Absolute Zero and the Conquest of Cold (26 page)

On the basis of finding a steady decline of resistance at liquid-nitrogen temperature, and obtaining apparent confirmation of the rate of descent in their liquid-oxygen tests, Dewar and Fleming had
predicted that at absolute zero, pure metals would be perfect conductors of electricity. Dewar had revised that prediction when readings taken at the still-lower liquid-hydrogen temperature did not fit. Certainly, resistance at the liquid-hydrogen level was appreciably lower than that at the liquid-oxygen level—Dewar beautifully demonstrated this in a 1900 lecture, first immersing a lamp with a copper resistance coil into liquid oxygen to show that the bath brightened its light, then removing the coil from the oxygen and reimmersing it in a lower-temperature liquid-hydrogen bath, with the result that the lamp's light shone more brilliantly. Dewar's calculations of the "downward slope of resistance" had initially suggested that the "resistivity" of a copper wire would drop to zero at—233°C (40 K), but it did not, and when at the liquid-hydrogen temperature of—253°C (20 K) it still had not dropped all the way to zero, he wrote that "we must infer that the curve co-relating resistance and temperature tends to become asymptotic at the lowest temperatures."

Dewar did not try to guess what might have caused the "asymptotic" resistivity readings. A year later, he changed his opinion on the disappearance of resistivity at absolute zero. Now he believed that below a certain temperature point, resistance levels might persist no matter how much further down the scale experimenters went.

Dropping resistance levels as temperatures seriously declined had been predicted in 1864 and documented in the 1880s by von Wróblewski and Olszewski—separately, of course. And so in the phase of exploration that Onnes pursued in the years immediately following his 1908 liquefaction of helium, it was natural for him to consider electrical resistance at liquid-helium temperatures as a main point of his research. But he also wanted to look into other properties of matter associated with liquid helium. Its low temperature could produce changes in the magnetization and in the specific-heat capacities of metals.

In trying to use liquid helium as a tool, Onnes ran into the same barrier that had once beset Dewar: the practical difficulty of han
dling the intensely cold liquid. The cryostats Dewar had invented to work with liquid oxygen proved inadequate for preserving liquid helium. Dewar had walked around a lecture platform carrying liquid hydrogen in an open vessel. That could not be done with liquid helium, because allowing even the slightest additional heat into a vessel could cause the liquid helium to again become a gas. Completely enclosed containers had to be made that could maintain the helium as a liquid while experiments were done with it. The process of creating the necessary equipment to handle liquid helium during experimentation took until 1911.
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Only then could Onnes use liquid helium to critically test the properties of matter.

Electrical resistance at ultra-low temperatures took center stage, not least because Onnes saw in it a chance to resolve questions for which competing theories and theorists proposed very different answers. Several leading authorities had made guesses as to what would happen to the resistance of a good conductor as the temperature was brought down near absolute zero. Lord Kelvin was the most prominent proponent of a belief, held by many other scientists, that the "death of matter" would occur at absolute zero, and that this would mean infinitely large rather than infinitely small resistance. Kelvin argued that because the electrical resistance of copper was still measurable at 20 K, the temperature of liquid hydrogen, when it had been predicted to disappear at 40 K, it was probable that as the temperature of the copper was lowered even closer to absolute zero, the electrons of the copper would freeze into place; that freezing would produce an upward "spike" in resistance, an indication that at absolute zero, resistance would be infinitely large. In Kelvin's view, while resistance might pass through a minimum on the way down the temperature scale, perhaps in the neighborhood of 10 K, when the temperature was further reduced, "electron condensation" would make resistance rise again.

For some time, Onnes agreed with Kelvin, possibly because the theory that Kelvin formulated regarded the electrons in the conductive metal as a substance that could be described by a van der Waals equation of state. But Onnes had to contend with the equally persuasive work of Nernst. The German physicist's theory implied that the resistance of a pure metal would disappear completely at absolute zero. Nernst had visited Onnes at Leiden, and the two men corresponded.

