(a) The magnetic field lines that pass through the square hole at the SQUIDÕs center determine the phases of electron waves circulating in the SQUIDÕs superconducting region (green): the wavesÕ interference is proportional to the magnetic flux over the hole. Since superconductors have no electrical resistance, the interference can be measured only by interrupting the superconductor with small regions that have electrical resistanceÑthe two Josephson junctionsÑso that voltage drops will develop across them. The voltage measured across the junctions is proportional to the magnetic flux over the SQUIDÕs square hole. The feedback coil magnetically couples the SQUID to the pickup coil in the SQUID sensor. A SQUID is typically 10 to 100 micrometers on a side.

(b) The colored contours show how the magnetic field produced by neural brain currents (dashed arrows) changes intensity and polarity over the skullÕs surface. In the red region, the field is most intense in a direction pointing out of the skull. In the blue region, the field is most intense in a direction pointing into the skull.

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