Review. In an experiment carried out by S. C. Collins between 1955 and 1958, a current was maintained in a superconducting lead ring for 2.50 yr with no observed loss, even though there was no energy input. If the inductance of the ring were 3.14 × 10−8 H and the sensitivity of the experiment were 1 part in 109, what was the maximum resistance of the ring? Suggestion: Treat the ring as an RL circuit carrying decaying current and recall that the approximation e−x ≈ 1 − x is valid for small x.
We consider a superconducting lead ring as an circuit. The current in this circuit decays, and after time , current in this circuit becomes .
Now, [where is the time constant of the circuit where ]