6 ms·

How would you prove there isn't any water in a cup?

• bagels 3y ago
  I didn't see any references to cups or water in these papers.
  • postalrat 3y ago
    If you can't answer the question then you won't ever understand how to measure zero resistance.
    • svnt 3y ago
      It's a limitation of the most basic approach to the problem (measuring resistance), but not a limitation of every approach (such as measuring current). You can inject a current into a superconducting coil and take measurements of the resultant magnetic field as the current circles for an indefinite period of time. I'm not able to see how this approach analogizes to water in a cup.
      • davrosthedalek 3y ago
        I don't think that works as advertised: Any measurement of the current will likely produce a magnetic field itself -- for example from the current in a Hall probe. This current will induct a small, opposite current into the superconducting coil. So the current will go down. And even if not, what you would need is to measure the change of the field over time. This has finite resolution, so you can't distinguish no resistance from very very small resistance.
        • svnt 3y ago
          There are several tests that have been ongoing over decades doing exactly this. The small loss of current during measurement occurs, but you only engage with the field intermittently, and you can calculate approximately what the loss should be. Yes it could be some tiny resistance, but the same issue occurs with the resolution/accuracy of the voltage or current measurement you would make.
          • davrosthedalek 3y ago
            Exactly my point. You cannot measure that the resistance is exactly zero, you can "only" give an upper bound. The upper bound depends on your approach, but no approach can give you a zero upper bound.
            • svnt 3y ago
              This is always true, though, for any value.
    • appplication 3y ago
      Would one method (assuming a pure sample) be to pass a high current through it and measure if it heats up? I know nothing about this, but that would be my first guess.
      • moqmar 3y ago
        It will heat up, as the contact points & probes will have resistance, not sure if there are good ways to correct for that.
        • jonny_eh 3y ago
          If the material were an unshielded wire, spool it, and send a current through it and measure if the V/I/W drops off as you unspool it?
  • jadamson 3y ago
    But I did eat breakfast this morning?
• fooker 3y ago
  That's a really good way to reason about the difficulties.