A SQUID (for superconducting quantum interference device) is a very sensitive magnetometer used to measure extremely subtle magnetic fields, based on superconducting loops containing Josephson junctions. A SQUID is formed by connecting two Josephson junctions in parallel. When current is passed into this arrangemnt, it splits flowing across the two opposite arc. The current through the circuit will have a periodicity which is very sensitive to the magnetic flux passing normally through the closed circuit. As a result, extremely small magnetic flux can be detected with this device. This device can also be used to detect voltages as small as 10-15 V. Magnetic field changes as small as 10-21 T can be detected. Weak magnetic fields produced by biological currents such as those in the brain can also be detected using SQUIDs. SQUID detectors are used to measure the levels of iron in liver – so that iron built up can be treated before much harm is done to the body.
