CURRENT MEASUREMENTS

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The ocean currents are complicated because, superimposed upon the major currents that transport enormous masses of water, there are irregular eddies that may reach to great depths, wind currents that are confined to the surface layers, and tidal currents or currents associated with internal waves which are present at all depths between the surface and the bottom but which change periodically. In many instances the major currents cannot be directly measured, but conclusions as to their directions and velocities must be based on the application of the laws of hydrodynamics to the observed distribution of density. The methods used in such studies are dealt with in chapter XIII. Here will be discussed only the methods for direct observation, and these can be conveniently classified into two groups: drift methods and flow methods. An excellent summary of all methods and a description of instruments used have been prepared by Thorade (1933).

In scientific literature the velocity of a current is given in centimeters per second (cm/sec) or occasionally in meters per second (m/sec), but in publications on navigation the velocity is stated in knots (nautical miles per hour) or in nautical miles per 24 hours. The term “knots” dates back to the time of the sailing vessels when the speed of the vessel was measured by chip log, log line, and sand glass. Along the log line distances from a zero mark were shown by short strings provided with knots. On the first string was tied one knot, on the second two, and so on. When the chip log was thrown overboard the log line would be pulled out and as the zero mark passed the rail the sand glass was turned. When the sand glass had run down, the log line was stopped and the number of knots on the nearest string were counted. The sand glass and the distances between strings were adjusted to give the speed of the vessel in nautical miles per hour; thus the number of knots gave the speed in this unit—that is, in “knots.”

The direction is always given as the direction toward which the current flows, because a navigator is interested in knowing the direction in which his vessel is carried by the current. The direction is indicated by compass points (for example, NNW, SE), by degrees reckoned from north or south toward east or west (for example, N 60°W, S 30°E) or in degrees from 0° to 360°, counting current towards north as 0° (or 360°) and current towards south as 180°.

Drift Methods

Information as to the general direction of surface currents is obtained from the drift of floating objects such as logs, wreckage from vessels, and fishermen's implements. Thus, glass balls used by Japanese fishermen and wrecked Chinese junks are sometimes found on the west coast

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of the United States, and from these findings it is concluded that a current flows from west to east across the North Pacific Ocean. Another example is the recovery in July, 1884, from a drifting ice floe off southwestern Greenland, of equipment and documents from the ill-fated Jeanette, which on June 12, 1881, had been crushed in the ice to the east-northeast of the New Siberian Islands in lat. 77°17′N, long. 153°42′E. The recovery of these objects established the fact that water extends from Siberia to Greenland, and, since the relics were picked up on a piece of floating ice, the average speed of the drift across this sea could be ascertained. In most instances, conclusions as to currents by the finding of accidental drifting objects are incomplete, because the locality and the time at which the drift started are not known, nor is it known how long the object might have been lying on the beach before discovery. It is also difficult to determine to what extent such drifting bodies have “sailed” through the water, being carried forward by winds.

More than a century ago, in order to overcome such uncertainties, drift bottles were introduced. These are weighted down with sand so that they will be nearly immersed, offering only a very small surface for the wind to act on, and they are carefully sealed. They contain cards giving the number of the bottle, which establishes the locality and time of release, and requesting the finder to fill in information as to place and time of finding and to send this information to a central office.

In order further to insure reduction of the direct effect of the wind, drift bottles have sometimes been provided with a kind of drift anchor—for example, a cross-shaped piece of sheet iron suspended about 1 m below the bottle. In other instances, two bottles have been used, one of which has been weighted so much that it is carried by the other, the connecting wire between the bottles being about 1 m long. Still other experiments have been conducted with two bottles, one containing a weak acid which in a given length of time corrodes a metal stopper, thus permitting the sea water to fill the bottle. When this takes place, the bottles sink to the bottom, where they are held by a piece of sheet metal that acts as an anchor. This device has been used in the shallow waters of the North Sea, where bottom trawls are used extensively by fishermen, who recover many of the bottles.

