4. Towards a determination to ...

Today, the SI units of measurement are extremely precisely defined. The metre, for example, is defined in terms of the speed of light in a vacuum; the second is linked to the frequency of radiation emitted by the caesium 133 atom during the transition between two energy levels. On the other hand, the basic unit of mass, the kilogram, was defined in 1889 by the first General Conference on Weights and Measures, as the mass of a standard cylinder made of an alloy of platinum (90%) and iridium (10%), preserved under vacuum and deposited at the Bureau International des Poids et Mesures for over a century. There are 80 copies of this cylinder around the world, enabling different states to have their own version of the reference standard. However, over the last few decades, significant variations in mass, of the order of 50 micrograms, have been observed between these different cylinders, suggesting...