Absolute Zero and the Conquest of Cold (15 page)

Thomson also made his own, more precise version of the Carnot-based proposition for the second law: "It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects." He implied that these laws of thermodynamics had been foreshadowed by the 102nd Psalm's prophecy: that the heavens and earth "all of them shall wax old like a garment" but that God would "endure."

Like Carnot when confronted with the implications of his work, Thomson was shortly thereafter compelled to another, more radical conclusion, one with cosmological implications. From what he called A
Universal Tendency in Nature to the Dissipation of Mechanical Energy,
he drew a consequence that he had never wanted to note but to which science and logic had brought him: that such dissipation meant the sun was "not inexhaustible." This in turn meant that "within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted," because the earth would be too cold to sustain life. Though Thomson could not bring himself to say so explicitly, his conclusion showed to others that the Bible's timetable for the creation of the earth and the heavens was not factually accurate. And when his conclusion based on the laws of thermodynamics was considered in conjunction with the evidence verifying Charles Darwin's contemporaneous theory of evolution, they together cast serious doubt on the existence of God as defined by the Bible.

Having finally accepted Joule's contentions about the conservation of energy, Thomson then yielded to Joule's entreaties to become a friend and began with him a long series of joint experiments and publications that firmly established the dynamical theory of heat, and that also made possible the next generation's explorations into the nether regions of temperature.

The theory, when fully expressed by Thomson, based on Joule's earlier work, unified the phenomena of heat, electricity, magnetism, and light. All these, it contended, were different forms of energy that were convertible into one another, and the relationship between forms of energy could be expressed by such numerical constants as Joule's mechanical equivalent of heat.

Since Joule had proved the existence of one constant, there must be others, and Thomson joined with Joule to establish them. Thomson suggested the specific experiments that Joule designed and conducted, discussing most of the details with Thomson in advance of the trials through exchanges of letters and the occasional visit by Thomson to Manchester. Delighted at this collaboration, Joule continually deferred to and attempted to accommodate Thomson, except where experimental data would not permit him to agree. Simultaneously with the joint series, the two colleagues continued their own studies, which were published under each individual's name but which were influenced by both men. Two of the experiments directly concerned the generation of cold and would provide the key to later mastery of cold.

In an early letter that Thomson went back and reread, Joule had brought to Thomson's attention the 1834 work of Peltier on thermoelectrics, in which heat or cold could be produced by electricity. Peltier had shown that heat could be either liberated or absorbed when an electric current flowed across two conductors made of different materials. Neither the shape nor the size of the conductors seemed to have anything to do with this "Peltier effect." Joule suspected that thermoelectric effects had to do with the interconvertibility of thermal energy and electrical energy. With a mind far nimbler than Peltier's, and using Joule's hunch, Thomson quickly showed additional reversible thermal effects occurring in thermoelectric circuits, and that the magnitude and direction of what came to be called "Thomson effects" depended on the composition and temperature of the conductor. If there was a difference of 1 degree Kelvin between the temperature at one end of the conductor and the other, when the current moved along in the same direction as the temperature gradient, from hotter to colder, it could produce heat, but when the current coursed
against
the temperature gradient, it could absorb heat, thereby producing cold. Previously considered no more than a curiosity, cold produced by electricity (using Thomson's findings) would by century's end lead to the construction of effective thermoelectric generators and refrigerators.

Joule and Thomson collaborated directly on a second method of producing cold, related to a phenomenon noted by many scientists, that some cooling resulted when gases were released from pressure. Joule had verified this in his two-vessel experiments. Thomson proposed making important alterations to the apparatus to measure this gas-expansion cooling. Joule eagerly agreed.
^*

At Thomson's suggestion, Joule replaced the copper vessels with long coils of metal piping, and he considerably narrowed the connector between the two coils, to try to prove that the cooling effect of the pressurized air as it expanded into the lower-pressure arena would be offset somewhat by the heating effect produced at the nozzle due to friction. The first experiments on this were inconclusive, so Joule tinkered further with the apparatus. A letter to Thomson written during this period displays Joule's confabulation of personal and scientific enterprises in the friendship:

The expected stranger arrived safely into the world yesterday morning. It is a little girl, very healthy and strong...[and] if, as I hope, you will make it convenient to be at the christening and stand godfather, we might at the same time endeavour to settle the question of heat and cold from air rushing through an orifice. Using a plug of guttapercha with a small hole I find the air to be cooled from 63 to 61/2 when rushing at a pressure of 4 atmospheres.

Thomson was busy—not only with his own work but also with pursuing a woman who would later become his wife—and did not attend the christening, or for some months offer any cogent work suggestions to Joule, though he did write a bit disparagingly to brother James of Joule's request that he stand godfather to the child. A more sympathetic friend of Joule's attended the ceremony as Thomson's surrogate.

Joule's restless mind further refined the apparatus, in the direction of smaller and smaller flow passages, until he was satisfied with a nozzle that fit the definition of a "porous plug." That did the trick. Testing many gases, Joule and Thomson found that air, carbon dioxide, oxygen, and nitrogen became colder during the expansion but that hydrogen became hotter. Experimenting further and graphing the results, they discovered a set of "inversion temperatures." At and below these temperatures, precooled gases were certain to cool further when expanded. What shortly became known as "Joule-Thomson expansion"—the expansion of pressurized, precooled gases through a porous plug into a lower-pressure vessel, producing a significant decrease in temperature—became the basis for many subsequent efforts in refrigeration, even those used today. We will encounter it as a key concept in the last stages of the drive toward absolute zero.

