In 1582, Galileo noticed something quite mundane. It may or may not be legend: that while sitting in his pew in the cathedral at Pisa he watched the lantern over the nave swinging back and forth, and doing so at a regular rate. He experimented with a pendulum and found out that the rate of the swing depended not on the weight of the pendulum bob, but on the length of the pendulum itself. The longer the pendulum arm, the slower and more languid the back-and-forth interval. A short pendulum would result in a more rapid tick-tock, tick-tock. By way of Galileo’s simple observation so length and time were seen to be linked—a linkage that made it possible that a length could be derived not simply from the dimensions of limbs and knuckles and strides, but by the hitherto quite unanticipated observation of the passage of time.
A century later an English divine, John Wilkins, proposed employing Galileo’s discovery to create an entirely new fundamental unit, one that had nothing to do with the then-traditional standard in England, which was a rod that was more or less officially declared to be the length of a yard. In a paper published in 1668, Wilkins proposed quite simply making a pendulum that had a beat of exactly one second—and then, whatever the length of the pendulum arm that resulted would be the new unit. He took his concept further: a unit of volume could be created from this length; and a unit of mass could be made by filling the resulting volume with distilled water. All three of these new proposed units, of length, volume, and mass, could then be divided or multiplied by ten—a proposal which made the Reverend Wilkins, at least nominally, the inventor of the idea of a metric system. Sad to say, the committee set up to investigate the plan of this remarkable figure 1 never reported, and his proposal faded into oblivion.
1 Wilkins, who was variously warden of Wadham College, Oxford, and master of Trinity College, Cambridge, was a polymath the like of which is little known today. Not only was he a practicing priest and college administrator, but he had a great interest in science: he suspected there might be life on the moon, imagined the existence of new planets, devised plans for submarines and aircraft and perpetual motion machines, and, in the same book that proposed a metric system based on the pendulum, proposed the establishment of a new universal language, because of the deficiencies of Latin. Also, during his time at Wadham, he created transparent beehives so that honey could be harvested without disturbing the bees.
Except that one aspect of the Wilkins proposal did resonate—albeit a century later—across the Channel in Paris, and with the support of the powerful cleric and diplomat Talleyrand. The formal proposal, which Talleyrand put to the National Assembly two years after the Revolution, in 1791, exactly duplicated Wilkins’s ideas, refining them only to the extent that the one-second beating pendulum be suspended at a known location along the latitude of 45 degrees North. (Varying gravitational fields cause pendulums to behave in varying ways: sticking to one latitude would help mitigate that problem.)
But Talleyrand’s proposal fell afoul of the postrevolutionary zeal of the times. The Republican Calendar had been introduced by some of the firebrands of the day, and for a while France was gripped by a mad confusion of new-named months (Fructidor, Pluviôse, and Vendémiaire among them), ten-day weeks (beginning on primidi and ending on décadi ), and ten-hour days—with each hour being divided into one hundred minutes and each minute into a hundred seconds. Since Talleyrand’s proposed second did not match the Revolutionary Second (which was 13.6 percent shorter than a conventional second of the Ancien Régime) the National Assembly, gripped by the new orthodoxy, rejected the idea wholesale.
And it would be more than two further centuries before the fundamental importance of the second was fully accepted. For now, in the minds of eighteenth-century French assemblymen, length was a concept vastly preferable to time.
about the author
Simon Winchester is the author of multiple New York Times bestsellers, including The Professor and the Madman , and A Crack in the Edge of the World .
For in dismissing Talleyrand so they turned instead to another idea, brand-new, which was linked to a natural aspect of the Earth, and so in their view more suitably revolutionary. Either the meridian of the Earth or its equator should be measured, they said, and divided into forty million equal parts, with each one of these parts being the new fundamental measure of length. After some vigorous debate, the parliamentarians opted for the meridian, in part because it passed through Paris; they then also decreed that to make the project manageable the meridian be measured not in its entirety, but only in the quarter of it that ran from the North Pole to the equator—a quarter of the way around, in other words. This quarter should then be divided into ten million parts—with the length of the fractional part then being named the meter (from the Greek noun μέτρον, a measure).
A great survey was promptly commissioned by the French parliament to determine the exact length of the chosen meridian—or a tenth part of it, an arc subtending about nine degrees (a tenth of the ninety degrees of a quarter-meridian), and which, using today’s measurement, would be about a thousand kilometers long. It would necessarily be measured in the length units of eighteenth-century France: the toise (about six feet long), divided into six piedsduroi , each pied divided into twelve pouces , and these further divided into twelve lignes . But these units were of no consequence—because all that mattered was that the total length be known and then be divided by ten million—with whatever resulted becoming the measure that was now desired, a creation of France to be eventually gifted to the world.
