Composite pickup coil for measurement of magnetic field components in a superconducting quantum interference device

A pickup coil for measuring the first order gradient of magnetic flux density comprising:

• four substantially planar loops (1, 2, 3, 4) of current conductors, said loops being positioned symmetrically about a central axis extending perpendicularly to the plane of the loops such that a first loop (1) is positioned adjacent a second loop (2) and a third loop (3), said second loop (2) is positioned adjacent said first loop (1) and a fourth loop (4), said fourth loop (4) is positioned adjacent said second loop (2) and said third loop (3), and said third loop (3) is positioned adjacent said fourth loop (4) and said first loop (1), said first and second loops being connected in series such that current flowing in said pickup coil flows in one direction around the first loop and in the opposite direction around the second loop, said third and fourth loops being connected in series such that current flowing in the pickup coil flows in one direction around the third loop and in the opposite direction around the fourth loop;
• and
• first and second terminals (5, 6) for connecting said pickup coil to current sensing means;
• the first and second series connected loops being connected in parallel with the third and fourth series connected loops across said first and second terminals, said first terminal (5) being located between said first and third loops and said second terminal (6) being located between said second and fourth loops, the connection of said loops to said terminals being such that current flows in the one direction in said first and third loops and in the opposite direction in said second and fourth loops.