SQUID (Superconducting Quantum Interference Device) is a very sensitive magnetic flux sensor based on magnetic flux quantization and the Josephson effect. This paper describes the principles of operation of SQUID and gives historical details of some RF SQUID prototypes. A general model for a SQUID magnetometer comprising a SQUID sensor, RF amplifier and phase-locked loop is described. Its flux resolution limit, maximum flux slew rate and input coil current sensitivity can be estimated, as described in the paper.
Various instruments which apply SQUID are now under development…SQUID can be used to measure voltage, current, noise temperature, magnetic susceptibility, and the magnetic fields of the earth and the human body. The paper shows a magnetocardiogram obtained with a SQUID magnetometer; this magnetometer was developed at the authors' company. The paper concludes by listing references which describe some of the many possible applications of SQUID.
