(skwid) A sensitive magnetometer which detects magnetic field changes by means of a superconducting loop containing one or two Josephson junctions. Acronym for ‘‘Superconducting Quantum Interference Device.’’ A SQUID carries supercurrent up to a certain critical value, beyond which a finite resistance appears in the loop. The value of this critical current depends upon the external flux as well as the geometry. In the rf-SQUID magnetometer, a loop with one Josephson junction is driven inductively by a high-frequency (typically 30 MHz) alternating current which periodically exceeds the critical current. The resulting voltage depends on the value of the critical current which in turn depends on the external flux. In the dc-SQUID magnetometer a dc current slightly greater than the critical current is fed into a loop containing two Josephson junctions. This produces high-frequency oscillations in the loop due to the ac-Josephson effect. Thus the current periodically exceeds the critical current and a voltage appears which depends upon the external flux. Both the rf- and dc-SQUIDS are usually incorporated in negative feedback circuits which detect and null the flux, and the output of the negative feedback circuit is proportional to the output of the SQUID. SQUID magnetometers are capable of detecting fields on the order of 10–5 nanotesla, and are used in magnetotelluric and controlled-source electromagnetic field techniques. See Clarke (1974) and Weinstock and Overton (1981).