Onnes and the Discovery of Superconductivity (1911)

On the afternoon of April 8, 1911, in a laboratory in Leiden, a notebook entry recorded that the resistance of mercury was "practically zero." Heike Kamerlingh Onnes had been trying to find out what happens to the electrical resistance of a pure metal as it is cooled toward absolute zero. Competing theories gave different answers. Mercury gave one nobody had predicted: at a temperature of about 4.2 kelvins (K), roughly 4 degrees above absolute zero, its resistance did not slowly shrink. It collapsed.

This article covers the experiment as it was actually done: the race to liquefy helium that made it possible, the apparatus, the numbers, a famous anecdote that the lab notebooks contradict, and the years it took for physicists to understand what Onnes had found. For the physics of why superconductors behave as they do, see our explainer on what superconductivity is.

The problem: what does a metal do when it gets very cold?

An electric current in a metal is a stream of electrons. Resistance is what happens when those electrons bump into things: atoms vibrating with heat, and impurities or defects in the crystal. Think of crossing a crowded train station. A heat-shaken lattice is like a crowd jostling in every direction, and impurities are like fixed pillars you have to dodge. Cooling the metal calms the crowd. The question around 1900 was how far the jostling would fall, and what would be left.

Physicists had measured resistance dropping steadily as metals were cooled with liquid air and liquid hydrogen, but nobody knew where the curve ended. The 1911 papers and later histories describe competing pictures. One said resistance would head smoothly to zero at absolute zero. Lord Kelvin proposed in 1902 that resistance would reach a minimum and then climb toward infinity at absolute zero. According to the detailed reconstruction in Physics Today, Onnes himself predicted that at 4.3 K mercury's resistance would be much smaller than at 14 K but still measurable, falling to zero, within experimental accuracy, only at much lower temperatures.

Only one place on Earth could test any of this. It took a gas that no one had yet turned into a liquid.

Leiden and the race to liquefy helium

Onnes founded a large cryogenics laboratory at Leiden in 1904, now named the Kamerlingh Onnes Laboratory. His method was relentless cascading: liquefy one gas, use it to cool the next. Air, then hydrogen, which boils at about 20.4 K, then the last holdout. Helium was the gas that would not condense. Its boiling point is so low that it stayed gaseous in every apparatus built to that point.

On July 10, 1908, Onnes liquefied helium for the first time, reaching its boiling point of about 4.2 K (minus 269 degrees Celsius). By reducing the pressure over the liquid he then pumped it down to near 1.5 K, the lowest temperature anyone had produced on Earth. Producing the liquid relied on the Joule-Thomson effect, in which a compressed gas cools as it expands through a narrow opening, the principle behind our article on the Joule-Thomson porous plug experiment. Onnes combined it with several stages of pre-cooling.

Cryogenics at that scale was an industrial effort, not a bench experiment. The Physics Today account names Gerrit Flim, the lab's technical manager, and the master glassblower Oskar Kesselring among the staff, and notes that the first helium transfer in March 1910 reached roughly 1.1 K. Studying matter at 4 K meant having a working helium liquefier, and very few laboratories in the world could say that.

Why measure resistance, and why mercury

Onnes wanted to settle the question of resistance, and he had done a clever bit of preparation. Mercury is a liquid at room temperature, which makes it easy to purify: distillation removes the impurities that would otherwise leave a floor of resistance. A thin metal wire has to be drawn through a die, a process that can contaminate or strain it. Mercury could instead be poured into a glass capillary and frozen in place. And mercury's resistance had shown a steep slope near 14 K that gave no sign of leveling off.

Before mercury, the lab had worked with gold and platinum. A platinum measurement in December 1910 showed the resistance becoming constant below about 4.25 K, which ruled out Kelvin's picture. That left the question of what a still purer metal would do, and mercury was the best candidate for the purest metal one could actually prepare.

