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The world depends on one chunk of metal in Paris for measuring the mass of everything on Earth. Every scale and every measurement of mass goes back to it. It's the original kilogram.
In 1875, 17 nations got together and decided they needed a standard definition of weight and length so they could all follow the metric system reliably. They made a platinum iridium cylinder that was to be the standard — the definition of the kilogram — for the planet. They kept the original in France, and then made official copies of it, and one of them is located at the National Institute of Standards and Technology in Gaithersburg, Md.
The piece of metal sits behind five locked doors, several panes of glass, a safe door, and two bell jars. The temperature is controlled to 0.01 degrees Celsius. Its appearance is metallic, silver looking, perfectly polished, and small — just a little bigger than a golf ball.
“It is a cylinder of platinum iridium… 90 percent platinum, 10 percent iridium, and made in 1889, says physicist Patrick Abbott.
This piece of metal is the official kilogram for the U.S. Every prescription produced, every pound of fruit measured, every ounce of self-loathing people feel when they look at the scale — it’s all calibrated to this piece of platinum iridium metal.
But there’s one small problem: the definition of the kilogram is changing.
Changing the standard
The original kilogram in France and the copies don’t match any more. Scientists believe the original is losing mass.
Now, how could a solid block of platinum iridium alloy locked behind closed doors and handled only once every few decades possibly lose mass?
“It could be losing mass as a result of things coming out of it,” says Abbott. “For instance, when it was made, it’s possible that gaseous impurities got in. Even when solidified, those gaseous impurities can diffuse out. So some people have speculated maybe its gas that’s been trapped since its been formed, and it’s diffusing out at a constant rate.” But, he says, no one really knows for sure.
It’s not changing by a whole lot though. “About 50 micrograms over the last hundred years,” says Abbott, which equates to about three pieces of dust.
However, just because it’s a tiny amount, doesn’t mean it’s insignificant.
“It’s important to me because of the precision and accuracy required for my laboratory,” says Mark Ruefanacht, who calibrates scales for a living. He’s with Heusser-Neweigh and sometimes teaches at the National Institute of Standards and Technology.
“We are always adjusting for the latest information, and in essence there's some frustration because we always feel there’s a moving target,” says Ruefanacht. “We’re already seeing the need for measuring these smaller things in the biotechnology and pharmaceutical world. The stem cell research community for example has interest in these very small weights that are accurate. Our pharmaceuticals are interested in it.”
Pharmaceuticals already measure things out in pretty small quantities. “If you think about a pill there’s only a few milligrams, or we’d say a few grains of salt of an active ingredient in an actual pill,” explains Ruefanacht. “The rest is just starch or filler to help us swallow down the ingredient.”
So what scientists and governments around the world are trying to do is get rid of the kilogram — not the idea of the kilogram, but the piece of metal. They’re trying to tie it to something cosmically stable. One idea is linking it to something called the Planck constant.
“The Planck constant is a constant that arises out of quantum mechanics,” says Ruefanacht. “It relates energy to things we can see and measure. For instance if you want to know the energy of a photon of light, it’s equal to the Planck constant multiplied by the frequency of that light.”
The point is, it’s not going to change over time. And they’ve done this for a few other units. For example, a second is defined by the time taken for a certain number of vibrations of a cesium atom. Or a meter is related to how far light travels in a certain period of time.
But for the kilogram, they can’t quite measure the physical constant they need for it to be worthy of being an international standard. It requires experiments of incredible complexity and precision to measure, and technology that isn’t there yet.
More and more countries are working to improve this over the next few years though, and until then, those platinum iridium cylinders are the standard even as they ever so slowly and mysteriously drift apart from one another.
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