Popular Science Monthly 



453 



tached to this secondary or "exploring" 

 coil indicates its position and con- 

 sequently the direction of the sending 

 station. 



A useful application of the direction 

 finder is the determination of whether 

 the ship is on a course which will take 

 it inside or outside a lightship or iso- 

 lated lighthouse. A few signals from 

 the fixed station will settle the question 

 as certainly as if the light were yisible. 

 Similarly, when making a harbor, a few 

 signals from a station within will show 

 immediately whether the ship has drifted 

 to one side of the entrance. When try- 

 ing to locate another vessel in a fog, the 

 indication of the direction finder may 

 show, by a steadily increasing strength 

 of signal, that the other ship is approach- 

 ing, but, since only the direction and 

 not the sense is given, it might leave 

 doubt as to whether it was approaching 

 on the port bow or the starboard quarter. 

 A wireless query as to her course, ad- 

 dressed to the other ship, would remove 

 the doubt at once. 



Following out the German idea and in- 

 stalling the compass on shore, relieves 

 the ship of a special aerial, but the 

 point against it is that there must be 

 numerous coast stations fitted out with 

 the transmitting apparatus. The Tele- 

 funken device is used along the German 

 coast, however, and at the outbreak of 

 the war, comprehensive schemes for in- 

 stallation at intervals of every 25 miles 

 along the northern and western coasts 

 were about to be carried out. 



United States Government engineers, 

 working from a slightly dififerent angle, 

 have suggested a plan which they be- 

 lieve will greatly reduce the fog peril 

 and yet require minimum investment in 

 men and money. It is merely a wire- 

 less transmitter, fitted with an auto- 

 matic sending device, and calibrated to 

 send only a limited distance. This radio 

 fog-signal may be installed with equal 

 facility on shipboard or at a land station. 

 The antennas are of the simplest type, 

 and the automatic transmitter makes it 

 possible for any person to operate it. A 

 ship making its way along the coast in 

 a fog may hear some lighthouse in his 

 vicinity, equipped with the radio fog- 

 signal, sending out a pre-arranged series 

 of signals, characteristic of that particu- 



The transmitting distributor of the Tele- 

 funk en compass 



lar lighthouse. The captain then knows 

 that he is within ten or twelve miles of 

 that particular point. His position is 

 further fixed as the ship proceeds, from 

 the change in intensity of the signals, 

 since, if the signals increase in strength, 

 the captain knows that he is getting 

 nearer the source of transmission. 



A bad coast may be fitted with the 

 radio signals at intervals close enough 

 so that the coast-wise vessel will 

 pass directly from the jurisdiction of 

 one to that of the next. In this way 

 there will be continuous protection for 

 the ship. Installed on shipboard it may 

 prove a valuable means of keeping ves- 

 sels from crowding on to one another. 

 The radio fog-signal is not a direction 

 finder, but is to be merely a warning to 

 ships passing along a dangerous coast, 

 or an inexpensive addition to ship's 

 equipment which way be used in time 

 of fog. 



The Earth's Conductivity 



TH E resistance of sea-water is 

 only about one-hundredth that 

 of fresh water. Damp earth often 

 oflFcrs less resistance to electric current 

 than docs fresh water, but dry earth 

 m(^asures over ten times as many ohms 

 between opposite sides of a cubic 

 section. 



