DESIGN: MAGNET SELECTION AND ORIENTATION

For our initial studies, we used grade C8 Barium-Ferrite (empirical BaFe₁₂O₁₉) permanent magnets, each with 15.24 cm x 10.1 cm x 1.27 cm dimensions. The cost, size, and handling safety makes the use of this type of maget very desirable over rare-Earth magnets of the same size. The strength of flux is sacrificed - The flux per unit area (B) is at least one order of magnitude weaker at a Barium-Ferrite's magnet's surface than a rare-Earth magnet

.Despite this limitation, we hypotheiszed that the wide-area flux created by these magnets, especially at distances less than 20cm, would orient the sharks away from the area in proximity to the magnets.

The flux per unit area (B) at the poles of a grade C8 Barium-Ferrite magnet was measured.  Using an F. W. Bell teslameter, the surface flux per unit area was measured to be 180 Gauss at each pole, nearly 360 times the strenght of the Earth's magnetic field. The flux per unit area was observed to increase exponentially (x³) as a pole was approached, as expected.

A grade C8 Barium-Ferrite block magnet  (dimensions: 15.24cm x 10.1cm x 1.27cm).

Magnetic flux per unit area (B) of a Barium-Ferrite magnet vs. a maximum distance of 150 cm from the surface of one pole (axially). . 

Placing the magnets with the side containing the least surface area oriented towards the surface of the water, only created one polar flux lobe, at approximately 180G. 

Conversely, when placing the magnets flat with the side containing the most surface area oriented towards the surface of the water, two lobes are measured at approximately 100G, corresponding to the field lines from each pole.  Positioning the magnets to create two 100G maxima in magnetic flux was hypothesized to be the most effective because having two electric field barriers instead of one would increase the exposure of the fields towards our subjects.  With one barrier, the shark could encounter the 180G field and accelerate through it.  But, with two barriers, the shark could encounter the first and detect that there was another electric field directly behind it.  This could cause the shark to slow down while encountering the first field and then turn around after detecting the second field.

Placing the magnets in the position displayed at right, a variety of elasmobranchs will be screened for behavioral responses to the electric fields created by electromagnetic induction.  Our tests will be conducted at the Bimini Biological Field Station in South Bimini, Bahamas.  Due to the relative abundance of Southern Stingrays (Dasyatis americana), Nurse Sharks ( Ginglymostoma cirratum) and Lemon Sharks (Negaprion brevirostris) at this field station, our experiments will focus on these species.

Representation of a nurse shark entering the field lines at one pole of a submerged permanent magnet. 

Representation of a nurse shark entering the field lines at two poles of a submerged permanent magnet

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