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The Kilogram Is Putting on Weight

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There's long been debate over the accuracy of the standardized kilogram. Now, though, scientists have shown once and for all that the lump of metal defining the unit of mass has been putting on some weight.


The original kilogram—the International Prototype Kilogram, or IPK, made of platinum and iridium alloy—is the standard against which all other measurements of mass are set. So, it's quite important. Forty replicas exist across the world, and the UK is in possession of replica 18. Researchers from Newcastle University, UK, thought they'd take a look at—and they found something amiss.

Using a state-of-the-art x-ray photoelectron spectroscopy machine— the only one of its kind in the world—they were able to analyze the block with unprecedented accuracy. Turns out age hasn't been kind. The x-ray measurements show that there's a build-up of hydrocarbons on the surface—weighing up to 100 micrograms.


While that would be fine if every block was aging in the same way—as we'd all still be working to the same standard, it'd just be subtly different than it used to be—the scientists don't think that's the case. Peter Cumpson, Professor of MicroElectroMechanical Systems at Newcastle University, explains:

"Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original... We're only talking about a very small change... but mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge."

So what's to do? Well, the researchers have also shown that by exposing similar blocks to a mixture of UV and ozone it's possible to remove the surface contaminants and—potentially—restore them to their original weights. So far nobody's attempted that task with the kilogram standards, though, so it remains to be seen if it will actually work in practice. Who's gonna try first? [Science Daily]

Image from Wikimedia Commons under Creative Commons license