In France, this sentence symbolised the continuous reign of the monarchy as the crown was passed directly from the deceased king to his successor. However, this is not the case for Louis XVI. His reign ended with the short, swift fall of the guillotine, taking with him the French monarchy as the French Revolution took hold at the end of the 18th century.

What does that have to do with the kilogram? Well, another effect of the French Revolution was to initiate the beginning of our present International System of Units, abbreviated worldwide as SI. In the spirit of the revolution, ‘freedom, equality, fraternity’, even measurement was something that should become equal and accessible for all, independent of royal requirements. This was a significant change – before, units such as mass or length were defined differently from kingdom to kingdom. Often the arm length of the king or duke was quite literally what defined unit of length.

During the next century the idea of the uniform metric system of units spread worldwide. In 1875, 17 states joined together to form the so-called Metre Convention, with the aim of ensuring a globally uniform system of measurement. Delegates of the member states still meet regularly at the so-called General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) to discuss the further development of the system of units and to decide on innovations.

At the first CGPM, in 1889, a new prototype for the kilogram was created – the International Prototype Kilogram or Le grand k. Also a cylinder, the revised prototype was remade using an alloy of platinum and iridium, which is more stable than pure platinum. This inconspicuous object, safely stored in a safe in a small village near Paris, was the reference for any mass measurement around the world for 130 years, from 1889 to 2019. Platinum is a very heavy element, and so the original kilogram measures only 39 mm in diameter and 39 mm in height – so actually very small and rather modest!

The kilogram is dead! Long live the kilogram!

Yet, the reign of Le grand k is over. From 20 May 2019 this last artefact is no longer the reference for international mass measurement. It will be replaced by a letter – h, which physicists refer to as the Planck constant.

A copy of the “provisional” metre installed 1796-1797 in Paris.