EARTH'S MAGNETIC FIELD 



No tubes are used to obtain directionality in the method of induction-flow 

 measurement that uses the earth's magnetic field. The induced voltages between 

 electrodes can be given by 



\' = U H Lx 10-8 (20) 



where V = fluid velocity (cm per second): H = earth's magnetic field (gauss); 

 L = distance between electrodes (cm); \' = induced voltage (volts); and where 

 U, H, and the line joining the electrodes are mutually perpendicular. 



The low strength of the earth's field causes the signals to be very small. 

 Large separation of electrodes thus becomes necessai-y to obtain reasonable 

 voltage levels. A typical signal for electrodes of 100-meter separation is 2.5 

 millivolts per knot. Since the earth's field is stationary, the dc voltages induced 

 cause some polarization problems. 



Modern electrometers with very high input resistance are available to 

 minimize loading problems. Calibration and testing in a tow tank is impossible 

 for electrode spacings of this size. Location has to be distant from large bodies 

 of iron such as tower structures or ships. Effects from the sea floor must be 

 considered. Generally the velocity measured will be that component which is 

 perpendicular to the line joining the electrodes and to the magnetic field. In 

 practice, the line joining the electrodes can be placed parallel to the horizontal 

 component of the earth's field. Then only the vertical component of the earth's 

 field will produce an output. The output will be proportional to only the horizontal- 

 velocity component perpendicular to the line joining the electrodes. Other orien- 

 tations may produce signals due to vertical as well as horizontal -velocity compo- 

 nents. The method is inherently capable of sensing direction of flow. The output 

 polarity at the electrodes is merely reversed. Zero drifts can be of the same order 

 of magnitude as the signal and are inherently difficult to determine. One method 

 for shunting the electrodes to obtain an electrical zero is given by Mangelsdorf.^^ 



MEASUREMENT OF V'ELOCITIES 



The measurement of velocities over large distances (100 meters) has a 

 possibility for increasing signal-to-noise ratios. Assume the surface swell con- 

 sists of plane waves of wavelength A. Let an instrument measure horizontal-flow 

 components in a direction parallel to the surface wave propagation. Measure this 

 flow over a distance of A or integral multiples thereof. Thus a rejection filter 

 results for the major source of noise, the swell of wavelength A. The filtering is 

 accomplished in the transducer itself and the method becomes Approach .3 (page 6 ). 

 Typical values of A would be over 100 meters. In practice, both the direction and 

 wavelength will change with time. Instrument placement would be determined by 

 the expected swell direction and wavelength. 



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