ABSTRACT
The macroscopic theory of superconductivity indicates that squid behaviour depends in a fundamental way on magnetic flux quantisation. The output state of a squid is a periodic function of applied flux, but the absolute origin of the flux scale cannot be identified. This means that the device is essentially a sensor of flux changes. There are few applications which require only flux changes to be measured, but by coupling a suitable transducer to the input of a squid a very wide range of physical parameters may be sensed with the highest sensitivity.