The proposed survey line ran from Dunkirk in the north to Barcelona in the south, each port city self-evidently at sea level. Since this nine-odd-degree arc was located around the middle of the meridian—Dunkirk is at 51 degrees North and Barcelona 41 degrees North, with the midpoint of 45 degrees North being the village of Saint-Médard-de-Guizières in the Gironde—it was thought likely the oblate nature of the Earth’s shape, the bulge that afflicts its sphericity and makes it resemble more of an orange than a football, would be most evident and so easier to counter with calculation. (To further confirm the Earth’s shape the French Academy of Sciences sent out two more expeditions, one to Peru and the other to Lapland, to see how long a degree of high latitude was: all confirmed the orange shape that Isaac Newton had predicted centuries before.)
The story of the triangulation of the meridian in France and Spain, and which was carried out by Pierre Méchain and Jean-Baptiste Delambre over six tumultuous years during the worst of the postrevolutionary terror, is the stuff of heroic adventure. On numerous occasions the pair escaped great violence (but not jail time) only by the skin of their teeth. The story is also outside the scope of this account, for what matters to precision engineers of the future—and to engineers all over the world, since that one remarkable survey led to the establishment of the metric system still in use today—is what the French did once the survey results were in. And that mostly involved the making of bronze or platinum rods.
The survey results were announced in April 1799. The length of the meridian quadrant was calculated from the extrapolated survey findings to be 5,130,740 toise . All that was required was that bars and rods be cut or cast that were one ten-millionth of that number—0.5130740 toise , in other words. And that length would be, henceforward, the standard measure—the standard meter—of postrevolutionary France.
The commissioners then ordered this length to be cast out of platinum, as what is known as an étalon —a standard. A former court goldsmith named Marc Étienne Janety had been selected to make it, and was called back from Marseille, where he had been sheltering from the excesses of the Terror. The result of his labors exists to this day—the Meter of the Archives, a bar of pure platinum that is twenty-five millimeters wide and four millimeters deep, and exactly, exactly, one meter in length. On June 22, 1799, this meter was officially presented to the National Assembly.
But that was not all: for in addition to the platinum rod that was the meter, so also there came with it a few months later a pure platinum cylinder which, it was explained, was the étalon of mass, the kilogram. Janety had made this one too, and also from platinum, 39 millimeters tall, thirty-nine millimeters in diameter, stored in a neat octagonal box with the label proclaiming, in good Napoleonic calendric detail, “Kilogramme Conforme à la loi du 18 Germinal An3, présenté le 4 Messidor An 7.”
2 The linkage of length and mass standards, and the concept of using water to come up with a standard of mass, was first put forward by the same John Wilkins who suggested using a pendulum for length determination.
The two properties of length and mass were now inextricably and ineradicably connected. For once the standard of length had been determined, so that length could be employed to determine a volume and, using a standard material to fill that volume, so a mass could be determined too. 2 And so in Paris at the exhausting end of the eighteenth century it was decided to create a new standard for mass based on a formula of elegant simplicity. One-tenth of the newly presented meter—and which would be technically a decimeter—could be set as the side of an exactly manufactured cube. This cubic decimeter would be called a litre measure, and it would be made as precisely as possible out of steel or silver. It would then be filled entirely with pure distilled water and the water held as close as possible to the temperature of 4 degrees Celsius, the temperature at which the density of water is most stable. The resulting volume, this one liter of this particular water, would then be defined as having a mass of one kilogram.
The platinum object made by the goldsmith M. Janety was duly cast, and adjusted until it exactly balanced the weight of that cubic decimeter of water. And that platinum object—very much smaller than the water, of course, since platinum was so much denser, by a factor of almost twenty-two—would from December 10, 1799, henceforward be the kilogram.
The Kilogram of the Archives and the Meter of the Archives, from which the kilogram had been determined, were thus the new fundamentals of what would soon be a new world order of weights and measures. The metric system was now officially born.
These two icons of its founding are still in existence, in a steel safe deep within the Archives Nationales de France in the Marais, in central Paris. One resides in an octagonal black leather-covered box, the other in a long and thin box of reddish-brown leather.
Except that—and this is a constant feature in the universe of measurement—these beauteous objects were eventually found to be wanting.
