RIVERS IN THE SEA — SMITH 439 



buoys, with deep keels or weighted poles to minimize wind action, 

 are now equipped to send out radio signals which carry information 

 about water temperature as well as signaling their changing positions. 

 Surface vessels are thus able to follow individual floats at their will, 

 simply by "homing" on the radio signals. Another type of float works 

 in reverse, as it were, by having a staff and metal flag that acts as a 

 radar target, so that the research vessel may find its free floating 

 buoys on its own radar screen. This system obviously suffers from 

 being unduly subject to the effects of wind on the target. Still an- 

 other type of free-floating buoy, developed at Miami, has a lightweight 

 anchor and cable. At the end of a specified period, say 30 hours, a 

 clockwork mechanism trips the anchor, thus mooring the float, and 

 erects a radar target, previously folded down out of wind action. 

 So far we have considered the more or less direct ways of measuring 

 ocean currents, but there are some interesting properties of sea water 

 that make it possible to use indirect methods, involving, odd as it may 

 seem, tide gages, thermometers, electric currents, and even chemical 

 analysis. Only by means such as these is the oceanographer able to 

 compute more or less accurately the volume of water flowing in the 

 sea which, in the case of the Gulf Stream, is many hundred times that 

 of River Mississippi in flood. 



ELECTROMAGNETIC MEASUREMENT 



An old principle has been recently applied to the measurement of 

 currents. This is based upon the electromagnetic properties that 

 underlie the dynamo from which we obtain our electricity for power 

 and light. When an electrical conductor moves across a magnetic field, 

 then an electric current is developed in the circuit containing the con- 

 ductor. The faster the conductor moves the more electricity is pro- 

 duced. Pretty much the same thing happens when sea water, itself a 

 conductor of electricity, moves across the earth's magnetic field. 

 And so, by measuring the small amounts of electricity produced at 

 sea, we have a means of measuring the rate of flow of the water. For 

 this purpose two stationary electrodes may be placed at suitable dis- 

 tances apart and the electrical flow between them measured. Or 

 they may be towed behind a ship. In each case they measure the 

 water flow between and at right angles to the line joining the elec- 

 trodes, independent of the ship's speed. The instrument used at sea 

 today, including a potentiometer, which records the speed of water 

 current continuously on paper, is known as the G. E. K., or to be 

 more exact the geomagnetic electrokinetograph. When the method 

 was originally tested in England by Faraday from a bridge over 

 the River Thames the results were not satisfactory, but in more re- 

 cent years Longuet-Higgins and subsequently Von Arx have developed 



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