WHEN charts showing ocean currents were first produced around the middle of the last century, the means of estimating the rate of movement of a current, or what the cognoscenti like to call its "drift", were relatively simple. The observer first used dead reckoning to establish the position of his ship, then some hours later, having estimated his speed and direction of movement during the intervening period, and made due allowance for leeway the drift from the intended course due to winds or tides he calculated what ought to be his new location, finally he once again "dead reckoned" his position, and any difference between the two was deemed to have been caused by currents.
This method is still often used today, using modern satellite navigation systems instead of "dead reckoning", but other more subtle ways of measuring the drift of currents have also been developed. A simple method is to drop a buoy or "log" into the ocean, and track it using satellites or radar. Moreover, currents some distance below the surface can be measured in this way by attaching a parachute or "drogue" at a specific depth to a device like this; the speed and direction of movement of the log will then be largely determined by the drogue below, and hence by the drift of the submarine current.
Currents in confined areas like estuaries, or near the shore, can be tracked by using harmless dyes whose progress is followed from an aircraft or a satellite. Alternatively, rotating current meters, each resembling a propeller and anchored by cable at a predetermined depth, can be employed these are based on the assumption that the number of revolutions of the propeller which are counted, is related to the speed of movement of the current. And more sophisticated methods involve the use of acoustic meters, which measure the time taken for a pulse of sound to travel a known distance underwater. The sound waves are either assisted or retarded by any movement of the water, and the time measured allows the drift of currents to be calculated.
Finally there are electromagnetic current meters. Sea water, because it contains dissolved salts, is an efficient conductor of electricity, and when any electrical conductor moves across a magnetic field, an electric current tends to flow in it. Hence tiny electric currents can be detected in moving sea water by suitably placed electrodes, being generated either by the earth's magnetic field or by an artificial one specially provided for the purpose. The electric current readings in these circumstances give a measure of any movement of the water.