Squids

SQUIDs, or Superconducting Quantum Interference Devices, are used to measure extremely small magnetic fields; they are one of the most sensitive magnetometers known, with noise levels as low as 3 fT·Hz−½. While a typical fridge magnet is ~0.01 tesla (10−2 T), some processes in animals produce very small magnetic fields; typically sized between a microtesla (10−6 T) and a nanotesla (10−9 T). SQUIDs are especially well suited for studying magnetic fields this small. Their sensitivity was recently (in 2002) surpassed by SERF atomic magnetometers.Uses for SQUIDs
The extreme sensitivity of SQUIDs make them ideal for studies in biology. Magnetoencephalography (MEG), for example, uses measurements from an array of SQUIDs to make inferences about neural activity inside brains. Because SQUIDs can operate at acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (kHz), MEG achieves good temporal resolution.
Probably the most common use of SQUIDs is in magnetic property measurement systems. These are turn-key systems, made by several manufacturers, that measure the magnetic properties of a material sample. This is typically done over a temperature range from that of liquid helium (~4K), to a couple of hundred degrees above room temperature.
Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravity waves. Four SQUIDs were employed on Gravity Probe B in order to test the limits of the theory of general relativity.