Tom Whipple in More Intelligent Life:
IN AN UNDERGROUND vault beneath a hill overlooking Paris, behind a steel door whose lock requires three keys—only two of which are in France—and protected by three glass covers, lives the kilogram. Not a kilogram but The kilogram. For the past 125 years, this sleek cylinder of platinum-iridium has defined mass for the world. The vault is buried under the Bureau International des Poids et Mesures (BIPM), a whitewashed stately home a discreet distance from Paris, with extensive gardens for physicists to roam in and views across a grand meander of the Seine. The BIPM exists to control, define and distribute the Standard International (SI) units by which science—and indeed non-science—makes its measurements: the second, the metre, the mole, the candela, the ampere, the kelvin and the kilogram. You might call it the spiritual centre of the metric system—if the rational world of revolutionary France’s metrification programme allowed room for anything as irrational as the spiritual.
…THE KILOGRAM AS we know it was created at the first General Conference on Weights and Measures, held in France in 1889 and attended by 20 of the world’s more science-minded nations. As the Enlightenment progressed, spreading knowledge and developing the modern empirical methods, it became clear that, for science to work, its practitioners had to agree on units. If an experiment in Rio de Janeiro used a gram of catalyst heated to 75 degrees, someone repeating that experiment in Tokyo needed to know that a gram and a degree meant the same on both sides of the Pacific. Following almost a century of discussion, the conference defined the key units of measurement, and the kilogram was forged and incarcerated. In the years since 1889, around 100 daughter kilograms have been made, some in platinum-iridium, others in stainless steel. Most have been distributed around the world to provide national mass standards. Six are kept here in Paris, to be used as a check on the main kilogram.