The transmitting and receiving aerials are the same as in wireless telegraphy, and like them are tuned to the same frequency. The receiving apparatus too is of the ordinary wireless type, with telephones and electrolytic or other detectors.
Wireless telephony has been used with considerable success in various German collieries, and at the Dinnington Main Colliery, Yorkshire. Early last year Marconi succeeded in establishing communication by wireless telephony between Bournemouth and Chelmsford, which are about 100 miles apart; and about the same time a song sung at Laeken, in Belgium, was heard clearly at the Eiffel Tower, Paris, a distance of 225 miles. The German Telefunken Company have communicated by wireless telephony between Berlin and Vienna, 375 miles, and speech has been transmitted from Rome to Tripoli, a total distance of more than 600 miles. These distances are of course comparatively small, but if the microphone trouble can be overcome satisfactorily, transatlantic wireless telephony appears to be quite possible.
CHAPTER XXI
WIRELESS TELEGRAPHY—PRACTICAL APPLICATIONS
A fairly good idea of the principles and apparatus of wireless telegraphy should have been gained in reading [Chapter XX]., but so far little has been said about its practical use. If we leave their power out of consideration, wireless stations may be divided into two classes: fixed stations on land, and moving stations, if the expression may be allowed, on ships. For moving stations wireless telegraphy has the field all to itself, but for communication between fixed stations it comes into conflict with ordinary telegraphy by wire or cable. As regards land messages over comparatively short distances, say throughout Great Britain, wireless telegraphy has no advantages over the older methods; and it is extremely unlikely that it ever will be substituted for the existing cable telegraphy. For long distances overland wireless has the great advantage of having all its apparatus concentrated at two points. A long land line passing through wild country, and exposed to all kinds of weather, requires constant labour to keep it in good repair, and when a breakdown occurs at any point, the repairing gang may be miles away, so that delay is caused. On the other hand, whatever may go wrong at a wireless station, no time is lost in effecting the necessary repairs, for everything is on the spot.
At present there is no great competition between wireless and ordinary telegraphy for overland messages of any kind, but the case is different when we come to communication across seas and oceans. Already the cable companies have been affected considerably, and there is little doubt that they will feel the competition much more seriously before long. The general public, always conservative in such matters, have not yet grasped the fact that telegrams can be handed in at any telegraph office in the British Isles, and at most telegraph offices in the United States and Canada, for wireless transmission across the Atlantic, via the Marconi stations at Clifden and Glace Bay. The cost is remarkably small, being eightpence a word for ordinary messages.
It is impossible to state with any accuracy how many land wireless stations there are in the world, but the list given in the Year-Book of Wireless Telegraphy for 1915 enumerates about 700 stations. This list does not include private or experimental stations, and also many stations used exclusively for naval or military purposes are not given. The information available about these 700 stations is incomplete in many cases, but about 500 are controlled by various departments of the governments of the different states. Of the remainder, about 100 are controlled by the Marconi Company, the rest being in the hands of various wireless, commercial, or railway companies.
Amongst the most important land stations are the Clifden and Glace Bay transatlantic stations. They are very similar in plan, and each has a separate aerial for sending and for receiving. Contrary to the usual practice, continuous current is used to charge the condensers. In [Chapter IV]. we saw how a current of high voltage could be obtained by connecting a number of cells in series, and at these stations the necessary high voltage is produced by connecting a number of powerful dynamos in series, on the same principle. Along with the dynamos a huge battery of accumulators, consisting of about 6000 cells, is used as a sort of reservoir of current. These stations have a normal range of considerably over 3000 miles. Last year a large transmitting station was completed at Cefndu, near Carnarvon. This station, which is probably the most powerful in existence, is intended to communicate directly with New Jersey, United States, as an alternative to the Clifden-Glace Bay route.
Other powerful stations are Poldhu, in Cornwall, of which we shall speak later; the French Eiffel Tower station; the German station at Nauen, near Berlin, which last year succeeded in exchanging messages with Windhoek, German South-West Africa, a distance of nearly 6000 miles; and the extremely powerful station at Coltano, Italy. France has three stations in West Africa with a night range of 1600 miles; and Italy one in Somaliland with a normal range of about the same distance. The recently opened Chinese stations at Canton, Foochow, and Woosung have a range of 1300 miles by night, and 650 miles by day. With the fall of Tsingtau, China, Germany lost a wireless station capable of signalling over 1350 miles at night. Japan has six stations with a night range of over 1000 miles. Massawa, on the Red Sea, has a range of 1600 miles, and New Zealand has two stations with ranges of 1200 miles by day, and 2500 miles by night. Australia has a large number of stations with a normal range of about 500 miles. In the United States, which has a very large number of stations, Arlington, Virginia, covers 1000 miles, and Sayville from 600 to 2300 miles. South America has not many high-power stations, but Cerrito, in Uruguay, has a range of about 1000 miles.
Until a thoroughly practical system of long-distance wireless telephony is developed, wireless telegraphy will remain the only possible means of communication between ships and shore, or between one ship and another, except where the distance is so small that some method of semaphore signalling can be used. In the days when wireless was unknown, a navigator was thrown entirely upon his own resources as soon as his vessel was out of sight of land, for no information of any kind could reach him. Even with a wireless installation on board, the captain of a vessel still needs the same skill and watchfulness as of old, but in the times of uncertainty and danger to which all ships are liable, he often is able to obtain information which may prevent disaster. In order to determine accurately his position, a navigator requires to know the exact Greenwich Mean Time, and he gets this time from his chronometers. These are wonderfully reliable instruments, but even they may err at times. To avoid the possibility of mistakes from this cause, wireless time signals are sent out at regular intervals by certain high-power stations, and as long as a vessel is within range of one of these stations the slightest variation in the chronometers may be detected immediately. Amongst these stations are the Eiffel Tower, giving time signals at 10 a.m. and at midnight; and Norddeich, Germany, giving signals at noon and midnight. These time signals have proved most useful also on land, more particularly for astronomers and for explorers engaged on surveying work.
In addition to time signals, other valuable information is conveyed by wireless to ships at sea. A ship encountering ice, or a derelict, reports its discovery to other ships and to the shore stations, and in this way vessels coming along the same route are warned of the danger in time to take the necessary precautions. Weather reports are issued regularly from various shore stations in most parts of the world. The completeness of the information given varies a good deal with different stations, but in many cases it includes a report of the existing state of the weather at a number of different places, a forecast of the winds likely to be encountered at sea, say at a distance of 100 miles from land, and warnings of approaching storms, with remarks on any special atmospheric conditions at the time of sending. In Europe weather reports are issued daily from the Admiralty station at Cleethorpes, the Eiffel Tower, and Norddeich; and in the United States more than a dozen powerful stations are engaged in this work.