A superconducting second-derivative gradiometer

A balanced superconducting second-derivative gradiometer has been constructed in order to measure magnetic phenomena in an unshielded environment. The essential design features of the gradiometer, including its insensitivity to rotation and its capability of discriminating effectively against magnetic fields from distant sources, are described. Several details of the measured response of the gradiometer are presented to establish a correspondence between the design and the actual performance. Two magnetocardio-grams and a measurement of remanent magnetization in the area of the chest resulting from a 100 Oe magnetizing field are shown as typical results obtained in an unshielded environment. The observation of a magnetic signal synchronized with the heartbeat but which depends upon orientation of the subject and apparatus in the earth's field is also reported.