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.

Superconductivity

What is superconductivity?
Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (-459 degrees Fahrenheit, 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen temperatures (-321 F, 77 K, -196 C), they have no electrical resistance. The temperature at which electrical resistance is zero is called the critical temperature (Tc) and varies with the individual material. For practical purposes, critical temperatures are achieved by cooling materials with either liquid helium or liquid nitrogen. The following table shows the critical temperatures of various superconductors:

Because these materials have no electrical resistance, meaning electrons can travel through them freely, they can carry large amounts of electrical current for long periods of time without losing energy as heat. Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss. This property has implications for electrical power transmission, if transmission lines can be made of superconducting ceramics, and for electrical-storage devices.

Superconductors

Uses for Superconductors
Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than superconducting magnets. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally-funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line which opened on April 3, 1997. In December 2003, the MLX01 test vehicle (shown above) attained an incredible speed of 361 mph (581 kph).
Although the technology has now been proven, the wider use of MAGLEV vehicles has been constrained by political and environmental concerns (strong magnetic fields can create a bio-hazard). The world's first MAGLEV train to be adopted into commercial service, a shuttle in Birmingham, England, shut down in 1997 after operating for 11 years. A Sino-German maglev is currently operating over a 30-km course at Pudong International Airport in Shanghai, China. The U.S. plans to put its first (non-superconducting) Maglev train into operation on a Virginia college campus. Click this link for a website that lists other uses for MAGLEV.