ABSTRACT
Superconducting quantum interference devices (SQUIDs) have been a key factor in the development and commercialization of ultrasensitive electric and magnetic measurement systems. In addition to measuring magnetic fields, SQUID sensors can be configured to measure a wide variety of electromagnetic properties. One important factor of SQUID design is such that the feedback electronics is able to follow changes in the shielding currents. If the shielding current changes so fast that the flux in SQUID loops changes by more than one-half a flux quantum, it is possible that the feedback electronics will lag behind the rapidly changing flux. SQUIDs offer the ability to measure at sensitivities unachievable by other magnetic sensing methodologies. However, their sensitivity requires proper attention to cryogenics and environmental noise. The SQUID can also be used as the null detector in an ac bridge circuit to measure both resistive and reactive components of complex impedance.