The interpretation of results of drift-bottle experiments presents difficulties. In general, a bottle has not followed a straight course from the place of release to the place of finding, and conclusions as to the probable drift must be guided by knowledge of the temperature and salinity distribution in the surface layers. Fairly accurate estimates of the average speed of the drift can be made if the bottle is picked up from the water, or if a special drift bottle is brought up from the bottom. Bottles that are picked up on frequented beaches can also be used for estimating the speed of the drift. Tait's conclusions (1930) from the

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results of drift-bottle experiments in the eastern North Sea afford an example of ingenious interpretation. Numerous bottles thrown out simultaneously in about last. 57°N, long. 4°E were found on the coast of Jutland, the apparent time of drift in most cases being a multiple of twenty days. Tait assumed that off Jutland there was an eddy, as indicated by the distribution of salinity, and that the time for completing one circuit in the eddy was about twenty days. If most of the bottles had been drawn into this eddy and had escaped after having completed one, two, or more circuits, the equal time intervals at which the bottles were washed up on the beaches would be accounted for.

Drift bottles have been used successfully for obtaining information as to surface currents over relatively large ocean areas, such as the equatorial part of the Atlantic Ocean (Defant, 1929, p. 34) and the seas around Japan (Uda, 1935). They have supplied numerous details in more enclosed seas like the English Channel and the North Sea (Fulton, 1897; Carruthers, 1930; Tait, 1930), but have proved less successful off an open coast (Tibby, 1939).

The drift method can also be used for obtaining information as to currents in a shorter time interval. The currents derived from ships' records are determined by this method (p. 428) and give the average surface current in twenty-four hours or multiples of twenty-four hours. From an anchored vessel, say a lightship, the surface current can be determined either by a chip log (Bowditch, 1934, p. 11) or by drift buoys, below which, in general, there is a “current cross” acting as a sea anchor. This type of drift buoy was used on the Challenger. The latter methods give nearly instantaneous values of the surface currents at the place of observation.

Near land the methods can be elaborated in such a manner that the drift of a body can be determined in detail over long distances and long periods. A drifting buoy can be followed by a vessel whose positions can be accurately established by bearings on known landmarks, or the buoy can be provided with a mast and the direction to the buoy can be observed. Its distance from a fixed locality can then be measured by a range finder. Both methods have been used successfully. The latter can also be employed in the open ocean by anchoring one buoy, setting another buoy adrift, and determining the bearing of and the distance to the drifting buoy from a ship that remains as close as possible to the anchored buoy.

Still another drift method has been used with advantage in order to determine the ice drift in shallow waters out of sight of land. The method consists in letting a weight drop so rapidly to the bottom that it sticks in the bottom mud. The time and the length of wire rope payed out are recorded, and then more wire rope is payed out according to the drift of the ice floe to which the vessel is tied up. After a given length of

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time the wire rope is tightened and the total length payed out is recorded as the weight is pulled out of the bottom mud. The direction of stray of the wire is also recorded, and from these data the drift of the ice can be computed.

Flow Methods

From an anchored vessel or float, currents can be measured by stationary instruments past which the current flows, turning a propeller of some type or exerting a pressure that can be determined by various methods. The advantage of these instruments is that observations need not be limited to the currents of the surface layers but can be extended to any depth. The obvious difficulty is to retain the instrument in a fixed locality so that the absolute flow of the water may be measured and not merely the flow relative to a moving instrument. In shallow water a vessel can be anchored so that the motion of the vessel is small enough to be insignificant or of such nature that it can be eliminated. In deep water, current measurements were first made from anchored boats, but in later years the technique of deep-sea anchoring has been advanced (p. 361) to such an extent that vessels like the Meteor, Armauer Hansen, and Atlantis have remained anchored in depths of from 4000 to 5500 m for days and weeks. In other instances, relative currents have been measured from slowly drifting vessels.