In the early 1860s, Thomson and Joule's new elucidation of what could produce cold came to influence the theory of heat. Rudolf Clausius found in Thomson's paper on thermoelectricity an important clue relating to an attribute of matter that deeply intrigued him. For years Clausius had been wondering what could explain or measure the apparently universal tendency toward dissipation. Thomson demonstrated in his article on thermoelectricity that materials possessed some internal energy and postulated that it was somehow used for molecular cohesion. Clausius had touched on a similar concept in his 1850 paper, but it was only after Thomson had produced a sort of experimental verification of internal energy that Clausius pounced on the idea as though it were the Rosetta Stone that could explain what had previously been right before his eyes but had been incomprehensible.

There were not two types of transformation of energy, Clausius wrote in 1865, there were three. Along with mechanical energy being transformed into heat, and heat being transferred from a hotter body to a colder body, there was the transformation that took place when the constituent molecules of a material were rearranged. From this notion, and from the accepted fact that the change from a solid to a liquid, and from a liquid to a gas, involved work or heat, he derived the concept of
disgregation,
the degree of dispersion of the molecules of a body. The disgregation of a solid was low, that of a liquid higher, and that of a gas higher still.

Clausius argued that when a gas was expanded but no work was performed, a transformational change in its energy condition still took place—an increase in its disgregation. To explain this further, Clausius introduced the term
entropy,
a measure of the unavailable energy in a closed system, or a measure of the bias in nature toward dissipation. The greater the disgregation, the greater the entropy.

Building on the work of dozens of investigators over forty years, Clausius finally concluded that the "fundamental laws of the universe which correspond to the two fundamental theorems of the mechanical theory of heat" were "1) The energy of the universe is constant; 2) The entropy of the universe tends to a maximum."

This ultimate, concise, eloquent expression of the forms of energy eviscerated what historian of thermodynamics Donald Cardwell called the "balanced, symmetrical, self-perpetuating universe" of Boyle and Newton, substituting a glimpse of something wholly modern, stripped of theological benevolence, and thoroughly disquieting: "a universe tending inexorably to doom, to the atrophy of a 'heat death,' in which no energy at all will be available although none will have been destroyed; and the complementary condition is that the entropy of the universe will be at its maximum."

In other words, everything will settle into a state that has a single uniform—but non-zero temperature. Within a half century of Clausius's pronouncement, the concept of entropy would provide the key to his intellectual heir, Walther Nernst, for refining the understanding of entropy in a way that would allow twentieth-century experimenters to reach to within a tantalizing two-billionths of a degree of absolute zero.

I
N
1865,
THE YEAR OF
Rudolf Clausius's seminal paper, his former student Carl Linde began working for a locomotive manufacturer and at the same time helped found the Munich Polytechnische Schule, the first of its kind in Bavaria. Linde later recalled that he had been expected to follow in his father's footsteps and become a minister, but an early infatuation led him to study the power of machines rather than the power of God. Learning physics from Clausius before becoming an engineer, Linde never lost his respect for theory. In 1870 a contest sponsored by the Mineral Oil Society of Halle caught his eye: the challenge was to design a system to maintain 25 tons of paraffin for as long as a year at a temperature of—5°C, achieved through artificial means.

Linde addressed the problem as a student of Clausius. He read all he could about the several extant refrigeration systems, including the Carré absorption machinery, which then dominated the field, having found success in the United States as well as in France. Then he subjected the systems to thermodynamic analysis. The one designed by the Geneva-based chemist Raoul-Pierre Pictet was the most efficient, a vapor-compression system that used sulfur dioxide as the cooling medium; it functioned at much lower pressures than its competitors, but the sulfur dioxide sometimes made contact with water and could transform into the very corrosive sulfuric acid, which ate away the metal of the machinery. Linde found that the other systems were based on principles that did not take advantage of what thermodynamics taught about the conservation of energy. So he designed a thermodynamically sound system of his own, without sulfur dioxide, along the lines of a Carnot cycle achieved through vapor compression.

Mechanical Effects of Extracting Heat at Low Temperatures,
his article detailing all this, appeared in a new and relatively obscure Bavarian trade journal, where it was noted by the director of the largest Austrian brewery company, who commissioned Linde to design a refrigeration system for a new brewery. Linde-designed refrigerators were so much better than the Carré- and Pictet-designed machines that within a few years his units had replaced the older ones, first in breweries and then in other industrial processes that required cooling, until there were more than a thousand Linde machines at work in factories all over Europe.

Artificially produced refrigeration has been the least noted of the three technological breakthroughs of great significance to the growth of cities that came to the fore between the 1860s and the 1880s. More emphasis has been given to the role played by the elevator and by the varying means of communication, first the telegraph and later the telephone. The elevator permitted buildings to be erected higher than the half-dozen stories a worker or resident could comfortably climb; telegraphs and telephones enabled companies to locate managerial and sales headquarters at a distance from the ultimate consumers of goods and services. Refrigeration had equal impact, allowing the establishment of larger populations farther than ever from the sources of their food supplies. These innovations helped consolidate the results of the Industrial Revolution, and after their introduction, the populations of major cities doubled each quarter century, first in the United States—where the technologies took hold earlier than they did in older countries—and then elsewhere in the world.

A spate of fantastic literature also began to appear at this time; in books such as Jules Verne's
Paris in the Twentieth Century,
set in 1960, indoor climate control was mentioned, though its wonders were not fully explored. From the mid-nineteenth century on, most visions of technologically rich futures included predictions of control over indoor and sometimes outdoor temperature.