April 8, 1911: the day, hour by hour

Physics Today's reconstruction of the day, built from the original notebook, gives a better picture than the legend does. The team started at 7 a.m. Helium circulation began at 11:20. By noon the gas thermometer read 5 K. The team then reduced the vapor pressure over the liquid helium to 197 millimeters of mercury, about a quarter of an atmosphere, which brought the bath to roughly 3 K. At 4 p.m. the notebook records the mercury resistance as practically zero. The notebook entry, in Dutch, reads "Kwik nagenoeg nul," mercury practically zero; the gold resistance was measured again at the same time.

The lowest temperature reached that day was about 1.8 K. At that point Onnes also noticed that the boiling of the helium suddenly stopped, which was the transition of helium to a superfluid state at about 2.2 K. He did not realize what he had seen; superfluid helium was not recognized as a distinct phase until the 1930s, with the discovery of superfluidity in helium-4 dated to 1937 in a review of the first hundred years of superconductivity. On the same afternoon, then, two of the great discoveries of low-temperature physics were in the same flask, and only one was recognized.

The apparatus

The resistor itself was built from seven U-shaped glass capillaries connected in series, each containing a small reservoir of mercury. Four platinum feed-throughs connected it to copper leads that ran out of the cryostat. Gilles Holst, the young physicist who made the resistance measurements, suggested solidifying the mercury in the capillaries with liquid nitrogen before the helium run, and Onnes followed his advice.

The measurement was an old one in principle: pass a known current through the mercury, measure the voltage across it, and divide. What was new was the temperature.

The "blue boy" story

A popular anecdote says that during the run a sleepy apprentice from Onnes's instrument-makers' school, the so-called blue boy, let the helium boil by mistake, which warmed the mercury above its transition and made the resistance reappear, and that this is how Leiden realized the effect was real. The story is told in many textbooks.

The notebooks say otherwise. According to the Physics Today reconstruction, a May 1911 notebook entry records the resistance beginning to reappear at 4.12 K as the temperature rose, which the authors say contradicts the anecdote. Onnes and his team did the obvious thing a careful experimenter would do: they warmed the sample in a controlled way and watched the resistance return. The anecdote survives because it makes the discovery sound like an accident, when it was the result of deliberate and increasingly refined work.

The numbers that made people believe it

An abrupt drop in resistance could be an artifact of a short circuit: a bit of wiring that touches the sample and bypasses the part you are trying to measure. Onnes's group checked for exactly that before trusting the result, and concluded that short circuits were not the cause, according to Physics Today. They also improved the measurement: in the May 1911 experiment the voltage resolution was about 30 nanovolts.

Date / source	What was measured	Result
April 8, 1911	Mercury resistance, about 3 K	Recorded as practically zero
May 1911	Resistance reappearing on warming	Back at 4.12 K
May 1911	Ratio R(T)/R₀ at 3 K (R₀ is the calculated resistance of crystalline mercury extrapolated to 0 °C)	Below 10⁻⁷
May 1911	Just above 4.2 K	About 10⁻⁵ of R₀, then a rise by a factor of almost 400 within 0.1 K
October 26, 1911	Resistance plotted against temperature	Jump within 0.01 K at 4.20 K, from below 10⁻⁶ ohm to 0.1 ohm

Key readings from the 1911 mercury experiments, as reconstructed in Physics Today's account of the discovery.

In his 1913 Nobel lecture, Onnes reported that at 4.20 K the resistance had fallen to a millionth of its value at normal temperature, and at 1.50 K to less than a thousand-millionth of the room-temperature value, as quoted in the first-hundred-years review on arXiv. He stressed that the disappearance did not happen gradually but abruptly.

That abruptness is the real surprise. Kelvin's picture predicted a smooth change. So did Onnes's own. Neither predicted a cliff.

One caution on the numbers: reference works quote slightly different transition temperatures for mercury. Some reference summaries, including Wikipedia's article on Onnes, give 3.6 K for the April 1911 observation, while the October 1911 plot places the jump at 4.20 K. Modern measurements put the transition of natural mercury near 4.15 K (4.156 K by Maxwell and 4.150 K by Reynolds and colleagues, both reported in 1950). The spread among the post-1911 figures reflects thermometry and the isotope composition of the mercury, not a dispute about whether the effect occurs.

