From the Metre of 1799 to the SI of 2019

A decimal system for everyone

Before the 1790s, France alone had hundreds of local units, and they varied from town to town. The revolutionary government asked the Academy of Sciences for a system based on nature and divided by tens.

• The metre was defined as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris.
• The kilogram was defined as the mass of one cubic decimetre of water at its maximum density.
• In 1799, platinum standards of both were deposited in the French National Archives.

The metric system became compulsory in France in 1840. On 20 May 1875, seventeen states signed the Metre Convention and created the International Bureau of Weights and Measures (BIPM) near Paris. That date is now marked each year as World Metrology Day.

The imperial system lives on, but it is now defined in metric terms. Since 1959, the inch has been exactly 25.4 mm and the pound exactly 0.45359237 kg.

Seven base units

In 1889, new platinum–iridium prototypes of the metre and the kilogram became the international standards. Over the following decades, the system grew. In 1960 it was named the International System of Units (SI), and in 1971 the mole completed the set of seven base units: the metre, kilogram, second, ampere, kelvin, mole and candela.

2019: units from constants

For 130 years, the kilogram was a cylinder of platinum–iridium kept at the BIPM. Comparisons showed that it and its copies were drifting apart by tens of micrograms.

The 26th General Conference on Weights and Measures voted in November 2018 to redefine the SI. From 20 May 2019, every unit is defined by fixing the value of seven constants:

• the caesium hyperfine frequency
• the speed of light
• the Planck constant
• the elementary charge
• the Boltzmann constant
• the Avogadro constant
• the luminous efficacy of 555 nm radiation

The kilogram is now realised from the Planck constant, using a Kibble balance or by counting atoms in a silicon sphere.

What it meant for calibration

For everyday calibration, nothing changed overnight. The values of weights, thermometers and gauges stayed the same. What changed is the top of the traceability chain. It now ends at a constant of nature, which any suitably equipped laboratory in the world can realise, rather than at one artefact in a vault.