The 20 May 2020 constitutes the 145th anniversary of the Meter Convention (French: Convention du Mètre). In 1875, metrologists celebrated this day when seventeen States signed an agreement on the world-wide uniformity of measurement. With this agreement, the States created the first international, intergovernmental scientific organisations: The International Bureau of Weights and Measures (BIPM), the General Conference on Weights and Measures (CGPM) and the International Committee on Weights and Measures (CIPM), which jointly coordinate metrology and the development of the metric system at an international level. The signing countries decided to produce measurement standards (“the original meter” and “the original kilogram”) for the units of measurement “meter” and “kilogram”.
The Meter Convention was initially drafted to provide standards of length and mass only. In the 1860s, James Clerk Maxwell and Lord Kelvin suggested a system with three base units – length, mass, and time. At the end of the 19th century, it became clear that the three fundamental units of length, weight and time could not explain electrical measurements. The Italian physicist Giovanni Giorgi, therefore, proposed to extend that the measurands meter, kilo and second by including ampere. The Giorgi System was the precursor of today’s International System of Units (SI) with seven basic units: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount in chemistry), and candela (brightness). These base units can partially be combined to describe different quantities, such as volume, energy, pressure, and speed, which are composed of more than one unit.
Diagram of the basic units of the International System (SI)
The World Metrology Day 2019 achieved a further milestone in the development of the metric system. On 20 May 2019, the redefinition of the International System of Units (SI) came into force. From that date onwards, all the units depend on the laws of nature, instead of depending on physical artefacts. The “Big Kilo” hosted at the BIPM in Paris, is no longer the reference for the “kilogram”. Today, the “mass” unit is based on the Planck constant; the “meter” is based on the speed of light. In the redefinition, four of the seven SI base units – the kilogram, the ampere, the kelvin, and the mole – were redefined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.
Two months ago, I had the opportunity to visit the National Metrology Institute of South Africa (NMISA). For the first time, I could see a Kibble-Watt balance there, one of the instruments that can measure the kilogram with ultra-precision. Similar to a mechanical scale, the device indirectly compares mechanical power with an equivalent amount of electrical power. More precisely, it measures the downward attraction of a mass due to a gravitational force by opposing it with an upward electromagnetic force. This design of the Kibble Balance provides the most accurate mass measurement possible.
In his message, the CIPM President, Dr Wynand Louw, emphasises the unexpected benefits of the 2019 revision of the System of Units. Today, National Metrology Institutes around the world can implement the basic units of measurement without the involvement of physical artefacts. In times of the Corona pandemic, it proves to be a particular advantage that the National Metrology Institutes no longer have to travel to the BIPM in France to recalibrate their national standards. It is impressive to see that an emerging economy like South Africa is technologically capable of measuring at the same level of precision as the world’s leading NMIs.
De Courtenay, N., et al. (2019). The Reform of the International System of Units (SI): Philosophical, Historical and Sociological Issues, Routledge.
Quinn, T. (2018). The Metre Convention and the creation of the BIPM. In: P. Tavella, M. J. T. Milton and M. Inguscio, Metrology: from Physics Fundamentals to Quality of Life. Amsterdam. 196: 203.
Feature image with permission and copyright of BIMP/ OIML.