What Onnes thought he had found

It is tempting to imagine Onnes announcing superconductivity on April 8. In fact the interpretation lagged the data. The drop was far too fast for his own model, which predicted a gradual approach. Onnes published a first paper in late April 1911, a second in May and another in November. On October 26 the group produced the plot that has become the iconic image of the discovery. About a week later, Onnes presented the work in Brussels at the first Solvay Conference.

Onnes described mercury as having passed into a "new state." He first called the phenomenon "supraconductivity," a word that later became "superconductivity." He treated it as a state, a phase with properties of its own, rather than as the limit of a smooth trend, and that framing turned out to be right.

Beyond mercury: tin, lead, and the first magnets

Over the next three years the Leiden group followed the lead wherever it went.

Other metals. In June 1912 the team discussed alloying mercury with gold and cadmium, and mercury alloys also became superconducting. By 1913, lead and tin had turned out to be superconductors too, with transitions near 6 K and 4 K. Superconductivity was a property of many metals, not a curiosity of mercury.

The magnet that was not to be. Onnes saw at once the engineering possibility: a coil with no resistance should carry huge currents and make powerful magnetic fields without melting. In January 1914 he found a snag. Lead lost its superconductivity in a magnetic field of only about 600 gauss at 4.25 K. A strong enough field switches the state off, which made high-field magnets from those metals unfeasible. The modern descendants, niobium-titanium magnets like the 8 tesla dipoles in the Large Hadron Collider, are made of a material with a much higher critical field.

Persistent currents. In 1914 Onnes demonstrated the most vivid property of all. A lead coil cooled to 1.8 K carried a current of 0.6 amperes that did not decrease perceptibly over an hour. A lead ring with a 3.0 by 3.5 millimeter cross section carried a persistent current of 320 amperes. The physicist Paul Ehrenfest was reportedly flabbergasted by the experiment.

Persistent currents have a showmanship all their own. In 1932, Flim carried a lead ring holding a 200 ampere persistent current to a lecture at the Royal Institution in London, so that an audience could see a ring carrying a current with no power source attached. Few experimental demonstrations in physics are that literal.

Even Onnes's own estimate of the current density was off. He calculated 30 amperes per square millimeter for the lead ring, a figure that Physics Today notes was far too low, because supercurrents flow in a thin surface layer a few hundred nanometers thick. That fact was explained only after the brothers Fritz and Heinz London introduced the idea of a penetration depth in 1935.

The Nobel Prize, and what it did not say

Onnes won the 1913 Nobel Prize in Physics "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium." The citation names helium. It does not name superconductivity, which had been discovered two years earlier and was still not understood. The committee honored the capability, getting matter to 1 K, over the single strangest thing that capability had revealed.

That is a useful corrective to the idea that the discovery was instantly recognized as foundational. It was recognized as a remarkable fact about cold metals. Understanding came in stages across half a century.

From perfect conductor to something new: 1933

For two decades superconductors were treated as perfect conductors, materials with exactly zero resistance. A perfect conductor has a specific magnetic behavior. If a perfect conductor is cooled in a magnetic field, the field is trapped inside. If a field is switched on after cooling, the field is kept out. In either case the state of the material depends on its history, on what was done first.

In 1933 Walther Meissner and Robert Ochsenfeld measured the magnetic field around superconducting tin and lead samples. They applied a field, cooled the samples below their transition temperature, and found the field inside was almost completely cancelled. Because magnetic flux is conserved, the missing field inside shows up as an increase in the field outside, and that is how they detected it. A superconductor does not just resist changes in flux; it expels flux, whatever its history.

