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If you live in the US, you’re probably used to talking about weight in pounds and ounces (and having to convert things when Australians like us start talking in kilograms and grams), but here’s the thing – all your measurements are actually based on one very metric standard: the international kilogram.
Veritasium host Derek Muller went to the US National Institute of Standards and Technology (NIST) to take a look at K20 – one of 40 identical kilogram weights made in France in the 1880s, which is still used to define what a kilogram means today.
America’s relationship with the metric system actually stretches back to the 19th century, when France presented the US with K20 after it signed the Treaty of the Metre in 1875. In it, 17 countries agreed to stick to the same metric standards for measurements.
The international prototype kilograms were made and distributed to help define the standard weight across the world. As the Veritasium video below shows, the K20 kilogram (one of two the US owns) is kept in a vacuum-sealed jar behind some tight security:
That’s because if a single atom gets added to or removed from the objects, the definition of a kilogram instantly changes, so it’s very rare to see K20 and its partner K4 removed from their bell jars.
Anyone who wants a definitive answer to the question of how much a kilogram weighs has to visit NIST and order a stainless steel cylinder calibrated against the precise weight of K20 or K4.
“Virtually every mass that has been accurately measured in the US over the past 130 years can trace its measurement back to this one kilogram hunk of 90 percent platinum and 10 percent iridium,” explains host Derek Muller.
Those elements were chosen for their high density and resistance to oxidation, by the way, as these kilogram samples were built to last. They’re now regarded as priceless, though modern copies can be yours for a mere US$100,000.
But K20 and K4 might not be in use for much longer.
As Derek explains, the kilogram is the last metric unit still defined by a physical artefact rather than a fundamental constant – that is, a fixed property found in nature – and that’s causing a problem for scientists.
Every few decades, K4, K20, and all the other kilograms made in the 1800s regularly take a trip back to Paris so their weights can be compared against the original International Prototype of the Kilogram.
It turns out that the weights of these objects have been slightly shifting, perhaps due to atmospheric contamination, though scientists aren’t really sure.
Now efforts are underway by NIST and others to settle on a fixed, physics-based definition of what a kilogram actually is. From 2018, the kilogram is going to be defined by the Planck constant, producing a fixed value from measurements taken worldwide.
So enjoy this view of K20 while you can… and the little-known fact that the non-metric standards used in the US are actually based on very metric measurements, like the kilogram shown here.
“All the units we commonly use, like feet, and gallons, and so on, are actually defined in terms of metric units,” says the NIST’s Patrick Abbott.
“It’s just a little translation that we do here – but our country is actually on the metric system.”