  Quantum mechanics predicts that the equilibrium state of a resistive
electrical circuit contains a dissipationless current. This persistent current
has been the focus of considerable theoretical and experimental work, but its
basic properties remain a topic of controversy. The main experimental
challenges in studying persistent currents have been the small signals they
produce and their exceptional sensitivity to their environment. To address
these issues we have developed a new technique for detecting persistent
currents which offers greatly improved sensitivity and reduced measurement back
action. This allows us to measure the persistent current in metal rings over a
wider range of temperature, ring size, and magnetic field than has been
possible previously. We find that measurements of both a single ring and arrays
of rings agree well with calculations based on a model of non-interacting
electrons.
