ultra-small changes in magnetic fields. A SQUID is built up from one or more
using a subtle quantum
effect on the supercurrent or the tunneling
current running between the two superconductors in the presence of an external
magnetic field. A SQUID is able to detect a change in the magnetic field in
the order of ft (femtotesla) which is much smaller than the magnetic field of
the earth. SQUID's are in use to measure the magnetic fields of mammalian
brains for example. See also "Josephson effect".