Defining critical current

The critical current of a practical superconductor can be defined in a variety of ways such as a specific voltage level, an apparent resistivity, or even the point of the irreversible superconducting-to-normal transition. The resultant values may differ very little or be meaningless for one given condition, but when comparing superconductors under a variety of conditions, such as different magnetic fields or applied stresses, these various definitions can give apparently different behavior. This is illustrated using data on the effects of stress on the critical current behavior in wires. As part of an initial effort at the National Bureau of Standards to develop standard practices and definitions for practical superconductors, several critical current criteria are proposed and discussed.