measure for measure Political union through science The is one of the enduring achievements of the French Revolution. Martin Milton recounts how it was also intended to unite nations.

oday, the name of Charles Maurice de ensured that the work required to prepare Talleyrand-Périgord is among those and distribute the first prototypes of the Tfamed for political intrigue at the time and the kilogram would be carried of the French Revolution. His reputation has out by the director of the newly established always been for clever diplomacy but he also organization with just two assistants and has a strong claim to be the initiator of the some technical staff. After completing the metric system. In March 1790, he submitted work, it was agreed that the staff would be a proposal to the National Constituent reduced to the minimum needed. Of course, Assembly — the government that had history worked out differently. emerged from the French Revolution — that The task proved to be more difficult than uniformity in weights and measures could first foreseen, and the twentieth century be achieved by referring to the length of a brought along new challenges. Inevitably, pendulum with a period of two seconds as the number of staff increased over the the unit of length. His ambition for what years. Today, the International Bureau would eventually become the metric system of Weights and Measures (BIPM) has 62 was motivated by science, but Talleyrand member states and 40 associate states and went a step further by suggesting that the economies that participate in its work. The idea should be developed collaboratively president of the Académie des Sciences, between the Académie des Sciences in Professor Philippe Taquet described it in and the Royal Society in London. He later 2014 as “an organization that is a perfect and wrote: “perhaps it is permissible to see, in magnificent example of a work of peace. The this competition of two nations questioning Credit: INTERFOTO / Alamy Stock Photo BIPM symbolizes in a unique manner what nature together to achieve an important people are able to achieve when they pool result, the principle of a political union their wills, their knowledge, their talents and through science.”1 from around the world came together in their abilities.”5 Unfortunately, the threat of war in Paris on 20 May 1875 to sign what is now At this time, when many claim that Europe derailed his plans, and his proposal known as the . The aim of multilateralism is in crisis, we should be for the unit of length was discarded the convention — in the spirit of Talleyrand’s reassured by the history of the metric because local variations in the strength proposal — appears prominently on the first system: it demonstrates vividly that of would be too large. Instead, the page, and states that the signatories desired collaboration to address scientific and Académie des Sciences set the metre as one “to assure the international unification technical challenges still leads to valuable ten-millionth of the quadrant of the Earth and perfection of the metric system”4 and agreements between nations. ❐ measured along the Paris . they undertook “to create and maintain, However, Talleyrand’s vision of an at their common expense, a scientific and Martin J. T. Milton ✉ international system of units as a way to permanent International Bureau of Weights International Bureau of Weights and Measures, involve “foreign nations and have them and Measures with its headquarters in Paris”. Sèvres, . agree on a measurement system”2 found This clear and noble-minded ambition ✉e-mail: [email protected] favour with the members of the Académie for the convention belies the extent of des Sciences. The commission that disagreement over the nature of the Published online: 8 February 2021 was appointed to develop the proposal commitment to which the states would https://doi.org/10.1038/s41567-020-01155-2 for a metric system included several agree. Although establishing an international References representatives from outside France. One standard for the metre and the kilogram 1. Bulwer, H. L. Essai sur Talleyrand 69 (Paris, C. Reinwald, 1868). of them, Jean Henri Van Swinden from was a shared goal, the signatory states were 2. Dupont, D. (ed.) Decree of 26 March 1791 adopted by the National the Batavian Republic, now part of the concerned that the formation of a permanent Constituent Assembly (Archives Parlementaires, Librairie , later recorded his enthusiasm organization would incur a commitment to Administrative, 1886). 3. Mémoires de l’Institut national des sciences et des arts, Sciences for the work: “It remains for us to express ever-increasing costs. After much discussion mathématiques et physiques Vol. 2 (Baudouin, Imprimeur de our hopes for this beautiful metric system to — largely facilitated by the representatives l’Institut National, Fructidor An VII). be adopted by all people.”3 from Germany and France, two nations 4. Preamble to the Metre Convention. Documents diplomatiques de la Conférence du Mètre (Imprimerie Nationale, 1875). Almost 100 years later, in a Europe that that had been at war less than five years 5. Proc. 25th Meeting of the General Conference on Weights and had been forced apart by war, 17 nations earlier — a compromise was struck. This Measures 289 (BIPM, 2014).

286 Nature Physics | VOL 17 | February 2021 | 286 | www.nature.com/naturephysics