DISCOVERY 



309 



task, and is, in most cases, practically impossible. It 

 is found, however, that signals can be received on a 

 closed loop of w ire, and that this can be used in certain 



cases instead of an ordinar}' type of aerial. The loop 

 consists of several turns of wire wound usually around 

 the periphery of a large rectangular wooden frame. 

 This is mounted so that it can be rotated conveniently 

 about a vertical axis of symmetry in the plane of the 

 coil. 



Expressed mathematically, the energy absorbed by 

 the coil PQ (Fig. 2) at receiving station R from the 

 transmitting station T is proportional to cos 6, where 

 6 is the angle which the coil makes with the straight 

 hne joining the transmitting and receiving stations. 

 This energy' will be a maximum when 61=0, i.e. when 

 the plane of the coil lies in the straight line joining 

 the stations. In other words, signals received at 

 R on coil PQ from station T will be loudest when 

 PQ coincides with RT. The application of this 

 principle to direction-finding is easy to see, for suppose 

 the position of T is unknown. An observer at R 

 listening to signals from T rotates the coil PQ until he 

 finds the position in which the signals are loudest. 

 He then knows that the coil PQ points to the unknown 

 station. This is not sufficient to determine the posi- 



tion of T, but if similar observations are made at a 

 second station not in the same straight line as TR, the 

 data are then sufficient to determine the exact position 

 of T. Thus the position of the unknown wireless 

 station X (Fig. 3) can be determined from observations 

 made at two fixed stations Y and Z (Fig. 3). Mhen 



X transmits, Y and Z, by means of their direction- 

 finding apparatus, read off the magnetic bearing to the 

 unknown station as determined by the position of the 

 rotating coil when signals from X are loudest. The 

 point of intersection of these two bearings gives the 

 position of X. Stations Y and Z are, of course, in 

 communication with each other, either by wireless 

 or by land-hne. It may here be remarked inciden- 

 tally that by such means the position of enemy sub- 

 marines, ships, headquarters, etc., using wireless during 

 the war were determined. 



With certain modifications and adjustments, the 

 above principle has been successfully applied to air- 

 craft, and at least two main methods are in use. In the 

 first of these the aircraft carries the direction-finding 

 wireless apparatus. To find his position at any time, 

 the aircraft operator selects two land stations whose 

 positions he knows accurately. \\'hen these stations 

 transmit, he takes a bearing to each of them by means 

 of his direction-finding apparatus. These two read- 

 ings (which only take a matter of some seconds) are 

 sufficient to enable him to plot his position accurately 

 on the map. A check can be obtained by taking the 

 bearing to a third station. This method is of great 



value, especially during war, as the aircraft can deter- 

 mine its position without transmitting and thus re- 

 vealing its own presence. It also makes flying 

 possible on cloudy nights when landmarks cannot be 

 seen. The stations to which bearings are taken 

 should obviously not be too close together. During 

 peace-time, and also during war-time, there are always 

 sufficient high-power wireless stations (such as Poldhu 

 in Cornwall and Eiffel Tower in Paris) at work to 

 enable these bearings to be taken. Also, for special 

 flights, special transmission, if necessary, can be easily 

 arranged. 



For the second method no special direction-finding 

 wireless apparatus is necessary in the aircraft. This 

 is installed in the ground stations, and only an ordinary 

 wireless installation is carried in the aircraft. When 

 its position is required, the aircraft X (in Fig. 3) calls 

 up stations Y and Z and asks for its position. Both 

 Y and Z take bearings to the aircraft by means of 



