critical current
/ KRIT-ih-kul KUR-unt /
It is tempting to think that since a superconductor has zero resistance, you could push as much current through it as you like. But there is always a ceiling. Like a rope that holds any reasonable weight yet snaps if you overload it, a superconductor carries current freely only up to a point, beyond which the magic abruptly stops.
The critical current is the largest current a superconductor can carry while staying in its lossless state. Two things impose this limit. First, every current creates its own magnetic field, and once that self-field reaches the critical field, superconductivity dies. Second, in type-II materials a large current pushes on the flux vortices threading the sample, and if the vortices start to move they dissipate energy, so resistance reappears even though the material is still cold and superconducting in principle.
This matters because the critical current decides how much power a superconducting wire can actually deliver, which is the bottleneck for real-world cables and magnets. The crucial engineering trick is to pin the vortices firmly using deliberately introduced defects, which can raise the critical current enormously. The common misconception — that zero resistance means unlimited current — is exactly backwards: the current limit, not the resistance, is usually what constrains a practical device.
A commercial niobium-titanium wire only a millimetre or two thick can carry hundreds of amperes without loss at low temperature — but push past its critical current and it suddenly heats, goes normal, and can be damaged in an instant.
Above the critical current a superconducting wire goes normal and heats — sometimes destructively.
Critical current is usually quoted as a critical current density — amperes per unit cross-sectional area — because a thicker wire simply carries proportionally more. The same material can have wildly different current limits depending on how well its vortices are pinned.