34 



DISCOVERY 



where signals become inaudible, or by introducing 

 another coil into the circuit and noting when its 

 introduction has no effect on the strength of signals, 

 but these are technical details which need not be con- 

 sidered here. This Bellini-Tosi arrangement was 

 used for several years before the war, but was not fully 

 developed until, during the war, the introduction of 

 the three-electrode thermionic tube as an amplifier 

 of weak signals completely revolutionised wireless 

 reception, and made it possible to employ small loop 

 aerials for the reception of signals transmitted from 

 even great distances. Small loops are now extensively 

 used for direction-finding purposes in aircraft, and 

 larger ones in ships. The adoption of this apparatus' 

 in ships is not so general as in aircraft, but a few 

 score of our merchant ships are already fitted. The 

 possible errors already mentioned largely account for 

 this comparatively slow growth, as well as the fact that 

 directional shore stations can give more accurate 



Fig. 3.— trianguIvAr loop aerials. 



bearings to ships than ships can obtain with their own 

 directional apparatus. 



In France, nine of these shore stations are already 

 in operation along the English Channel alone, and in 

 the United States about a hundred are contemplated, 

 of which more than half are working. In this country 

 there are only four in operation. These are at present 

 under Admiralty control, but as has recently been 

 announced in the Press, it is intended to transfer this 

 work to the Post Office, the department responsible 

 for our commercial wireless communications. In 

 shore stations greater accuracy is obtained than in 

 ships, as there are less obstructions to the path of the 

 waves in the form of masts, stays, etc., and more 

 highly trained operators, as well as conveniences in 

 the matter of space, quietness, and stability, are 

 available. 



A ship or aircraft fitted with directional apparatus 

 can obtain a bearing relative to its fore and aft line 

 from any wireless station which it hears transmitting. 



and by taking such bearings from two, or preferably 

 three, stations, it can obtain its position, usually to a 

 degree of accuracy sufficient for the purposes of 

 navigation when other means have failed, provided 

 that the stations happen to be suitably placed for 

 obtaining a good cut with the lines of bearing. For 

 distances up to about 100 miles the bearings can 

 be plotted on the usual Mercator's chart, but for 

 greater distances a chart on the gnomonic projection 

 must be used. In these gnomonic charts, great 

 circles appear as straight lines instead of curves as 

 in Mercator charts, and the necessity of using the 

 gnomonic projection for long ranges is evident when 

 it is remembered that wireless waves travel along 

 great circles over the surface of the earth. As a 

 matter of fact, in ship work, bearings from stations 

 at distances of over 100 miles are very seldom 

 required, so that the usual Mercator chart gives 

 sufficiently accurate results under favourable con- 

 ditions ; but, as mentioned above, the results 

 obtained by ship sets are not so reliable as those 

 obtained by direction-finding shore stations. In the 

 latter case, the ship, which does not require any 

 directional set of its own, calls up a statibn and asks 

 for its bearing. The ship then transmits a succession 

 of letters, the station takes the bearing and signals 

 it to the ship. Sometimes two or more stations work 

 in conjunction ; each takes a bearing, and the con- 

 trolling station then signals to the ship the position 

 which is obtained from tho bearings taken. It will 

 be noticed that when shore stations do the work, the 

 responsibility for the results rests with them, not with 

 the ship, as would be a more satisfactory arrangement 

 if the results obtained by ships were as reliable as those 

 obtained by shore stations. In this country, a charge 

 of five shillings is made for a bearing given by a 

 direction-finding shore station, and similar charges 

 are in contemplation, or are already made, in several 

 other countries. 



The " radiophare," or wireless lighthouse, is another 

 form of directional shore station, and some of these 

 are in operation in France and the United States, but 

 not in this country. A radiophare is a directional 

 transmitting station which sends out continuously 

 a beam of wireless waves in a certain direction, strong 

 enough to be received by a ship up to a range of, say, 

 thirty miles. The transmission consists of the call 

 signal of the station sent continuously, so that when 

 a ship is receiving this signal it knows that it must 

 be somewhere in the beam sent out by that particular 

 station, and details are published of the direction of 

 the beam sent out by each radiophare. This form of 

 station is being found useful for navigational purposes 

 during fog in narrow waters, such as river mouths 

 and channels. 



