What Is the Metric System? Units Built From Nature Instead of Kings

Pre-metric measurement was local, inconsistent and frequently a source of dispute and fraud. Units varied between towns and between trades, the same name meant different quantities in different places, and conversion between them required memorised tables with awkward ratios: twelve inches to a foot, three feet to a yard, sixteen ounces to a pound, fourteen pounds to a stone. That made arithmetic difficult for anyone doing it by hand and made cheating easy, since a merchant could exploit the gap between the measure used for buying and the one used for selling. It also obstructed science, since results could not be compared reliably across borders. Revolutionary France, dismantling the authority of the old regime, treated measurement reform as part of the same project, and the commission that designed the new system included Lagrange, Laplace and Lavoisier, which is an indication of how seriously it was taken.

The design principles were that units should come from nature rather than from an authority, and that they should relate to each other simply:

• •The metre, defined as one ten-millionth of the distance from the north pole to the equator along the meridian through Paris, measured by a survey from Dunkirk to Barcelona that took seven years and was conducted partly during the Terror
• •The gram, defined as the mass of a cubic centimetre of water at a specified temperature, tying mass to length through a substance anyone could obtain
• •The litre, as a cubic decimetre, tying volume to length as well
• •Decimal prefixes from Greek and Latin roots for multiples and fractions, so that converting between units requires only moving a decimal point
• •A decimal calendar and decimal time were also introduced and abandoned within a few years, which is instructive about which reforms stick
• •Physical prototypes, a platinum metre bar and kilogram, were made because reproducing the survey was impractical, which quietly reintroduced the problem of a unit defined by an object

The prototypes were a compromise that lasted far longer than intended and caused real trouble. The metre bar was superseded in 1960 by a definition based on the wavelength of light from a krypton lamp, and again in 1983 by fixing the speed of light exactly, which makes the metre the distance light travels in a specified fraction of a second and means the metre is now defined in terms of the second. The kilogram was the last holdout, remaining a lump of platinum-iridium in a vault near Paris until 2019, and it was known to be drifting relative to its own official copies by tens of micrograms, which meant the world's definition of mass was changing in an unknown direction. The 2019 redefinition fixed the Planck constant exactly and derived the kilogram from it, using an instrument called a Kibble balance that relates mechanical to electrical power. Every base unit is now defined by fixing the numerical value of a constant of nature, so the entire system can in principle be reconstructed anywhere without reference to any artefact.

Adoption was slow and political everywhere, including in France, where it was resisted, briefly abandoned under Napoleon and reimposed in 1840. The system spread through the nineteenth century via the Metre Convention of 1875, which created the international bureau that still maintains it, and is now official in nearly every country. The United States is the significant exception in everyday use, having legally authorised metric units since 1866 and never mandated them, and Liberia and Myanmar are usually listed alongside. Britain occupies a hybrid position, having converted officially while retaining miles on roads, pints in pubs and stones for body weight. The costs of inconsistency are not theoretical: the Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the navigation software expected newton seconds, destroying a spacecraft over a unit conversion. Scientific and medical practice is metric worldwide regardless of what the surrounding culture uses, which is the point the system was built to serve.

The metric system replaced local units that varied by town and trade with units derived from nature and related by powers of ten, designed in revolutionary France by a commission including Lagrange and Laplace. The metre began as a fraction of the distance from pole to equator and the gram as the mass of a cubic centimetre of water, but physical prototypes reintroduced dependence on objects. Since 2019 every base unit is defined by fixing a constant of nature, so nothing needs to be kept in a vault.