SQUIDs (Superconducting QUantum-Interference-Devices) are the most sensitive sensors for magnetic fields. They make clever use of the properties, which are specific for superconductors: zero electrical resistance, expulsion of magnetic fields and quantization of magnetic flux.
Using a superconducting ring with a weak link (a Josephson contact) and a coupled rf-resonance circuit allows to detect changes of the magnetic flux which are smaller than 10-6 Phi0 (Phi0 denotes the magnetic flux quantum of 2*10-15 Tm2 or 2*10-7 Gauss cm2).
To get an idea of these changes are: The flux of the earth’s magnetic field through an area of 1 mm2 sums up to approx. 104 Phi0.