I see. V=IR=0 means R can measure as 0, but they did measure small current. Presumably they know the voltage. That seems odd if they can’t measure the resistance accurately if their instrumentation is rated to measure that current.
It is not possible to determine that a resistance is exactly zero through direct measurement, because any kind of probes that can be used to contact the material will have a high contact resistance, which will vary from one probe contact to another.
Even when the resistance is measured with a 4-point probe, the only conclusion that can be reached is that the resistance is smaller than the experimental error.
However it should be possible to determine that the resistance is zero through various indirect effects, for instance by establishing a closed current in the material and verifying that it does not decrease in time.
Even such experiments must be done carefully because some materials with unusual magnetic properties may behave apparently in a similar way to superconductors in some experiments, e.g. both a magnetized ferromagnetic material and a superconductor ring carrying a current will have a remanent magnetic field that is constant in time and in both cases the magnetic field will disappear when the temperature is raised above a threshold.
Thanks that’s interesting. If it’s so hard to determine if a substance is superconducting, does that mean many substances that are apparently but not quite superconducting have similar applicability? I.e., if you need superconductors for X application, can you not just put the substance in question in that application and observe if it works or not? Sorry for the dumb questions.
