Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions. They're sensitive enough to measure fields as low as 5 aT (5×10-18T) within a few days of averaged measurements. Their noise levels are as low as 3 fT·Hz-½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10-2 T), and some processes in animals produce very small magn… (More on Squid)