Years after they had been fashioned, the meridian line on which they had been based was resurveyed, and to widespread chagrin and dismay it was discovered that there were errors in Delambre and Méchain’s six-year eighteenth-century survey, and that their calculation of the length of the meridian was off. Not by much, but by enough for the physical Meter of the Archives to be shown to be two- tenths of a millimeter shorter than the newly calculated version. And it follows that if the meter was wrong, then the cubic meter and the cubic decimeter and the liter-of-water equivalent in platinum, which would be the kilogram, would be wrong also.
So a cumbersome process was set in train to create a set of wholly new prototypes, which would be as perfect in their exactitude as late nineteenth-century science could manage. It took more than seven decades for the international community to agree, and many further years to make the requisite cache of bars and cylinders. The need to make the standards as near-perfect as imaginable was to become the stuff of obsession. Fifty international delegates—all of them men, all of them white, and almost all of them with lengthy beards—gathered for the first meeting of the International Metre Commission in Paris in September 1872 to begin the process. They met in the former medieval priory of St. Martin des Champs, later to be turned into the Conservatoire National des Arts et Métiers, one of the world’s greatest repositories of scientific instruments. 3
3 One fewer after an accident in mid-May 2010 when the original Foucault’s pendulum of 1851, housed in the Conservatoire for decades, crashed to the floor, irreparably damaging its bob. The cable had snapped; some said that attendees at private parties held in the museum were known to have played with the solemnly swinging pendulum, weakening its stays.
The countries that would decide the future of the world’s measurement system included all the then-great Western powers—Britain, the United States, Russia, Austria-Hungary, the Ottoman Empire—but pointedly, neither China nor Japan. Their sessions, and those of their associated conferences—most notably the Diplomatic Conference of the Metre, which was more concerned with national policies, less with the technical aspects of making prototypes—went on for what at this remove seems an interminable period.
All of the meetings would, however, lead eventually to the signing, on May 20, 1875, of the Treaty of the Metre. It would mandate the formation of the BIPM, the present-day International Bureau of Weights and Measures, which would have as its home the Pavillon de Breteuil, outside Sèvres, and which it still inhabits today. Between them these bodies, at various times and in various ways, would commission the making of a set of vital new prototypes.
It took nearly fifteen years for the defining set of internationally agreed standard measures to be created, for the new standard artifacts to be cast, machined, milled, measured, polished, and offered up for the world’s approval. On September 28, 1889, a ceremony was held in Paris to distribute them.
The two best made, each so perfect in their appearance and exact in their dimensions, and which in consequence were nominated to be the international prototypes, had by now been chosen: they were the International Prototype Meter, to be known hereafter by the black-type letter M, and the International Prototype Kilogram— Le Grand K —designated by the black letter K. Both of these platinum-iridium alloy objects were to remain for all future time under heavy security in the basement of the Pavillon de Breteuil.
All the others were then, and for this September day only, on display in the Pavillon’s observatory. The stubby little kilograms gleamed under glass cloches (the national standards under a pair of glass cloches, the IPK itself under three), the slender meter bars in wooden tubes that were further enclosed in brass tubes with special fixtures to keep them safe while they traveled.
Certificates of authenticity had been engraved on heavy Japanese paper by the Parisian society printer Stern. Each of these certificates had a formulaic rubric that gave the properties of the body it accompanied: platinum-iridium cylinder No. 39, for example, had the notation “46.402mL 1kg - 0.118mg,” which is decoded as meaning the cylinder had a volume of 46.402 milliliters and was lighter than 1 kilogram by 0.118 milligrams. Certificates for the meters were a little more complicated: for instance, one of the meter bars was noted as being “1m + 6μ.0 + 8μ.664T + 0μ.00100T2,” which meant that at 0 degrees Celsius it was 6 micrometers longer than 1 meter, and at a 1 degree Celsius its length would be greater by a little more than 8.665 micrometers.
Three urns stood on a dais in the room, and officials had put into each paper slips bearing the numbers of the remaining standards—they were to be distributed among the member states by lottery.
And so, in midafternoon of that warm autumn Saturday, the world lined up as if bidding for the distribution of sporting season tickets. Officials called out the countries’ names, in alphabetical order, in French. Allemagne was first, Suisse last. The draw took an hour. When it was all over the United States had received Kilograms 4 and 20, and Meters 21 and 27. 4 Britain had acquired Meter 16 and Kilogram 18; Japan (which by this time had signed the 1875 treaty), 5 Meter 22 and Kilogram 6.
4 After serving as the U.S. standard for the meter for seventy-one years—and being taken to Paris four times during that period, for comparison with LeGrandK—No. 27 was retired in 1960 and sits in a glass case in a museum at the National Institute of Standards and Technology in Gaithersburg, Maryland, outside Washington, DC.
