Excerpt from The Cold Wars: A History of Superconductivity by Jean Matricon and Georges Waysand

A trace of the wonder surrounding the discovery of superconductivity persists even today. Books about it begin like fairy tales: "In Leiden in 1911, Kamerlingh Onnes . . ." Only the "Once upon a time" is missing. The discovery that the resistance of mercury suddenly drops to zero around _269_C, close to what is now called absolute zero, was not a trivial accomplishment. The first milestone was reaching this temperature, the temperature of liquid helium at atmospheric pressure. Even in the most rigorous Siberian winter the temperature only very rarely gets below _60_C. The history of superconductivity begins in Leiden because it was the only place in the world at that time where anyone knew how to create such intense cold. The ritual was carried out every week, driven only by scientific curiosity.

The theoretical basis for an absolute temperature scale is commonly attributed to Lord Kelvin. Most physicists believe that credit for the idea of an absolute zero of temperature belongs to J. A. C. Charles and J. L. Gay-Lussac, who showed by a simple extrapolation of their measurements that the volume of any gas would shrink to zero near _273_C if the pressure was held constant. Even earlier, however, Guillaume Amontons, who was born in Paris in 1663, seems to have been the first to come up with this concept, well before the birth of thermodynamics in the nineteenth century.1 Amontons noted that the pressure of a constant volume of air decreased by the same amount for each degree that he lowered the temperature. As a result, he estimated that the pressure would go to zero near _240_C. This zero was purely hypothetical until around 1850. Step by step, however, scientists would get closer and closer in the years that followed.

Establishing a temperature scale requires a choice of units. In most countries, except the United States, the standard unit is the centigrade degree, obtained by dividing the temperature interval between the melting point of ice and the boiling point of water at atmospheric pressure into one hundred equal parts. Both the Celsius scale and the Kelvin scale use centigrade degrees. On the Celsius scale the temperature of the ice is labeled 0_C and the temperature of the steam 100_C. One hundred centigrade degrees on the Kelvin scale still separate the temperatures of the melting point of ice and the boiling point of water at atmospheric pressure. But the zero on the Kelvin scale is absolute zero, which happens to be 273.16 centigrade degrees below the temperature of the melting ice. Thus, the temperature of ice on the Kelvin scale is 273.16K (and steam is 373.16K). We can convert from one scale to the other with the equation T (kelvins, K) _ T (degrees Celsius, _C) _ 273.16. The conversion between the Fahrenheit scale used in the United States and these scales is slightly more difficult because a Fahrenheit degree is smaller than a centigrade degree, and the zero has no direct physical significance. Thus, absolute zero is _459.7_F, ice melts at 32_F, and water boils at 212_F.

The success in liquefying helium in Leiden in 1908 was the last step in a vast nineteenth-century enterprise, the liquefaction of gases. What started off as an intellectual exercise became one of the century's great driving forces of technology. Louis Cailletet's machines and James Dewar's containers, examples of the many instruments and special devices that had to be developed, were crucial to the discoveries described later in this chapter. The achievements were not merely technical, however; essential theoretical advances accompanied each step.

Michael Faraday (1791-1867) was the first scientist to become interested in liquefying gases. His fame is tied more closely to his work in electromagnetism (the farad, e.g., is a unit of capacitance). In 1823, however, he succeeded in compressing gaseous chlorine until it liquefied in the closed end of a V-shaped test tube. Simply compressing any gas, it seemed, should yield a liquid. In fact, Faraday liquefied "almost all the gases" (and discovered benzene in oil tar), according to Le Petit Larousse. What is not said in this excellent book is that Faraday realized very quickly that his failure to liquefy all gases was not an accident. He turned this failure into a success when he coined the term permanent gases to label his discovery of gases that could not be liquefied by compression. The designation "permanent gases" became quite widespread; among them were oxygen and hydrogen, both essential for chemistry and industry. Compressed to three thousand times atmospheric pressure, they remained gaseous even as they got more and more dense.

Economic interests were now at stake in this activity, even though scientists like Faraday did not raise such issues. In 1856 Henry Bessemer had suggested using oxygen to improve steel refining. Large quantities of the pure gas would be needed, and liquefaction was known to be a way to purify it. Before the first drop of liquid oxygen was made, then, the process already had industrial applications. Such interplay between science and industry will often capture our attention in this book. In fact, steelmaking with pure oxygen did not become an industrial reality until well after World War II, when it indeed yielded better steel, without carbon contaminants. Bessemer's suggestion is a well-known example of an important technical idea that arrived before its time. His idea was carried out much later than one might have thought, primarily because of the large amount of capital that is needed for steelmaking.

Physicists in the middle of the nineteenth century understood that it wasn't just the pressure that was important in liquefying gases; it was also the temperature. They thought that, if they could lower the temperature sufficiently, they might liquefy oxygen. In 1852 James P. Joule and William Thomson (Lord Kelvin) each showed independently that a sharp decrease in pressure led to rapid cooling, but the pressures they could achieve were not sufficient for conclusive tests.

In 1863 Thomas Andrews, an Irishman, following up on the work of Charles Cagniard de La Tour (the inventor of the siren), provided a detailed description of the necessary conditions for gas and liquid states to coexist. Cagniard, an attaché in the Ministry of the Interior in Paris (the ministry responsible for the police, thus the siren), had suggested that there exists a temperature above which a gas can never be liquefied, no matter how high the pressure. This temperature is called the critical temperature (for liquefaction); above it, the liquid phase cannot exist, even as tiny droplets. Andrews showed experimentally that this was true for carbon dioxide, and he surmised that it was true for all gases.

The problem that had already grabbed Andrews's attention, one that would capture more and more physicists interested in condensed matter, was understanding phase transitions, the passage from one phase to another. The term phase designates a state of matter, here liquid or gaseous, later on resistive or superconducting. The physical quantities that define the state of a gas are its temperature and pressure. For a given amount of gas at a fixed temperature, there is a relation between its pressure and its volume that can be represented by a curve on a graph of pressure versus volume. The curve is called an isotherm (equal temperature) because the temperature is the same everywhere along it. Andrews plotted many isotherms for carbon dioxide at different temperatures. He noted that they looked very much alike below the critical temperature but very different from those above it.

For all temperatures below the critical temperature, simple compression, generally to pressures less than a hundred times atmospheric pressure, will liquefy a typical gas. From the moment some liquid begins to form, the pressure remains constant as long as some unliquefied gas remains, and the volume decreases as the material becomes more dense. If we plot this (constant) pressure against this (decreasing) volume, we get a horizontal straight line. The change in volume is larger at lower temperatures, so that the length of the straight line on the plot is then longer. At higher temperatures, however, the line gets shorter and reduces to a point as the critical temperature is reached. We are all familiar with the idea that a physical quantity might stay constant during a phase transition. It happens during a change in phase that we know well, the liquid-to-gas transition that occurs when we boil water for coffee in the morning. As long as some liquid remains, the water temperature stays constant while it boils, no matter how high we turn up the heat.

Discovering that a critical temperature for liquefaction exists did not solve the problem of liquefying oxygen, which remained a complete mystery. At best one could imagine that it would require a clever mixture of temperature and pressure. Liquefying oxygen was becoming a scientific challenge, and passions were rising.