A Chronology of Measurement

The dates that follow mark out the path of the book; the sign ”~” indicates approximate datings, and future events are declared as expected. For the institutional history of the metric system the reference of choice remains Quinn’s account [Quinn 2011]; for the standing definitions of the units, the official text of the International System [BIPM 2019] .

Antiquity

• ~2700–2600 BC — Egypt: the royal cubit of seven palms is the unit of state; standards in granite and wood, nilometers for the flood, “rope-stretcher” surveyors.
• ~2100 BC — Lagash: the statues of Gudea with the graduated rule on their knees, the oldest known ruler as an attribute of sovereignty.
• ~2000 BC — Indus valley: cubic weights of chert in regular ratios, identical from city to city; metrology legible where the script is not.
• 18th century BC — The Code of Hammurabi imposes capital penalties for fraud in weight; the base sixty of minas and shekels still governs our minutes and our degrees.
• 1st millennium BC — Deuteronomy forbids the “two weights, a large and a small”: the false measure as sin before crime.
• ~430 BC — Athens: the metronomoi, magistrates by lot for the market’s weights and measures; public standards in the agora.
• 221 BC — Qin Shi Huang unifies weights, measures, coinage, writing, and cart gauge: the uniform measure as the sign of a single order.
• ~240 BC — Eratosthenes estimates the Earth’s circumference from the shadows of Syene and Alexandria.

Middle Ages and the modern era

• 789 — Charlemagne’s capitularies order “equal and just measures” throughout the empire; the dream dissolves with the dynasty.
• 9th century — The caliph al-Ma’mun has a degree of meridian measured in Mesopotamia; in the cities of Islam the muhtasib watches over weights and balances.
• 1215 — Magna Carta, clause 35: one measure of wine, ale, and corn for the realm — a promise repeated for centuries.
• ~1340 — Pegolotti’s Pratica della mercatura: the ready-reckoner of conversions from London to the Sea of Azov.
• 1535 — Köbel’s manual: sixteen men leaving mass to make the “right and lawful” rod.
• 1668 — The toise du Châtelet walled in at Paris; a century later it will be found deformed and deposed (1766).
• 1670 — Gabriel Mouton proposes the unit from the meridian with decimal division: the programme of the metre 120 years early.
• 1672 — Richer at Cayenne: the pendulum runs slow near the equator; the “natural” standard depends on place.
• 1675 — Foundation of the Greenwich observatory, “for the perfecting of the art of navigation.”
• 1701 — Newton’s scale of “degrees of heat,” from winter air to glowing coals.
• 1707 — The Scilly disaster: Shovell’s fleet lost for ignorance of longitude.
• 1714 — Longitude Act: twenty thousand pounds for longitude at sea; Newton consultant to the committee.
• ~1714–1724 — Fahrenheit imposes the mercury thermometer and his scale.
• 1733 — De Moivre meets the bell curve in games of chance.
• 1735–1759 — Harrison builds H1, H2, H3 and at last H4, the clock that carries the hour across the ocean.
• 1742 — Celsius’s centigrade scale (inverted: 0 at boiling; overturned after his death).
• 1761–1764 — The trials of H4 toward Jamaica and Barbados: errors of a few seconds, three times within the prize.
• 1767 — Maskelyne publishes the first Nautical Almanac: the lunar distances become a procedure.
• 1773 — Parliament settles with Harrison: the longitude prize is never formally “won” by anyone.
• 1775 — Turgot charges Condorcet with a reform on the seconds pendulum at 45°: the minister falls, the project falls.
• 1786 — Laplace estimates the population of France from a sample, with a declared margin of error.

