persistent current
/ per-SIS-tunt KUR-unt /
Set a top spinning on a real table and it slows and topples within seconds, because friction steadily drains its motion. Now imagine a top that never slows at all, spinning on and on without anyone touching it. A current looping inside a superconducting ring is just like that frictionless top — once set going, it simply keeps circling.
A persistent current is an electrical current that flows around a closed superconducting loop indefinitely, with no battery and no power source to keep it alive. Because the ring has zero resistance, there is nothing to convert the current's energy into heat, so it never decays. The flux it threads through the loop is locked in by flux quantization, fixed at a whole number of quanta, and the current quietly adjusts itself to hold that flux constant.
This matters as the most vivid everyday proof of zero resistance, and it is also genuinely useful: MRI magnets are charged up once, then switched into a persistent loop where the current circulates on its own, holding a steady field for years while the power supply is disconnected. The honest caveat is the cold — the loop must be kept continuously below its critical temperature, so the running cost is refrigeration, not electricity. Let it warm up and the trapped current dies in an instant.
A hospital MRI magnet is energized once and then closed into a superconducting loop; the current flows for years without any external power, which is why such magnets are described as running in 'persistent mode'.
MRI magnets run in persistent mode: charged once, then circulating on their own for years.
The persistence is not literally forever — it is limited by the slow, almost imperceptible creep of flux vortices and by tiny imperfections — but the predicted decay time can exceed the age of the universe, so in practice it is as good as eternal.