the geomagnetic electrokinetograph^, will be called the von Arx current 

 meter, or simply the current meter, for the sake of brevity in this dis- 

 cussion. The electric current which flows as a result of the generated 

 electromotive force is short circuited by the surrounding media (which 

 for practical purposes is the sea in deep water, and is the sea and bottom 

 in shallow water). In the von Arx current meter a pair of electrodes 

 a fixed distance apart are towed from the ship by means of an insulated 

 wire cable of sufficient length to place the electrodes astern of the area 

 disturbed by the ship. The cable is connected to a potentiometer which 

 records the potential difference between the electrodes. The potential 

 difference is proportional to the distance between the electrodes, the 

 vertical intensity of the earth's magnetic field and the component of the 

 ocean current velocity normal to the line between the electrodes. The 

 dimensions have been so selected that one milhvolt corresponds to an 

 ocean current of about one knot. The potentiometer indication is good 

 to about 0.05 millivolt. 



Prior to the beginning of these field tests it was considered that the 

 theoretical and experimental development of the instrument had pro- 

 ceeded far enough to demonstrate its validity as a current meter within 

 the limitations imposed by magnetic storms and uncertainties as to the 

 departure of the proportionality factor from unity. However, no ex- 

 tensive comparison had yet been made between ocean currents as 

 measured by the current meter and as deduced from dynamic topography. 



The essential difference between the two methods is that the current 

 meter measures instantaneous values of current, whereas dynamic topog- 

 raphy gives average values on the assumption of a steady state. Thus, 

 if the instantaneous current differed from the mean current either be- 

 cause of periodic (such as tidal currents) or aperiodic (such as transitory 

 wind currents) disturbances, the two methods would be expected to give 

 different results. It was expected that in the ice patrol area the shal- 

 lower parts of the area would be found to be characterized by tidal 

 currents and that in the deeper waters off shore little or no tidal effect 

 would be encountered. It was hoped that current mapping might be 

 speeded up by running that offshore portion of a survey where no appre- 

 ciable tidal effects existed by means of the current meter, and by resum- 

 ing use of straight dynamic topographic methods in the inshore portion 

 of the survey when water affected by tidal currents was encountered. It 

 was also hoped that surveys in which both methods were used through- 

 out would delineate the practical boundary between the two parts of the 

 area. 



As the current meter gives the component of the current in a direction 

 normal to the line between the electrodes, standard procedure was to 

 run on the base course for 26 minutes and on a jog at right angles to the 

 base course for 4 minutes each half-hour. Successive jogs were made 



^ von Arx, William S.: "An electromagnetic method for measuring the velocities of ocean currents 

 from a ship under way." Papers in Physical Oceanography and Meteorology published by M.I.T. and 

 W.H.O.I., vol. XI No. 3 (1950), Cambridge and Woods Hole, Mass. 



93 



