Superconducting instruments

Instruments utilizing superconducting quantum interference devices (SQUIDS) operating at liquid helium temperature offer the greatest available sensitivity for many different electromagnetic measurements. Their equivalent noise temperature of less than one mK and low drift allow low level measurements of ac and dc current, voltage and resistance. We describe SQUID arrays to measure small vector magnetic fields and spacial gradients associated with geophysical and biomedical phenomena and present information on the design, advantages and limitations of these systems.