would improve survey efficiency. A high incidence of loss currently is 

 sustained with free-drifting buoys. This loss may be minimized with 

 the use of radio transmitters. Radio locating techniques also will allow 

 a greater number of simultaneous observations. 



2. Flow methods 



The most direct method of measuring currents is to detect their 

 rate of flow past a fixed geographical point. Stationary platforms are 

 required for this measurement, but anchored ships or buoys frequently 

 are used. However, anchored vessels do not completely satisfy the 

 requirement for a stationary platform. Therefore, the minimum 

 accuracy obtainable from an anchored ship or buoy is limited with any 

 flow instrument. The geomagnetic method of determining currents, 

 however, is a notable exception to this statement. Although this method 

 essentially measures the electromotive force induced by sea water 

 flow past a given geographical point, it is possible to sample this 

 force without the movement of the ship influencing the measurement. 



Flow may be measured by instruments which operate on various 

 principles. The most popular instruments employ rotating elements 

 or impellers whose speed of rotation is a function of current speed. 

 Other principles include the measurement of dynamic pressure against 

 a pendulum, plate, or pitot tube. Acoustic devices have been used for 

 measuring phase differences between the source and receiver for sound 

 traveling in various directions relative to the current flow. Measure- 

 ment can be made of the electromotive force generated when an 

 electricity- conducting fluid such as sea water moves through an 

 artificial magnetic field; this force is a function of velocity. Even hot 

 wires or thermistors, whose cooling rate is a function of flow, have 

 been used. 



All of these instruments, when used from a moving ship, must 

 depend upon an internal compass to indicate direction. 



C. AVAILABLE CURRENT METERS 



Hundreds of current meters have been described in the literature, 

 but only a relative few can be purchased as production items. Some 

 of these are listed in Table VI- 1 with approximate prices of the basic 

 sensing units. However, the initial cost of a current meter is a small 

 part of the total cost per observation of current data. Much larger and 

 more pertinent are the costs of operation, maintenance, and data 

 reduction; unfortunately, such comparative costs are lacking for 



VI- 4 



