# What is the highest upper limit on resistance of superconducting phase?

What is the highest precision achieved in the measure of the upper limit of the resistance of the superconducting phase (for any superconductor)?

• Are you asking about instrument limitations or whether a superconductor is actually a superconductor? – honeste_vivere Feb 20 '16 at 15:34
• The most accurate way (As far as I know) of measuring the resistance of a superconductor is in measuring the decay time of a persistent current in a superconducting ring (which is supposed to be infinite, but experimentally you cannot measure it for an infine time). I remember from some classes I had a couple years ago that the most precise measurement was done by letting a persistent current flow for something like an year, but I cannot find any reference, and the error they obtained. That's what I'm asking. – PhilipV Feb 20 '16 at 20:06
• Ah okay, perhaps edit your question to clearly state that so that the reader knows precisely to what you refer. – honeste_vivere Feb 20 '16 at 20:09