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A second-order SQUID gradiometer operating at 77 K

We describe a portable second-order electronic SQUID gradiometer cooled with liquid nitrogen and capable of operating in unshielded space. The measured magnetic-field resolution is <300 fT Hz-1/2 and <170 fT Hz-1/2 When operating in the first-order mode. The gradiometer was successfully used for the recording of screening magnetocardiograms of over 200 human subjects at four different geographic locations.