LABORATORY STUDY OF 



AN ELECTROMAGNETIC CURRENT METER 



This article is a brief summary of some recent work 

 done at the Beaoh Erosion Board's laboratory with a view 

 to devising an instrument that would measure and record 

 internal water velocities associated with wave motion. 

 The work was accomplished by H. A, Taylor and G, M, Hare 

 under the direction of J, M, Caldwell, Chief of the 

 Research Division of the Beach Erosion Board. 



In order to obtain more complete measviremsnts of water wave char- 

 acteristics, an instrument is desired which will accurately measure and 

 record the orbital velocities of the water particles mthin a wave 

 formation. It appeared that an all electrical instrument would eliminate 

 bulkiness and have a high degree of flexibility, so the investigation 

 was limited to an instrument utilizing the basic principle of electro- 

 magnetic induction. The motion of the water as associated with wave 

 action would serve as a moving conductor in which an electro-motive 

 force i'EiW) would be induced in the presence of a magnetic field. A 

 pair of electrodes, in the water connected to a suitable recording 

 dovice would pick up and record the induced voltage, which if the field 

 strength and electrode separation were maintained constant, would be 

 directly related to the water velocity. The electrode alignment woula 

 be perpendicular to the field direction, and thus only the velocity 

 component perpendicular to both field and electrode alignment would 

 contribute to the voltage picked up by the electrodes. This suggests 

 the possibility tl:iat two mutually perpendicular pairs of electrodes 

 could be used to measure and record the components of a velocity both 

 parallel and perpendicular to a given base line, and from these 

 simultaneous values the magnitude and direction of the incident velocity 

 could be computed. 



After study of published works of other experimenters, it was de- 

 cided to investigate the performance of an instrument similar to one 

 proposed by Guelke and Vanneck,* Their instrument consisted of a 

 toroidal coil to provide the magnetic field and pick-up electrodes in 

 a plane parallel to the plane of the coil, suspended at any given dis- 

 tance along the coil axis. Guelke and Vanneck utilized alternating 

 currents to energize the field coil as the use of direct current usually 

 causes polarization of the electrodes. However, the use of aLtei'nating 

 current appeared to have the following disadvantages: (1) an alternating 

 field would induce an alternating voltage In any loops formed by the 

 electrode leads, the magnitude of which could exceed that expected for 



*■ The Measurement of Sea-Water Velocities by Electromagnetic Induction, 

 R. W. Guelke, C. A. Schoute -Vanneck, Journal, Institution of Electrical 

 Engineers, London, Vol. 9!+, Pt. 2, No. 37, February 19*^7. 



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