Page:Popular Science Monthly Volume 88.djvu/481

 Popular Science Monthly

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��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.

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