5 China would not be a party to the treaty until 1977—by which time, as we shall see, the entire system of measurement had changed.
By the end of the day, so the delegates set off from Paris with their invaluable bounties—all packed away in boxes (the kilograms removed from their cloches for travel), and with all the bills paid. They were not insubstantial: the cost of a platinum-iridium meter was 10,151 francs; the kilogram a comparative steal at 3,105 francs. Within days or weeks (the Japanese took theirs back by ship) the new standards were safely in the metrology institutes that were by now being established in capitals all around the world. They were all kept safe and sound—though none so safe and sound as the International Prototypes, M and K, which were now to be taken to the basement and plunged into sempiternal darkness, incomparable, accurate, and fantastically precise. In safes nearby were six so-called témoins —witness bars, which would be regularly compared against the masters. These too would remain exact and perpetually inviolate.
Except, not exactly. Not so fast. The overseers of metrology’s fundamentals had been charged with the task of eternal vigilance, of always looking for still better standards than these. And in time they did indeed find one.
The first clues that there might be a better system had come some years before, in 1870, long before these platinum talismans were being wrought into their final definitive shapes and sizes. The Scots physicist James Clerk Maxwell, at the British Association for the Advancement of Science annual meeting in Liverpool, had made a speech that threw a wrench into everything that had been done. His words still ring in the ears of metrologists around the world. He reminded his listeners that modern measuring had begun with the survey and then the resurvey of the French meridian, and the derivation of the metric units from the results:
Yet, afterall, the dimensions of our Earth and its time of rotation, though, relatively to our present means of comparison, [are] very permanent, [they] are not so by physical necessity.The Earth might contract by cooling, or it might be enlarged by a layer of meteorites falling on it, or its rate of revolution might slowly slacken, and yet it would continue to be as much a planet as before. But a molecule, say, of hydrogen, if either its mass or its time of vibration were to be altered in the least, would no longer be a molecule of hydrogen.
If, then, we wish to obtain standards of length, time and mass which shall be absolutely permanent, we must seek them not in the dimensions, or the motion, or the mass of our planet, but in the wavelength, the period of vibration, and the absolute mass of these imperishable and unalterable and perfectly similar molecules.
What Maxwell had done was challenge the scientific basis for all systems of measurement up to that moment. It had long been self-evident that a system based on the dimensions of the human body—thumbs, arms, stride, and so forth—was essentially unreliable, subjective, variable, and useless. Now Maxwell was suggesting that standards previously assumed reliable, like fractions of a quadrant of the Earth’s meridian, or the swing of a pendulum or the length of a day, were not necessarily usefully constant either. The only true constants in nature, he declared, were to be found on a fundamental, atomic level.
And by this time scientific progress was providing windows into that atom, revealing structures and properties hitherto undreamed of. These very structures and properties that were by their very nature truly and eternally unvarying, Maxwell was saying, should next be employed as standards against which all else should be measured. To do otherwise was simply illogical. Fundamental nature possessed the finest standards—the only standards, in fact—so why not employ them?
It was the wavelength of light that was the atomic fundamental first used to try to define the standard measure of length, the meter. Light, after all, is a visible form of radiation caused by the excitation of atoms—excitation that causes their electrons to jump down from one energy state to another. Different atoms produce light ranging over different spectrums, with different wavelengths and colors, and so produce different and identifiable lines on a spectrometer.
It took a further hundred years to convince the international community of the wisdom of linking length to light and its wave-length. To the graybeards who then ran the world, abandoning the certitudes of Earth for the behavior of light was akin to believing that the continents could move—a simply preposterous idea. But just as in 1965, when the theory of plate tectonics was first advanced and continental drift was suddenly seen as obvious, a reality hidden in plain sight, so it became as much the same in metrology as it had been for geology: the notion of using atoms and the wavelength of the light they can emit as a standard for measuring everything snapped into place in a sudden moment of rational realization.
It was a late nineteenth-century Massachusetts genius named Charles Sanders Peirce who had that first moment, who first tied the two together. Few men of his generation can have been more brilliant—or more infuriatingly, insanely troublesome. He was many things—a mathematician, a philosopher, a surveyor, a logician, a philanderer of heroic proportions, and a man crippled with pain (a facial nerve problem), with mental illnesses (severe bipolar disorder most probably), and with a profound inability to keep his temper in check. On the plus side of the ledger: he could stand before a blackboard and write a mathematical theory on it with his right hand on the right side and, simultaneously, write its solution with his left hand on the left. On the minus side: he was once sued by his cook for hitting her with a brick. He drank. He took laudanum. He was much married, and was pathologically unfaithful.