The Revolution and the nineteenth century

• 1789 — The cahiers de doléances: “one king, one law, one weight, and one measure.”
• 1790 — Talleyrand proposes the new measure (with an opening to London); Jefferson presents his decimal plan to Congress: the Atlantic fork.
• 1791 — The Assembly chooses the quarter of the meridian: the metre will be its ten-millionth part.
• 1792–1798 — Delambre and Méchain triangulate the arc Dunkirk–Barcelona through Revolution and Terror.
• 1793 — Provisional metre by law; debut (and rapid eclipse) of decimal time.
• 1795 — Law of 18 Germinal Year III: metric nomenclature (metre, gramme, litre) and prefixes.
• 1796 — Maskelyne dismisses Kinnebrook for his transits “running late”: the case that will generate the personal equation.
• 1799 — The mètre and the kilogramme des Archives in platinum delivered to the Archives: “for all times, for all peoples.”
• 1801 — Piazzi discovers Ceres; Gauss finds it again by calculation: the treatment of error wins its first public battle.
• 1805–1809 — Legendre publishes least squares; Gauss claims them “since 1795” and ties the bell to errors of measure.
• 1812 — Napoleon reintroduces the “customary measures”: the metre retreats at home while it advances in Europe.
• 1832 — Gauss measures the Earth’s magnetism in “absolute” units: length, mass, time.
• 1833 — The Greenwich time ball: the first public time signal.
• 1840 — The metric system becomes compulsory again in France; the Great Western adopts London time: “railway time” is born.
• 1841 — Whitworth proposes the unified screw thread: precision enters the factory.
• 1848 — Thomson defines absolute temperature, independent of every thermometer.
• 1851 — The Great Exhibition of London sets the world’s measures side by side.
• 1855–1866 — The epic of the transatlantic cables: the failure of 1858 (Whitehouse against Thomson) and the success of 1866.
• 1861 — The committee of the British Association on the ohm: Maxwell, Jenkin, and the factory of electrical standards.
• 1864 — Sellers’s screw thread: the American continent screws itself onto a standard of its own.
• 1867 — The European geodesists ask for an international metre: science demands the refounding of the standard.
• 1875 — 20 May: seventeen states sign the Metre Convention at Paris; the Bureau of Sèvres is born.
• 1881 — Paris electrical congress: baptism of ohm, volt, ampere, coulomb, farad.
• 1883–1884 — The “day of the two noons” in the United States; the Washington conference chooses Greenwich as prime meridian.
• 1887 — The Physikalisch-Technische Reichsanstalt opens at Charlottenburg: the state becomes a laboratory.
• 1889 — First General Conference: the international prototypes in platinum-iridium sanctioned; distribution by lot; vault of the three keys.
• 1893 — Mendenhall Order: American yard and pound defined on the metre and kilogram; Michelson counts the metre in wavelengths at Sèvres.
• 1896 — Johansson’s gauge blocks: the metrological laboratory in a workshop box.
• 1900–1901 — NPL and NBS are born; at the Reichsanstalt the black-body measurements lead Planck to the quantum.

The twentieth century

• 1905 — Einstein defines simultaneity through the synchronization of clocks: coordinated time becomes physics.
• 1909–1912 — The “international candle”; the Eiffel Tower transmits the time signals; the Bureau International de l’Heure is born.
• 1911 — France adopts Greenwich time (“Paris mean time retarded by 9 minutes and 21 seconds”).
• 1921 — The mandate of the Bureau of Sèvres extends to the electrical and photometric magnitudes.
• 1927 — First International Temperature Scale; Bridgman formulates operationism.
• 1948 — The electrical units return to being absolute; the candle is anchored to the black body.
• 1955 — Essen and Parry light the first caesium clock at the NPL.
• 1959 — The inch becomes exactly 25.4 millimetres throughout the Anglo-Saxon world.
• 1960 — The 11th General Conference christens the “International System” (SI); the metre passes to the wavelength of krypton-86.
• 1967 — The second is redefined on caesium: 9,192,631,770 periods.
• 1975 — Bryan Kibble conceives the balance that will tie mass to the Planck constant.
• 1979 — The candela is redefined by radiometric means: the last flame pensioned off.
• 1980 — Von Klitzing discovers the quantum Hall effect: resistance in packets of nature.
• 1983 — The metre is redefined by fixing the speed of light: 299,792,458 m/s, exact.
• 1988–1992 — Third verification of the kilogram: the copies and the prototype diverge by ~50 micrograms a century, direction unknowable.
• 1990 — Volt and ohm of every laboratory pass to the quantum standards (conventional values): the “silent schism.”
• 1999 — The mutual recognition arrangement among metrological institutes; the Mars Climate Orbiter probe is lost to a confusion of units.
• 2005 — Nobel for the frequency comb: the gear between light and electronics.
• 2010 — The thirty-three-centimetre experiment: two optical clocks measure general relativity over a stool’s height difference.

The SI of the constants

• 2017 — The CODATA special adjustment fixes the definitive values of h, e, k, and the Avogadro number.
• 2018 — 16 November, Versailles: the 26th General Conference unanimously votes the SI founded on seven exact constants.
• 2019 — 20 May: the new SI enters into force; the Grand K, pensioned off, acquires for the first time an uncertainty.
• 2022 — The General Conference begins the path toward the optical second and resolves the abandonment of the leap second by 2035.
• 2025 — European atomic clocks installed on the International Space Station for comparisons from orbit.
• ~2030 — Expected horizon (not yet decided) for the optical redefinition of the second.