No drinker likes to be short-changed, as the Magna Carta recognised more than 800 years ago: “Let there be one measure for wine through our whole realm, and one measure for ale, and one measure for corn, to wit ‘the London quarter’; and one width of cloth, whether dyed, russet or halberget?.?.?.?”
Friday will mark another weighty moment for metrology, the science of measurement. Subject to an international vote, the definition of the kilogramme will change. The standard unit of mass will no longer be the International Prototype Kilogram, a 19th-century ingot of platinum-iridium encased in three bell jars in an underground vault near Paris. Instead, the kilo will be calculated using Planck’s constant, an unbelievably tiny number that plays an outsized role in quantum physics.
It is a historic moment: the kilo is the last remaining major unit to be based on a physical object. Friday’s vote at the International Bureau of Weights and Measures in France will see the measurement, which seems so solid, benchmarked against a fundamental but intangible constant of nature.