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A first-order off-diagonal gradiometer was fabricated and tested. The gradiometer consisted of two field pickup coils and a planar DC SQUID (superconducting quantum interference device) with two superconducting loops connected in parallel, two multiturn input coils, and a modulation-feedback coil. The size of the pickup coils was 6*6 mm/sup 2/ with a base line of 8 mm. The overall size of the gradiometer was 15*7.5 mm/sup 2/. The resolution of the magnetic field gradient of the gradiometer increased from 11 to 1.8 pT/m square root Hz (the noise limits), corresponding to the frequencies from 1 Hz to 600 Hz. The resolution became nearly white in a frequency range above 600 Hz. The intrinsic balance of the gradiometer was better than 1000 p.p.m. for the field perpendicular to its plane.>
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Koyanagi et al. (1989) studied this question.
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