But it was Peirce who in 1877 first took a pure and brilliant source of incandescent yellow sodium light, and tried as hard as he might to measure—in meters, thereby establishing the dimensional link between light and length—the black spectral line it produced when run through a diffraction grating, a kind of high-precision prism. It was one of the numberless misfortunes of his seventy-five years that this experiment never quite succeeded—there were problems with the expansion of the glass of the grating, problems with the thermometers used to measure the temperature of the glass. But he nevertheless published a short paper in the American Journal of Science, and by doing so laid historical claim to being the first to try. Had he succeeded his name would be on the lips of all. As it was he died obscurely in 1914, and in abject poverty, having to beg stale bread from the local bakery. He is long forgotten, except by a very few who agree with such as Bertrand Russell, who called Peirce “the greatest American thinker, ever.”
By 1927, after much badgering by scientists who were convinced by Maxwell’s argument that this was the best approach to setting an inviolable standard, so the world’s weights and measures community came, if somewhat grumpily, to an agreement. They first accepted, formally, that one particular element’s wavelength had thus been calculated, and in fractions of a meter—a very small number. Further, they then agreed that by multiplication, the meter could be defined as a certain number of those wavelengths—by comparison a very big number, and to at least seven decimal places. Multiply the one by the other and one gets, essentially, one meter.
The element in question was cadmium—a bluish, silvery, and quite poisonous zinc-like metal that was used for a while (with nickel) in batteries and to corrosion-proof steel and now is used to make (with tellurium) solar panels. It emits a very pure red light when heated, and from its spectral line the wavelength could be determined—so accurately that the International Astronomical Union used its wavelength to define a new and very tiny unit of length, the Ångstrom—one ten-billionth of a meter, 10 −10 m.
The wavelength of cadmium’s red line was measured and defined as 6,438.46963 Ångstroms. Twenty years later, with the weights and measures officials in Paris now accepting both the principle and the choice of cadmium (although making its red-line wavelength slightly fuzzier by losing the final number 3, rendering it as 6,438.4696Å), the meter could have been very easily defined by simple arithmetic as 1,553,164 of those wavelengths. (Multiplying the first figure by the second gives 1.000, essentially.)
But—and in the tortuous history of the meter, this is hardly surprising—cadmium then turned out to be not quite good enough. Its spectral line, when examined closely, was found not to be as fine and pure as had been thought—the samples of cadmium were probably mixtures of different isotopes of the metal, spoiling the hoped-for coherence of the emitted light. And so it happens that the meter never was formally defined in terms of cadmium. Much else was, but not the sacrosanct meter. The platinum-iridium bar clung on gamely through all the various meetings of the weights and measures committees, surviving all the siren-like temptations of other radiations—until finally, in 1960, there came agreement.
6 The unstable isotope krypton-85, which has a half-life of eleven years, is a by-product of nuclear explosions and fuel reprocessing—and the presence of plumes of the gas in the upper atmosphere has been detected by satellites orbiting over North Korea.
The world settled on krypton. This inert gas, which was only discovered in trace amounts in the air in 1898, is perhaps best known as the most commonly used gas in neon signs, which are seldom filled with neon at all. More important, in this long quest to define the meter in terms of wavelength, krypton has a spectral signature with extremely sharp emission lines. Krypton-86 is one of the six stable isotopes that occur naturally, 6 and on October 14, 1960, the International Committee on Weights and Measures decided, nearly unanimously, that this gas, with its formidable coherence and with the exactly known wavelength of its emissions of reddish-orange radiation (6,057.80211.) would be the ideal candidate to do for the meter what cadmium had done for the Ångstrom.
And so, with the delegates observing that the meter was still not defined with “sufficient precision for the needs of today’s metrology,” it was agreed that henceforward the meter would be defined as “the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.”
And with that simple declarative sentence so the old one-meter platinum bar was pronounced, essentially, useless. It had lived since 1889 as the ultimate standard for all length measurement: Ludwig Wittgenstein had once observed, with confusing but accurate drollery, “There is one thing of which one can say neither that it is one meter long, nor that it is not one meter long, and that is the standard meter in Paris.” No longer, for from October 14, 1960, onward, there was no standard meter remaining in Paris, nor anywhere else. This measurement had left the physical world and entered the absolutism and indifference of the universe.
From the book The Perfectionists by Simon Winchester. Copyright 2018 by Simon Winchester Published by Harper, an imprint of HarperCollins Publishers. Reprinted by permission.
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