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Calculated magnetic field B as a function of position along the centerline of the magnet, for 500 (◇), 1000 (◻), 1500 (▵), and 2000 (×) A-turns. The values of B in this figure are all high enough to ensure that a superparamagnetic bead would have constant m. Consequently, the force would be proportional to ∂Bx∕∂y.
