ABSTRACT
This chapter can be considered as an introduction to the principal concepts of SQUIDs and the application of these devices in a variety of measuring systems (there are many excellent review papers available; see, for instance, Clarke (1993). Many further valuable papers can be found in the book edited by Weinstock (1995)). SQUID is an acronym for superconducting quantum interference device. These devices are the most sensitive detectors for magnetic flux currently known. All physical quantities that can be converted to a magnetic flux can be measured with extreme sensitivity. SQUIDs can be used, for instance, for measuring magnetization, magnetic susceptibility, magnetic fields, current, voltage and small displacements. To mention a few applications, SQUID systems are used for detecting the very small magnetic fields due to spontaneous or evoked brain activity (Hämäläinen 1993). These fields are of the order of 10−14 T. SQUIDs are also applied for observing the very small displacements of a resonant mass antenna at millikelvin temperature after the passage of a gravitational wave (for a review see Blair (1993)). Displacements of 10−19 m can be detected and even 10−21 m will be reached in the near future.