Property	Perfect conductor	Superconductor
Resistance	Zero	Zero
Cooled in a field	Field stays trapped inside	Field is pushed out (Meissner effect)
Magnetic state	Depends on history	Same however it was prepared; a true thermodynamic equilibrium state

Why zero resistance alone cannot define a superconductor. The Meissner-Ochsenfeld result showed that the superconducting state is a distinct phase, not just an idealized conductor.

In 1935 the London brothers gave the effect a phenomenological description: the magnetic field decays exponentially from the surface of the superconductor over a short distance, the London penetration depth. Their equations could describe the zero resistance and the Meissner effect, but they did not explain where it came from. That took until 1957, when John Bardeen, Leon Cooper, and John Schrieffer published their theory of paired electrons (BCS theory, Nobel Prize 1972).

The story kept surprising people. In 1986 Georg Bednorz and Alex Müller at IBM in Zurich reported superconductivity in a lanthanum barium copper oxide at about 35 K, earning the 1987 Nobel Prize in Physics. Soon after, yttrium barium copper oxide became the first material found to superconduct above 77 K, the boiling point of liquid nitrogen, which is far cheaper to produce than liquid helium. Our explainer on what superconductivity is takes up that later history and the physics.

Fifty years in one table

The discovery sits inside a long chain of steps, each of which answered a question the previous one raised.

Year	Step	Who
1904	Cryogenics laboratory founded at Leiden	Kamerlingh Onnes
1908	First liquefaction of helium (July 10)	Kamerlingh Onnes and the Leiden team
1911	Mercury resistance practically zero (April 8); abrupt jump at 4.20 K plotted (October 26)	Onnes, Holst, Flim, and colleagues
1913	Nobel Prize in Physics for low-temperature research and liquid helium	Kamerlingh Onnes
1914	Critical field of lead; persistent currents in lead rings	Kamerlingh Onnes
1933	Magnetic field expelled from tin and lead	Meissner and Ochsenfeld
1935	Penetration depth and the London equations	Fritz and Heinz London
1957	Microscopic theory of paired electrons	Bardeen, Cooper, Schrieffer
1986	Superconductivity at about 35 K in a copper oxide	Bednorz and Müller

The first row is the only one that depended on a single laboratory's equipment. Every later row built on Leiden's measurements.

A useful way to hold the persistent-current result in mind is a flywheel. A spinning flywheel on a good bearing keeps turning long after you stop pushing, and its stored energy is easy to see. A superconducting ring does the same with electrons: set a current going, and it keeps flowing, because nothing in the ring takes energy out of it. In an ordinary copper loop the current dies away as resistance turns its energy into heat.

Where superconductors went

Superconducting wire is now an industrial product. Magnets cooled by liquid helium are at the heart of clinical MRI machines (see our article on how MRI machines work), and they are the bending magnets of particle accelerators, such as the LHC's niobium-titanium dipoles mentioned by Physics Today.

For readers who want the underlying physics of electricity and magnetism that Onnes took for granted, our pieces on how electricity works and how magnetism works fill in the basics. The older history of how electric current and magnetism were found to be linked is in our article on Oersted's discovery of electromagnetism, and another landmark measurement of how current behaves in a conductor is described in our piece on the Hall effect discovery of 1879.

What the experiment showed about method

A few features of the work are worth borrowing for anyone thinking about how experiments succeed.

Materials first. The key decision was not the cold; it was choosing a material whose purity could be controlled. By using mercury, a liquid that can be distilled and then frozen into a capillary, Onnes removed the main reason resistance might fail to fall. Years of purification work went into an afternoon's result.

Infrastructure as advantage. The discovery depended as much on the helium liquefier, the glassblowers, and the technical manager as on any single idea. The experiment could not have been done anywhere else.

Checking the boring explanations first. An abrupt drop in resistance invites suspicion. Onnes's team looked for short circuits and warmed the sample to see the resistance return, a pair of checks any skeptic would ask for.

Measuring more than the surprise. The sharp drop got the attention, but the team also measured the temperature dependence, the effect of impurities, the critical field, and persistent currents. Those turned a single observation into a body of facts that theorists could use.

