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.
The national voltage standard has recently been maintained by using the Josephson effect in more than ten countries. The voltage standard had been maintained by the standard cell for a long time. The stability of the voltage standard has been improved by the introduction of the Josephson voltage standard. The Josephson voltage standard is based on the constancy and universality of the fundamental physical constant 2e/h. The adopted value of 2e/h in ETL is 483 594GHz/V. The accuracy of the Josephson voltage standard is 10-7 to 10-8 in the practical use. The further improvement of the accuracy would be expected. The commercial Josephson voltage standard system will be widely used in near future. In this article, the above profile of the Josephson voltage standard is described.