Various methods have been devised to enable currents to be measured directly. Much useful information has been gained by observing the effect of surface currents upon the navigation of ships. Drift bottles and drift cards have also been used, the cards being placed either in a ballasted bottle or in a plastic waterproof envelope and released at sea. The finder fills in details of the recovery position and forwards the card to the investigator. Much of the knowledge gained about the coastal currents of New Zealand was obtained by the use of drift cards.

Subsurface currents are generally measured with current meters. These instruments, which measure current velocity and direction, are used more particularly in coastal waters since they are operated either from a ship or a buoy. Their principal disadvantage is the fact that they measure the current velocity relative to the ship or buoy and even an anchored ship moves at the end of its anchor line, particularly in deep water. A promising method for measuring deep currents has recently been developed in England. A small float fitted with a device which produces an ultrasonic signal, is ballasted to float at the required depth. A ship fitted with a suitable sound receiver can then track the movements of the float and so obtain a direct measurement of the deep currents. This method demands precise position-finding by the ship because the movements of the float are measured relative to the ship's movements.

In view of the difficulties encountered in measuring currents directly, oceanographers often employ an indirect method in which hydrodynamical equations of motion are used. The dynamic computation of currents depends upon the accurate measurement of temperature, salinity, and depth, and techniques are used which enables these measurements to be made. A metal bottle fitted with a valve at each end is attached to a wire and lowered with the valves open to the approximate desired depth. A weight which slides down the wire is then released. This strikes a release mechanism on the bottle which turns over and the valves close to obtain a sample of water. Normally a number of bottles are attached to the wire at various depths. The samples obtained can then be analysed for any desired properties. Salinity can be determined by chemical analysis but physical methods are often used, and a standard of accuracy of ±0.02%0 can be readily obtained. The average value of salinity within the oceans is 35%0 (%0 means parts per thousand). Reversing thermometers are mounted on the sampling bottle and when the bottle turns upside down the mercury columns break and record the temperature at the depth of sampling. Corrections are applied to the thermometer readings to allow for the expansion of mercury after it is brought on deck. Two types of thermometer are used on each bottle. One type is protected from the water pressure by a strong exterior glass sheathing. The other type is not protected from the pressure and the bulb of mercury is therefore compressed by an amount which depends upon the depth. Thus the unprotected thermometer gives a higher reading than the protected thermometer and the difference amounts to about 0.01°C for each metre of depth. The standard of accuracy for each type is about ±0.01C.