14 Methods of Observation 



ambiguity concerning the zero point of all velocity measurements made at 

 sea. Even anchored ships or buoys move about at the end of their cable 

 so much as to be unreliable reference points in deep water. 



Towed electrodes. — The component of a ship's velocity relative to the 

 earth and perpendicular to the ship's heading may be determined by 

 measm-ing the electromotive force induced by the earth's magnetic field 

 in a 100 m. length of ulre towed astern. The complete vector velocity may 

 be obtained by occasionally altering the ship's course. As developed by 

 William von Arx, this method has proved very valuable for practical 

 survey purposes in indicating the presence and direction of surface currents 

 (see Longuet-Higgins et al., 1954; and von Arx, 1950). The time required 

 is about 5 min. per measurement, because the ship must be maneuvered in 

 directions other than the regular heading. 



The chief difficulty of this method is that it does not give uniformly 

 accurate surface velocities. The reason for this deficiency can be easily 

 made clear by a concrete example. Assume that a ship and the cable 

 behind it are swept sideways by the component of surface current velocity 

 perpendicular to the ship's heading. An electromotive force equal to vH^l 

 is induced in the cable, where v is the sideways velocity of the ship and 

 cable, H^ is the vertical component of the earth's magnetic field, and I is 

 the length of the cable. If electrodes making good contact with the sea are 

 fitted at both ends of the cable, and were there no potential induced in the 

 sea by virtue of its motion, the electromotive force induced in the cable 

 could be measured by a potentiometer and the velocity determined by the 

 relation expressed above. In general, however, there is a potential gradient 

 component developed in the ocean in the same direction as that in the 

 cable; and since there is no way of knowing exactly what this ocean 

 potential is except by knowing a great deal about the ocean velocity field, 

 the method cannot give true surface velocity. In ocean current systems 

 that are broad compared to the depth of the ocean, the potential gradient 

 developed in the ocean is approximately vH^, where v is the velocity 

 averaged over the total depth. Since in the Gulf Stream the strongest 

 currents extend to only a fraction of the total depth, v<v, the towed 

 electrodes give a good approximation to surface velocities, the deficiency 

 varying between 2 and 40 per cent. 



Air-borne radiation thermometer. — Stommel and Parson (see Stommel et 

 al., 1953) conducted some field experiments to determine the feasibihty of 

 making rapid synoptic surveys of the surface temperature over the Gulf 

 Stream by airplane, using an infrared bolometer. The bolometer is exposed 

 alternately to the sea surface and to a black body in the plane. The chopped 

 radiation, falling on the bolometer, is restricted to the band of maximum 

 water- vapor transmission (8-13/^) which contains about 30 per cent of the 