Maintaining a vessel at anchor for a long time is expensive, and devices have therefore been developed for anchoring automatic recording current meters that can be left for weeks at a time (p. 370). Measurements of currents very close to the sea bottom cannot be made safely from an anchored vessel, no matter how securely it is kept in position, because an instrument suspended from the vessel cannot be retained at a constant distance from the bottom owing to the motion due to swells and tides. This difficulty was first overcome by Nansen, who lowered a tripod to the sea bottom and suspended a current meter from the top of the tripod. The same method was later used by Stetson (1937), Revelle and Fleming (p. 480), and Revelle and Shepard (in press). The latter suspended three current meters from the top of the tripod and thus were able to obtain simultaneous measurements of currents at three levels within less than 2 m of the bottom.

Current Meters

Current meters differ in design, but all propeller or cup instruments have some device for counting the number of revolutions of the propellers or cups in a given time interval, a vane for orienting the meter in the direction of the current, and a more or less complicated mechanism for recording this direction, either relative to the magnetic meridian (by compass) or relative to some fixed plane (bifilar suspension). Instruments that measure the pressure may not have a vane, because the

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direction can be obtained by the deflection of some kind of pendulum. An advantage of the propeller or cup instrument is that in general a linear relation exists between the velocity of the current, ν, and the number of revolutions per minute, n: where a and b are constants which must be determined by calibration for each instrument and for each propeller used in that instrument. For a well-balanced propeller running in the best bearings the constant a is about 0.5 cm/sec, but currents of velocities less than 2 cm/sec are not reliably recorded. The pressure exerted against surfaces facing the current is more nearly proportional to the square of the velocity. The actual relationship between the velocity of the current and the indication of an instrument measuring pressure must be ascertained by calibration.

Use of a compass for determining the direction of the current has the disadvantage that near a steel vessel a compass needle will be greatly influenced by the ship's magnetism. The deviation due to the ship's magnetism may be as much as 180°; that is, off the side of a vessel a compass needle may be completely reversed. This deviation, which decreases rapidly with depth, depends upon the heading of the ship, the latitude, and the depth of the current meter. It changes with time because a great part of the ship's magnetism is not permanent. It is a hopeless task to determine this deviation for all headings of the ship and all depths of measurement, and therefore a compass should not be used for observing directions from a steel vessel at depths of less than 50 m. Even from wooden vessels directions by compass have to be carefully examined, particularly if the depth is less than 20 m. For the upper layers, a bifilar suspension is recommended with an arrangement for recording the direction of the current relative to the orientation of the bifilar frame, which again can be determined by means of the ship's heading. A compass instrument can be used near the surface from an anchored boat, particularly if a manila rope is used for anchoring and not a steel rope or an anchor chain.

All propeller and cup instruments suffer from the disadvantage that drifting material may impede the motion of the screw or stop it completely. Instruments designed to be left in position for a long time should be checked at frequent intervals to insure perfect functioning. It should also be borne in mind that jellyfish or similar organisms may be caught on the wire rope and prevent a messenger from passing.

Owing to its simplicity and reliability, the Ekman current meter (Ekman, 1905, 1932) has been and still is widely used. This instrument is described in detail below. The greatest disadvantage of the Ekman meter is that only one instrument can be attached to the wire and that it has to be hauled up to be read after each period in which the propeller

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has been free to turn. In order to facilitate work at great depths, Ekman constructed a more complicated repeating current meter, the propeller of which can be released and arrested by messengers. Thus a series of observations can be obtained before the instrument has to be hauled up, either at one and the same depth or at a number of different depths. At the Scripps Institution of Oceanography, C. A. Johnson has developed a modified suspension for the Ekman meter so that several instruments can be attached to the same wire. The propeller of each instrument can be released and arrested by messengers in a manner similar to that employed when using the Nansen water bottles, and thus nearly simultaneous measurements at several depths can be obtained.