Onnes resolved his inability to choose between Nernst and Kelvin by doing a few experiments using liquid helium; these showed that the resistance of certain metals continued to drop or (in the case of pure platinum) to remain constant approaching absolute zero. Concluding that the evidence "excite[d] a doubt of Lord Kelvin's opinion," Onnes then partially abandoned his belief in infinite resistance at absolute zero. But he could not agree entirely with Nernst, either, and so made up his own theory, based on the 1864 contention that at the lowest temperatures, impurities in a metal would prevent resistance from disappearing entirely.
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Over the course of his many years of research on various subjects, and hundreds of experiments, Onnes fashioned dozens of working hypotheses. The evidence he turned up disproved nearly all of them. This does not indicate that Onnes was a poor theorist; rather, it suggests an important ability to make educated guesses and a willingness to toss them aside when they no longer seemed viable, to search for better explanations of the experimental results. The capacity to let go of working hypotheses when they proved inadequate was a reflection of Onnes's scientific worth and integrity.

To prove or disprove the hypothesis that the impurities in a metal kept its resistance from vanishing as the temperature reached a few degrees above absolute zero, Onnes gave up on platinum and began to experiment with "the only metal which one could hope to get into wires of a higher state of purity, viz. mercury." A fortuitous choice. The other metal he had been working with, in a very pure state, was gold, and had Onnes concentrated on gold and not mercury, he would not have obtained the same startling result. He could refine mercury at room temperature, and he did so repeatedly, until certain that he had removed all possible impurities.

In December 1910, Onnes was thrilled by the award of the Nobel Prize to his friend van der Waals, for his theoretical contributions. There were rumors that Onnes might be next in line, but there was also a feeling among some scientists that liquefying helium was simply a technological feat, not a discovery or theoretical advance considered worthy of a Nobel. Onnes shrugged off both notions, and continued working.

In April 1911, time became of the essence. Onnes learned from his journal reading that Nernst was obtaining preliminary results on the conductivity of metals at high and at low temperatures. Onnes was also concerned about Einstein, who was known to be investigating the elastic constants of metals at both high and low temperatures. Either of these formidable scientists could beat Onnes to the next accomplishment in low-temperature research.

It took until the summer for Onnes and his colleagues at Leiden—Flim, Gilles Hoist, and Cornelius Dorsman—to refine their mercury to their satisfaction and to set up an experimental apparatus to test its electrical conductivity at very low temperatures. The mercury was held in a U-shaped tube with wires running out of both ends, from which they would measure the metal's electrical resistance; as the temperature was lowered by means of liquid helium, the mercury congealed to a solid. Onnes and Flim worked with the cryogenic apparatus in one room, while in a dark room more than 150 feet away, Hoist and Dorsman sat and monitored the resis tance readings taken by a galvanometer, an instrument that records minute changes in electrical current by means of a coil of wire surrounding a magnet. As the temperature was pushed below 20 K above absolute zero, the resistance continued to decline but slowed its pace of descent, with each 1-degree drop in temperature no longer matched by equivalent percentage drops in the resistance.

During these electrical-resistance experiments there was not the same aura of excitement and expectation as had suffused the Kamerlingh Onnes laboratory three years earlier. Back then, Onnes had had the equivalent of a detailed treasure map in hand—composed of van der Waals's theory and Onnes's own isotherms—to guide him to the liquefaction of helium. Part of the thrill of that discovery was finding that the treasure was located precisely where the map had predicted it would be. In mid-1911, while Onnes and his colleagues did have a map of sorts—his hypothesis that the resistance of mercury would be prevented from vanishing at a few degrees above absolute zero because of impurities in the metal—they recognized that other maps pointed to different locations for the treasure. Suspecting that all the maps could well be wrong, they no longer labored in expectation of knowing just when a big event would take place or what it would consist of. So the thrill, when it came, was the unforeseen nature of what they discovered in the furthest region of the country of the cold.

