken. In this way a rapid succession of signals can be sent.
In 1899 Marconi conducted in England an exhaustive series of successful experiments, sending messages across the English Channel from the South Foreland to the French coast near Boulogne, and extending his results until much longer distances were covered. The process of development was continued until, to the world’s astonishment, signals were sent across the Atlantic and, finally, commercial messages were transmitted over this distance.
Marconi’s system is based on the property supposed to be exerted by the vibrations or waves of electric currents passing through a wire of setting up similar vibrations in the ether of space. These waves extend in every direction from the point of departure, and by ingenious and very delicate receiving instruments their presence in space is indicated and they are taken up in sufficient strength to repeat their pulsations and in this way reproduce the signals sent from the transmitter. One difficulty hitherto has been that a message may be received by hundreds of receiving instruments in all directions, thus preventing secrecy. Many efforts have been made to overcome this defect, but as yet with only partial success.
The distance to which messages can be sent has so far depended largely on the height to which the wires extend above the earth’s surface, lofty poles being erected at the stations. The height of these has been gradually increased until the Eiffel Tower at Paris has been utilized as a sending station. The strength of the electric waves has been similarly increased to add to their space-penetrating capacity. The record of wireless telegraphy has been in this way improved until now it has come into daily competition with other means of news sending. Methods of tuning the instruments have been adopted which limit the influence of the currents to properly tuned receivers and in this way some degree of secrecy is attained.
Marconi Wireless Station
Though the honor of inventing the art of wireless telegraphy is generally ascribed to Marconi, this is to give him more credit than he deserves. The principles involved were discovered by others and the utmost done by him was to invent a practical method of applying them. There are other systems of wireless telegraphy of later invention than that of Marconi, through a different application of the same principles.
Messages have been sent to enormous distances, far surpassing the width of the Atlantic, as from Nova Scotia and Ireland to Argentina, a distance of 5,600 miles. Under exceptional conditions a distance of 6,500 miles has been attained, but the daily effective range of the best equipped stations is little over 3,000 miles. For overland messages the limit of distance is about 1,000 miles.
There are a number of kinds of interference which arise from electrical disturbances in the earth’s atmosphere. A flash of lightning is liable to give rise to a wave of enormous power which will set half the aerials on the earth vibrating in spite of the differences of pitch to which they are tuned. Thunderstorms are at their worst in the summer in temperate latitudes, but they occur to some extent all the year round, and those in the tropics are of extreme violence. As a consequence it is frequently almost impossible to decipher earthly messages owing to the imperious signals from the clouds. Of the various methods adopted for choking off the “atmospherics,” as the disturbances are called, one is to use receiving circuits which respond only to a narrow range of oscillations very different from those produced by a lightning flash. The employment of a high-pitched musical note in the telephone is also an advantage because its extreme regularity distinguishes it from the marked irregularity of the stray waves.