Continuously recording current meters have obvious advantages, but they are complicated and expensive. The recording device may be mechanical, requiring a clockwork that will function reliably in sea water, or electrical, requiring contacts that are insulated from the sea water, or photographic, requiring a watertight chamber which can withstand the pressure and in which the photographic equipment is enclosed. Detailed descriptions of various designs are given by Thorade (1933). Here only a brief summary is given of the most important features of instruments that have been or are in use.

Ekman Current Meters (Ekman, 1905, 1932). The essential parts of this instrument are the propeller, the revolutions of which are recorded on a set of dials, the compass box, with the device for recording the orientation of the meter, and the vane which orients the instrument so that the propeller faces the current (fig. 88). The free swing of the instrument is ensured by mounting it in ball bearings on a vertical axis. The wire for lowering is fastened to the upper end of this axis, and a suitable weight is attached below the axis. The instrument is balanced in water so that the axis is vertical. The carefully balanced propeller, with four to eight thin, light blades, runs inside a strong protective ring but can easily be removed for inspection or transportation. The axis of the propeller runs with tantalum points on agate bearings. Inside the protective ring is a lever that can be operated by messengers. With the lever in its lowest position the propeller is arrested, and in this state the instrument is lowered. When the desired depth is reached, a messenger weight is dropped, pushing the lever up to its middle position and releasing the propeller, the turns of which are recorded on a set of dials. After a number of minutes a second messenger weight is dropped which pushes the lever up to the highest position and stops the propeller. In later types the propeller is also shielded in front when the instrument is lowered in order to prevent fouling of the propeller by such organisms as medusae. This front shield is opened by the first messenger.

The direction of the current is recorded by an ingenious device that is simple and reliable. A tube extends from above the dial box to a

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disk on the axis of the cogwheel, which turns once when the propeller makes one hundred revolutions. This tube is filled with phosphorbronze balls about 2 mm in diameter. In the disk are three indentations corresponding to the size of the balls. When one of these indentations passes below the tube, a ball drops into it and is carried around with the disk until it drops into a second tube that extends downward and ends above the center of the compass box. In the compass box, which can be easily removed from the bar to which it is fastened, a system of magnets swings freely on a pin that runs on agate. The frame to which the magnets are fastened carries a bar that is shaped like a wide, inverted V. The upper side of one arm of the bar is trough shaped, so that a ball dropping through a hole in the center of the lid of the box runs down in this trough and falls to the bottom of the box, which is divided into thirty-six compartments, each corresponding to an angle of 10°, and marked N, N 10°E, N 20°E, and so on. The compass box is rigidly connected to the vane of the meter, but the magnets of the compass adjust themselves in the magnetic meridian. The compartment into which the ball drops therefore indicates the direction of the vane—that is, the direction of the current at the moment the shot fell. In general, several shot fall during one observation, since one ball drops for each thirty-three revolutions of the propeller. The average direction of the current is obtained by computing the weighted mean according to the distribution of the balls. If the direction has varied widely during a short period of observation, the average direction will be uncertain or

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even indeterminate, in which case the average velocity as computed from the revolution of the propeller has no meaning.

Ekman Repeating Current Meter (Ekman, 1926). In this instrument the propeller is released and stopped by messengers. When the propeller is stopped, three numbered balls are released from a container. One ball drops down into a compass box, giving the direction of the current at the time the propeller was stopped, and two balls are guided into other slots by the position of dials turned by the propeller. From the slots into which the balls fall, the positions of the dials can be found and thus the number of revolutions of the propeller can be obtained. The messengers are designed to split when they strike the instrument, and the two parts are caught in a container. The operation can be repeated forty-seven times, when the store of numbered balls is exhausted.

Carruthers Residual Current Meter (Thorade, 1933). This instrument is designed for giving the residual current over a long period of time. It has no device for directly recording the revolutions of the cups, but after a certain number of turns a ball is released and drops down into a compass box similar to the compass box of the Ekman meter. The velocity is obtained from the number of balls released. The balls are supplied from a large box containing more than 22,000. At the end of the period of measurement, which may comprise one or more days or even weeks, the number of balls in the slots of the compass box are counted. From these data and the calibration results the average direction and velocity can be found.

