Speakers at the Open Session of the 26th General Conference on Weights and Measures (CGPM) described the revision of the International System of Units (SI) as “the greatest revolution in metrology since the French Revolution.” We usually think of science as intrinsically forward looking. Yet, the Nobel laureate Bill Phillips concluded his presentation on the revision by congratulating his colleagues on fulfilling a dream dating from the eighteenth century. At the CGPM, held in Versailles on 16th November 2018, delegates from the fifty-nine Member States to the Metre Convention approved unanimously to redefine four of the seven base units of the SI according to physical constants. The star of the reform was the kilogram. Since 20th May 2019, when the revision became effective, the definition of the kilogram is based on the value of the Planck Constant from quantum mechanics. Previously, the unit was defined as the mass of the Grand K, a platinum cylinder manufactured in 1889. While this definition was easier to grasp for the general public, its instability was becoming problematic: the Grand K was progressively losing matter. As a consequence, everything else on the planet was slowly gaining weight.
In the 1790s, the French government and state scientific academy created the first version of the metric system, in an unprecedented effort at standardizing weights and measures. Early modern European marketplaces were a jungle of units: different products (for example wine and milk) were measured with different units; the same material was measured with different units at extraction, wholesale, and retail; units varied at local level and, often, within parish. To make things more complicated, each of these units was defined as the length of a specific rod or as the capacity of a certain container. These precious objects often got altered or lost. Absurd from a present-day perspective, the system reflected a different culture of measuring. Early modern units expressed more than quantity: they reflected the quality of the product, its process of production, and the power relations of the feudal and corporative system. 
Enlightened reformers and compilers of the cahiers des doléances (lists of grievances compiled across France in spring 1789 at the order of Louis XVI) agreed that this fragmented system was chaotic and encouraged cheating. Its reform suited the agenda of the revolutionaries, who were seeking to give the people tangible evidences of rupture with the Ancien Regime. Most contemporary French science stars, such as Lavoisier, Lagrange, Laplace, Coulomb, took part in the project. They agreed to base the units on natural constants, defining the meter as 1 tenth-million of the distance between the pole and the equator and deriving from it the units to measure weight, capacity, and surface. The impartiality of nature was a key concept of the Enlightenment. Nature was the source of anything rational, universal, and lasting. Only a system of units based on nature could survive wars and earthquakes.
Thus was born the metric system. On 22nd June 1799, after almost a decade of cutting-edge scientific work, a team of experts from France and allied countries delivered the official platinum objects embodying the meter and the kilogram to the French government. But the new units did not match the Enlightenment dream. The unit of length was once again the length of a rod. Even if an earthquake destroyed the rod, the meridian would not be a direct source from which to reconstruct the meter.  To better preserve the value of the unit, in fact, the international team had linked the length of the rod to the length of the pendulum beating seconds at the latitude of Paris. Going back to the meridian would not have helped to rediscover the length of the meter: the scientists who remeasured the meridian in the nineteenth century found out that their colleagues had made mistakes.
At first, enthusiasm for the metric system hardly spread outside of the Parisian scientific milieu. Using the new units meant changing mindset and altering a network of practices, without any apparent benefit for the general public beside a rational aesthetic that failed to move Breton salt merchants. In the early nineteenth century the technological benefits of standardization that we enjoy today, like the mass production of laptops and cars, were unknown.
Today, writing the history of the metric system is writing the history of a winner: all countries have adopted the SI, except the United States, Burma, and Liberia. Even there, it is employed in science, medicine, and many sectors of industry. From this standpoint, it is easy to assume that the metric system had from the start what it took to win, but that the ignorant and lazy masses delayed its victory. Zooming out from the sources produced by the reformers, however, reveals a different picture: many French and foreign experts in science and policy thought that technical, as well as cultural, challenges made standardization of weights and measures unachievable. The public had to learn how to use the new decimal measures, but it was not the only issue. The process of standardizing weights and measures required the active involvement a variety of actors such as ministries, local governors, architects, artisans, merchants, policemen, and, as the attempt was unprecedented, so were the challenges to be faced. The victory of the metric system has more to do with the persistence of those who worked towards its diffusion over decades, if not centuries, than with its own intrinsic qualities.
