The International System of Units
Where the world
agrees on measure.
A metre is the same in Katowice, Kyoto and Quito. That quiet agreement is looked after in a small park near Paris, at the International Bureau of Weights and Measures. This site explains how it works, in plain language.
Exact by definition
c = 299 792 458 m/s
The speed of light in vacuum is fixed exactly. The metre follows from it.
Source: BIPM, SI Brochure, 9th edition
Seven units, seven constants
Since 20 May 2019, every SI unit rests on a constant of nature.
Start with a question
Six good places to start, each with a short answer on this site.
Science profile
Sèvres in science
Three fields put Sèvres on the map of science. Each has its own institution here, and each is explained on this site.
Metrology
Where the units are kept
The International Bureau of Weights and Measures works here, with four scientific departments: physical metrology, chemistry, ionizing radiation and time.
Metrology in SèvresTime and frequency
Where world time is computed
Coordinated Universal Time (UTC) is calculated at the BIPM from the readings of atomic clocks around the world.
Time in SèvresCeramics and materials
Where porcelain is made
The national porcelain manufactory has worked in Sèvres since 1756, with a ceramics museum of nearly 50 000 works.
Ceramics in Sèvres
Sèvres, the place
Porcelain and precision
Sèvres is known for two things: its porcelain manufactory, and the Pavillon de Breteuil, which France granted to the International Bureau of Weights and Measures in 1875, the year the Metre Convention was signed.
For over a century the kilogram itself was a metal cylinder kept here. In 2019 it was retired as the reference, and the kilogram became a number.
Articles
Short answers to practical questions about units and measurement, each based on primary sources.
What is UTC? How world time is made
World time is computed in Sèvres from hundreds of atomic clocks, and every country’s clocks are compared against it.
Why the kilogram is no longer a metal cylinder
For 130 years the kilogram was one cylinder in a vault in Sèvres. Since 2019 it is fixed by the Planck constant, and nothing on your scales changed.
20 °C or 20°C? How to write degrees Celsius
Put a space between the number and the symbol: 20 °C. The only exception is the degree of angle.