Böhnecke Mechanical Recording Current Meter (Thorade, 1933). In this current meter the propeller drives a set of horizontal dials with raised numbers on their vertical rims. A similar dial is attached to the magnet of a compass. A strip of tin foil passes the vertical rims of these dials, being rolled by clockwork from one spool to another. At intervals of five or ten minutes a hammer presses the tin foil against the raised numbers on the rims of the disks, thus recording the positions of the dials, which are turned by the propeller and the magnets of the compass. The instrument can aIso be suspended in a bifilar frame and the direction relative to the orientation of that frame can be recorded. The instrument has not been widely used, probably because it is difficult to find material for the spring in the clockwork, which will be exposed to sea water.

Witting Electrical Recording Current Meter (Witting, 1923). In this instrument the axis of a wheel that is turned by a propeller carries an eccentric disk operating a fork-shaped lever. Through a semiwatertight connection, one part of the fork is brought into a circular box filled with petroleum, where it raises or lowers a magnet. In raised position the magnet is free to swing, and no electrical current passes through the system. In lowered position the frame to which the magnets

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are fastened serves as a key that closes the electric circuit by providing contacts between an inner solid contact ring and an outer contact ring that is split in segments. One electric conductor is connected with the inner contact ring, and the other conductor is connected with the segments by resistances of different magnitudes. Thus, the total resistance in the electrical circuit, the current in which is supplied from a storage battery of nearly constant voltage, depends upon what segment of the outer ring the magnet frame touches when lowered—that is, upon the direction of the current. The recording instrument is a milliammeter, the pen of which records a deflection when the magnet is in lowered position. The time intervals between deflections give the velocity of the current and the magnitude of the deflections give the direction.

Sverdrup-Dahl Electrical Recording Current Meter, Compass Type (Sverdrup And Dahl, 1926; Sverdrup, 1929). This instrument is similar to the Witting current meter except that the contact rings are not enclosed in a semiwatertight box, but are arranged at the top of an inverted brass cylindrical container that serves as a diving bell. Before the instrument is lowered, petroleum is introduced into a vessel in the cylinder. When lowering, the petroleum floats on top and protects the contacts against the sea water, which, with increasing pressure, rises higher and higher. The magnet rests on a pin that is lifted and lowered as the propeller turns. The electric wiring differs somewhat from Witting's, but the record is similar.

Sverdrup-Dahl Electrical Recording Current Meter, Bifilar Type. In this current meter the vane of the instrument turns a sliding contact that rests against a resistance ring rigidly connected with the bifilar suspension. The electric circuit is closed and opened by a key operated by the propeller, remaining closed for about eighty revolutions of the propeller and open for about twenty. The electric contacts are arranged at the top of a “diving bell,” and a recording milliammeter serves as recording instrument. The last-named three instruments require only one insulated conductor, the suspension wire serving as a second conductor.

Ott Electrical Recording Current Meter (Thorade, 1933). In this meter the current velocity is recorded on a chronograph on which a mark is made electromagnetically when the propeller has completed a given number of turns. The direction is not recorded continuously, but by a somewhat complicated arrangement it is shown on a dial whenever the observer on board ship presses a button. All electrical contacts are enclosed in petroleum-filled chambers. Two insulated conductors are required.

Rauschelbach Electrical Recording Current Meter (Rauschelbach, 1929). This instrument is designed for bifilar suspension. The velocity is recorded on a chronograph by contacts made for every

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ten or twenty revolutions of the propeller, and the direction is also recorded on the same chronograph, using a number of contact segments that are arranged in such manner that the direction is obtained with an accuracy of ±1.5°. The contacts are closed every five or ten seconds, and thus the currents are recorded in great detail. The instrument requires two electric cables with seven wires in each.

