CHAPTER 9 

 CURRENT MEASUREMENTS 



9-1 GENERAL REMARKS.— Probably 

 more types of instruments are used for measur- 

 ing currents than for any other single oceano- 

 graphic measurement. Devices range from the 

 simple drift bottle to the automatic radio trans- 

 mitting and monitoring system of Roberts 

 radio current meters. Although many types of 

 current-measuring instruments have been de- 

 veloped most of them are limited in their use, 

 either by cost or design. A simple, rugged, and 

 accurate instrument that can indicate weak as 

 well as strong current speeds, direction, and 

 depth is still one of the most important needs 

 of oceanographic instrumentation. 



Types of current-measuring instruments may 

 be divided into three broad and general cate- 

 gories: free floating, shallow water, and ship- 

 board. Those in the first category include 

 drift bottles, dye marks, and floats that could 

 be observed from ship, shore, or aircraft. 

 Those in the second category include instru- 

 ments that are attached to piers, buoys, or 

 beacons, or are placed on the bottom in rivers, 

 bays, estuaries, and other nearshore areas at 

 more or less set depths. Those of the third 

 category include instruments that can be op- 

 erated when the ship is underway and those 

 operated when the ship is anchored. Repre- 

 sentative instruments of the second and third 

 categories are discussed in this chapter. 



To take surface current measurements when 

 underway. Navy survey ships use the GEK. 

 From this instrument the speed and direction 

 of surface currents can be calculated rapidly. 

 Subsurface currents at various depths are 

 determined from a ship at anchor with the 

 Ekman, Price, or Roberts current meters. 

 The Ekman and Roberts meters record both 

 speed and direction while the Price indicates 

 speed alone. 



9-2 INSTRUCTIONS FOR OPERATING 

 THE GEOMAGNETIC ELECTROKINETO- 

 GRAPH (GEK).— The GEK is a shipboard 

 surface-current measuring device designed to 

 record the electrical potential developed by 

 the movement of an electrolyte (ocean current) 



H. O. 607 



through a magnetic field (the earth's) in depths 

 of more than 100 fathoms. 



The essential physical equipment constitut- 

 ing the instrument is: 



1. A matched pair of electrodes mounted 100 

 meters apart on a 2-conductor cable long 

 enough (ordinarily 3 times the length of the 

 ship) to stream them astern, away from the 

 magnetic influences of the ship. 



2. A recording potentiometer assembly to 

 which the cable is connected. 



3. A gjTOcompass repeater, mounted above 

 or close to the recorder assembly. 



With the above equipment, observations of 

 the potential difference between the electrodes 

 along the ship's course and at right angles to it 

 are made underway. These potential differ- 

 ences are due to the motion of the water 

 through the earth's magnetic field. They are 

 rigidly related to the set and drift of the ship 

 and to the electrodes. The potential differ- 

 ence changes sign when currents set the ship 

 to port or starboard. The magnitude of the 

 potential difference depends on the rate of drift 

 normal to the course, the length of cable be- 

 tween electrodes, the local strength of the 

 vertical component of the earth's magnetic 

 field, and to some extent on the vertical distri- 

 bution of water velocities in the vicinity. 

 Through measurements of the potential dif- 

 ferences on 2 courses nearly at right angles, the 

 drift or component velocities in these 2 direc- 

 tions are known. The vector sum or resultant 

 of these velocities is the surface current vector 

 for that locality. 



9-3 Electrical Power Requirements. — The 

 power required to operate the GEK is usually 

 110 volts, 60 cycles, AC, with a power consump- 

 tion under 100 watts. 



9-4 Isolation Transformer. — It is necessory 

 to isolate the power with an isolation trans- 

 former to block any possible DC leakage that 

 may be present on the lines. The voltage and 

 frequency of the input power should be moni- 

 tored to arrest any variation of timekeeping 

 on the synchronously driven strip chart. More- 



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