Discovery of Superconductivity: Superconductivity Explained
Experience science history & the experiments leading to the discovery of superconductivity by famous scientists Heike Kamerlingh Onnes and Gilles Holst at The Emergent Universe, an online interactive science museum about emergence.
It's 1911. You are in the lab of Prof. H. Kamerlingh Onnes.
In 1911, Gilles Holst, who will later become the first head of Philips Physics Laboratory, is a doctoral student in Prof. Onnes lab.
Prof. Kamerlingh Onnes
Heike Kamerlingh Onnes, who will win the Nobel Prize in Physics in 1913, heads a laboratory in Leiden specializing in ultra-cold temperatures and liquefying gasses.
Onnes: Mr. Holst. I need you to determine how the electrical resistance of mercury changes at very low temperatures. Use only very pure mercury – any impurities will invalidate our results.
Holst: OK. I’ve already determined how to prepare very pure mercury, and now that we can reliably liquefy large amounts of helium, I can reach very low temperatures, down to 4° Kelvin. But why are you interested in studying resistance?
Onnes: Because there are 3 different theoretical predictions for resistance as the temperature approaches zero. We need to know which theory is most correct.
[visitor must click on valve; pressure gauge goes up; graph paper appears (scrolls out?) with 3 predicted curves (of Dewar, Matthiessen, Kelvin); Holst calls out temperatures and resistances, and a hand plots the data on the curves; show’s the SC drop to zero at ~4K.]
Holst: Oh, hello. Can you help me with the experiment? Just turn that valve.
Holst (6): I’m doubtful about these results…
Onnes: A sudden drop to zero?!? Not possible!! There must have been a short circuit or something. I advise you to be more careful. Check EVERYTHING and do this again. And this time, do it right!
Holst: I know I was very careful. But now I’ve checked and re-checked everything, just to be sure. Can you turn that valve again?
Holst: I’ve completely reproduced the data. I’m quite sure this is correct.
Onnes: Interesting. Why don’t you also try it with lead and tin?
Overlay box: Time passes. Experiments are conducted.
Onnes: So you’ve found for tin, lead, and mercury that the resistance drops suddenly to zero at some critical temperature. I suspect we’ve found an entirely new state of matter, which I’ll call “superconducting” because it appears to conduct perfectly, with no resistance. But we need to be sure. If the resistance really is zero in the superconducting state, then a current started in a ring of superconducting material should flow round and round forever, with no loss. Can you test this?
Holst: I set up an electric current in a superconducting lead ring and then disconnected the battery. I watched it for an entire hour, and the current kept flowing around the ring. There was no measurable loss of current at all!
Overlay box: The superconducting state is indeed a new state of matter. Found at temperatures below a critical temperature Tc, the superconducting state has zero resistance. When the superconducting ring experiment was repeated in 1962, there was no detectable loss of current during the entire two year experiment.
Very Low Temperatures: Absolute zero and the Kelvin scale
Absolute zero is the temperature at which all matter would be in its lowest energy state. It occurs at -273.15° C (-459.67° F). For temperatures near absolute zero, the Kelvin temperature scale is more convenient: it uses degrees of the same size as the Celsius scale but is set to zero at absolute zero (just as the Celsius scale is set to zero at the freezing point of water).
What's Electrical Resistance?
The flow of electrons along a wire creates electric current. Resistance tells us how much the wire itself slows the electrons’ net motion, and thus the current, along the wire. It is caused by the scattering of electrons from atom vibrations, material defects, and impurities in the wire. This scattering can also transfer energy from the electrons to the wire, turning electricity to heat. This is what heats the filaments in your toaster and the electric burners on your stove.