As the mercury reached the temperature of 4.19 K, the electrical resistance of the mercury solid fell abruptly—as though it had been driven off a cliff—to a level so low that the galvanometer no longer registered any resistance to the current. At 4.19 K, mercury's electrical resistance just disappeared. Not confident in the accuracy of this result, Onnes tried the experiment over and over again. Every time, the findings were the same: no resistance at 4.19 K above absolute zero. Onnes and his colleagues then assumed that their apparatus might be subject to a short circuit and took a few days to replace the U-shaped tube with a W-shaped one that had electrodes extruding
from all five points, which gave them more places between which to measure resistivity. Even with this more sensitive setup, when the temperature of the apparatus was lowered to 4.19 K, the resistance readings on the galvanometer fell to zero.

Flim later told a physicist who joined the laboratory in the 1930s that during the 1911 experiments, one of the blue boys was assigned to maintain pressure in the apparatus; however, because watching a dial was a very boring job, in the course of one run the young man fell asleep, and when the dial began to move, he did not see it and so did not alert his superiors to properly adjust the apparatus. The pressure dropped, the temperature in the apparatus rose above 4.2 K—and in the galvanometer room, Onnes's colleague Hoist saw the resistance reading suddenly jump into the measurable range.

This last, reverse demonstration of the transition that seemed to be occurring at 4.19 K may have been the one that finally convinced Onnes that he had discovered a novel property of matter at extreme temperatures, a property he did not even name in his first articles, including that entitled
On the Sudden Rate at Which the Resistance of Mercury Disappears.
But he did call attention to the astonishing fact that in this state, "the specific resistance of a circuit becomes a million times smaller than the best conductors at ordinary [room] temperatures."

Onnes initially thought the drop in mercury's resistivity at 4.19 K confirmed his theory about resistance being linked to the purity of the metal, but the steepness of the cliff over which the resistance fell showed him he had been wrong. It had nothing to do with purity. He could not explain the "disappearance" of resistance in mercury, nor the drop with tin and lead, nor that resistance did
not
drop abruptly in gold, where he expected it. Physicist Kurt Mendelssohn later suggested that Onnes's puzzlement was reflected in his dearth of published articles about the phenomenon in 1912, the year after he had first announced and correctly described the drop-in-resis tance phenomenon, a time when Onnes continued to conduct experiments. An equally likely reason was Onnes's deteriorating health, which increasingly kept him confined to his home and bed. Not until his second paper of 1913 did he use the word
supraconductivity
to describe the phenomenon, a term he later discarded in favor of
superconductivity.

Onnes may have been cautious because his peers did not initially recognize superconductivity as of great importance. At a conference in 1912, when Onnes reported on the discovery, his audience did not show much interest; only two questions arose on the subject. Moreover, when James Dewar—who had pioneered work on electrical resistance at low temperatures—heard about superconductivity, he made no comment that has been recorded, and he did not send Onnes a telegram of congratulations, as he had done several times earlier. Perhaps Dewar, too, did not realize the magnitude of the discovery.

As for Kamerlingh Onnes, though he might not have understood right away all he had accomplished, he did envision a future for superconductivity, and by 1912 he had constructed an experiment to provide fodder for it: he introduced a current into a superconducting circuit he had formed into the shape of a ring, then removed the battery that had generated the current. Inside the ring, the current continued to run, and run, and run, with no measurable change in intensity. Years later, one leading physicist who visited the lab wrote a letter about this demonstration to another, saying, "It is uncanny to see the influence of these 'permanent' currents on a magnetic needle. You can feel almost tangibly how the ring of electrons in the wire turns around, around, around—slowly and almost without friction." Max Planck, inventor of the quantum theory that was about to revolutionize physics, was also extremely interested in what Onnes had discovered. And Onnes himself would shortly predict that someday superconducting wires would enable human society to transport electricity in ways much more efficient than those
then in use: electrical power plants could be situated hundreds of miles away from the places where most power was to be used; transmission costs would drop precipitously, lowering the cost of electricity to its users; and the world would have a virtually unlimited supply of electric power.