Pettersson Photographic Recording Current Meter (O. Pettersson, 1913, H. Pettersson, 1915). The propeller of this meter is mounted on a vertical axis below a watertight cylindrical chamber. Outside this cylinder the propeller, through a reduction gear, turns a strong magnet which by induction turns a similar magnet in the cylinder. The magnet in the cylinder carries a disk with a transparent division along the rim. The compass carries a smaller disk with transparent division, the two disks being concentric. A short cylinder of soft iron shields the compass magnet from the magnets rotated by the propeller. Every half hour the positions of the two disks are photographed on a film that is advanced by clockwork. The clockwork also turns on and off a small electric bulb, the power for which is supplied by a storage battery. The meter is designed for suspension below a buoy that can be anchored 10 m or more below the sea surface and left for two weeks.

Idrac Photographic Recording Current Meter (Idrac, 1931). The cups of this instrument are mounted on a vertical axis and operate an electric contact which is placed at the top of an inverted cylindrical container similar to that used in the Sverdrup-Dahl meter. When the electric circuit is closed, a lamp is lighted and a mark is made on a film that is advanced at a constant speed by clockwork. The clockwork and the camera are enclosed in a watertight cylinder, and a storage battery is enclosed in a similar cylinder. The current velocity is obtained from the number of marks on the film in one hour. The direction is recorded continuously. Attached to the magnet of the compass is a black disk on which are marked two white concentric circles connected by a white spiral. The spiral rises from the outer to the inner circle in exactly 360 degrees. The camera is mounted above the disk and is provided with a narrow slit through which a very narrow strip of the disk is photographed, the two white circles and the white spiral appearing as points. If the orientation of the meter relative to the magnetic meridian remains constant, all points will produce straight lines on the moving film, but, if the meter turns, only the two circles give straight lines and the spiral gives a curve from which the orientation of the meter—that is, the direction of the current—is obtained.

Winters Photographic Recording Current Meter (Thorade, 1933). The Winters meter is constructed on principles similar to those of the Pettersson meter, but photographs of the counting device and of the position of the compass card are obtained at intervals of five minutes.

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The power for the light is supplied from the vessel or the float from which the meter is suspended.

Nansen Pendulum Current Meter (Nansen, 1906). This instrument is designed for mounting on a tripod and for measuring very weak currents near the sea bottom. A light pendulum swings above a slightly concave disk which is carried by a magnet and which is covered by graduated waxed paper. At short intervals of time a clockwork lowers the pendulum and a fine stylus attached to its bottom marks the paper, indicating the direction and velocity of the current.

Jacobsen's Bubble Current Meter (Jacobsen, 1909). This current meter is designed for use from such vessels as lightships. It employs no propeller or compass, but measures the magnitude and direction of the deflection of a pendulum relative to the ship from which the pendulum is lowered into the water. Cylinders open at both ends are used as pendulums. The line from the cylinder is fastened to a rod, the orientation of which as to inclination and plane of inclination is observed by a bubble in a segment of a sphere.

Buchanan-Wollaston Mechanical Recording Current Meter (Buchanan-Wollaston 1925, 1930). In this instrument the current velocity is recorded by the pressure exerted by the current against two perforated disks which always remain in a vertical position and which are made to face the current by a vane. The recording unit is enclosed in a watertight cylinder that is mounted horizontally and that turns around a horizontal axis when pressure is applied to the two perforated disks. The turning of the cylinder is recorded, and at intervals of twenty minutes the direction by compass is indicated. The instrument has the advantage that it carries no screw to be clogged by drifting objects, but it is not sensitive to weak currents, since the minimum velocity that can be recorded is about 12 cm/sec.

Analysis of Records of Currents

Different methods of representing surface currents are discussed on pp. 427–430. For currents of a periodic character, it is convenient in most cases to compute the north-south and east-west components of the currents or the components referred to some other coordinate system, say parallel to and at right angles to the coast. The components can readily be subjected to harmonic analysis or to other forms of statistical treatment, subsequent to which the results can be represented in a simple manner (see fig. 145, p. 573). Harmonic analysis has been widely applied and is a very useful tool (see Thorade, 1933).