Admitting the interpretation lagged. Onnes did not understand what he had found, and his reports show it. The data were good enough to outlive the explanation, which is a sign of honest measurement.

What the 1911 experiment did not show

Several statements about this discovery go beyond what the experiment established.

It did not show that resistance is exactly zero. A measurement can only set an upper bound; Onnes's bounds were a millionth and later a thousand-millionth of the room-temperature value, set by the sensitivity of his instruments. Persistent-current experiments, starting with Onnes's own in 1914, probe the question far more sensitively than a voltage reading can. It did not show why the effect happens; that came in 1957. It did not show that the state was thermodynamically distinct; that came with the Meissner-Ochsenfeld result in 1933. And it was not an accident, whatever the blue-boy story suggests.

Questions people ask

Q: What did Kamerlingh Onnes actually discover in 1911? A: He found that the electrical resistance of mercury drops abruptly to a value too small to measure when the metal is cooled to about 4.2 K with liquid helium. The notebook entry for April 8, 1911 records it as practically zero. He described mercury as having passed into a new state, later called superconductivity.

Q: Why did Onnes use mercury? A: Mercury is a liquid at room temperature that can be purified by distillation, and it can be frozen inside a glass capillary without the strain and contamination of drawing a wire. Onnes expected the resistance of an extremely pure metal to keep falling at helium temperatures, and mercury was the purest metal the Leiden lab could prepare.

Q: How did he cool the mercury to 4 K? A: He used liquid helium, which he had first liquefied on July 10, 1908, after years of cascading liquefaction of air and hydrogen. Pumping on the liquid helium lowered its pressure and temperature further, down to about 1.5 K in 1908 and lower later. The mercury sat in a vacuum-insulated glass cryostat and was cooled by the helium bath.

Q: Is the story about the sleepy assistant true? A: Probably not. The popular tale says a nodding apprentice let the helium boil, which warmed the mercury and made the resistance come back, convincing the team the effect was real. A May 1911 notebook entry instead records the resistance beginning to reappear at 4.12 K as the temperature rose.

Q: Did Onnes win the Nobel Prize for superconductivity? A: Not by name. The 1913 prize was for his investigations of the properties of matter at low temperatures, which led, among other things, to the production of liquid helium. Superconductivity was a major result of that work, but the citation highlights helium.

Q: What is the Meissner effect, and why does it matter? A: In 1933 Walther Meissner and Robert Ochsenfeld showed that a superconductor cooled in a magnetic field pushes the field out of its interior. A mere perfect conductor would trap the field instead. The effect proves a superconductor is a distinct phase of matter, not just a metal with zero resistance, and it is a defining test for superconductivity.

Q: Can I read more about how the experiment sits in the wider history? A: Our complete guide to science experiments places it among the landmark experiments by field, and the complete science guide covers the surrounding physics.

What this means

Onnes set out to answer a question about a trend and found a cliff. The result looks inevitable in hindsight, and it was nothing of the kind: Kelvin's theory and Onnes's own both predicted something else. What carried the day was preparation, a purified material, a liquefier nobody else had, and a team that checked the dull explanations before the exciting one.

The larger lesson is about timing. Onnes recorded the phenomenon in 1911, the abruptness was clear by October, persistent currents showed its power in 1914, the Meissner effect in 1933 showed it was a new phase, and BCS theory in 1957 explained it. A discovery can be solid long before it is understood, and the careful record keeping of the first year is what made the next 46 years of theory possible.

Sources

• van Delft, D. and Kes, P., "The discovery of superconductivity," Physics Today: physicstoday.aip.org.
• "Remarks on the first hundred years of superconductivity" (includes quotations from Onnes's 1913 Nobel lecture): ar5iv.arxiv.org/html/1111.5318.
• Wikipedia, "Heike Kamerlingh Onnes" (helium liquefaction date, Leiden laboratory, Nobel Prize): en.wikipedia.org.
• Wikipedia, "Meissner effect" (1933 experiment, London equations, perfect conductor comparison): en.wikipedia.org.