Ever since the 1790s, the revolutionary dream of a system of units “for all times, for all peoples” derived from natural parameters has fascinated generations of scientists. When scientific and technological development required more precise measurements, international metrological institutions redefined the units with the eighteenth-century conception of nature guiding their actions. This is not a straightforward choice. The dream might never be fulfilled. As of today, the preferred method to relate the kilogram to the Planck constant involves one of the most difficult experiments in physics. The protagonist of the experiment is the Kibble balance, a sophisticated instrument that can relate mass to electric current.
The new definition of the kilogram is not “or all times, for all peoples.” It is for societies who have knowledge in quantum mechanics and advanced technology. Within those societies, only few understand the definition. This is the cost of making units of measurement independent from physical objects. Scientists gathered in Versailles on 16th November acknowledged the challenge and the importance of finding a way to explain the new definitions to the general public. We usually think of science as intrinsically forward looking. The history of the metric system shows how cutting-edge scientific projects can be rooted in past dreams of revolutions.
Emma Prevignano is a PhD student at the University of Cambridge, with a project titled “Early attempts at implementing the metric system in France and bordering countries (1795-1845)”. At Cambridge, she completed an MPhil in Early Modern History in 2018, with a dissertation on “International collaboration in the creation of the metric system (1790- Republican Year 7). In 2017/2018, she was a Prize Research Student at the Joint Centre for History and Economics (University of Cambridge and Harvard University). Twitter: @emmaprevignano.
Title image: Bill Phillips’s lecture at the 26th CGPM (November 16, 2018). Image from Wolfram Blog.
Ken Alder, The measure of all things: the seven-year odyssey and hidden error that transformed the world (New York: Little Brown, 2002)
Witold Kula, Measures and Men (Princeton: Princeton University Press, 1986).
Simon Schaffer, “Les cérémonies de la mesure.” Annales. Histoire, Sciences Sociales 70e année, no. 2 (2015): 409–35.
Simon Schaffer, “La nouvelle définition du kilo est un révolution théologique,” Le Monde (Paris), November 12, 2018. Accessed May 16, 2019. https://www.lemonde.fr/sciences/article/2018/11/12/la-nouvelle-definition-du-kilo-est-une-revolution-theologique_5382601_1650684.html.
 Ken Alder, “A Revolution to measure: the political economy of the metric system in France”, in The values of precision edited by M. Norton Wise (ed.), 39-71 (Princeton: Princeton University Press, 1995), pp. 44-45; Robert Crease, World in the Balance: The Historic Quest for an Absolute System of Measurement (New York: Norton & Co, 2011), pp. 75-78.
 Maurice Crosland, “‘Nature’ and measurement in eighteenth century France,” Studies on Voltaire and the eighteenth century, no. 87 (1972): 277–309.
 The best accounts are Ken Alder, The measure of all things: the seven-year odyssey and hidden error that transformed the world (New York: Little Brown, 2002) and Charles Gillispie, Science and Polity in France the Revolutionary and Napoleonic Years (Princeton: Princeton University Press, 2004): 223-285, 458-493.
 Corps Législatif. Conseil des Anciens, Discours prononcé à la barre des deux Conseils du Corsp législatifs, au nom de l’Institut national des sciences et des arts, lors de la présentation des étalons prototypes du mètre et du kilogramme (Paris, 1799). The image of earthquakes destroying units of measures comes from here.
 Alder, The measure of all things.
 Jean Dhombres, “Résistances et adaptation du monde paysan au système métrique issu de la révolution : les Indices d’évolution d’une culture de la quantification.” Annales de Bretagne et Des Pays de l’Ouest 100, no. 4 (1993): 427–39.
 Marie-Hélène Froeschlé-Chopard, Michel Froeschlé-Chopard, “ Une double image de la Révolution : le calendrier et le mètre,” Annales historiques de la Révolution française, no. 279 (1990): 74–88.