Say Au Revoir To That Hunk Of Metal In France That Has Defined The Kilogram

Nov 13, 2018
Originally published on November 14, 2018 10:19 am

The world is about to say au revoir to Le Grand K, a cylinder of platinum and iridium that has long reigned over the world's system of weight measurement.

Le Grand K was forged in 1879 and is held in a locked vault outside Paris — revered and kept under lock and key because its mass, a little over 2 pounds, is the official definition of the kilogram.

But this is will soon change. On Friday, the international General Conference on Weights and Measures will meet in Versailles, France, to vote on whether to redefine the kilogram.

The vote is expected to be unanimous, a mere formality after years of work. Going forward, the world's system of mass measurement will not be based on some special hunk of metal, but rather on unalterable features of the universe — such as the speed of light, time and Planck's constant, a number that helps scientists figure out the energy of a photon of light, given its wavelength. (The approximate numerical value of Planck's constant is 6.626 x 10-34 joule-second.)

"It's fantastic! It's great! It's history in the making," enthuses Zeina Kubarych of the National Institute of Standards and Technology in Maryland, which specializes in the science of measurement.

"We're changing a mass realization system that we've had for 129 years," she says. "It's a huge event."

Kubarych works in a secure, climate-controlled lab at NIST that holds a pair of squat metal cylinders, each under — not one but two — old-fashioned, glass bell jars. The United States got these official duplicates of Le Grand K after signing the Treaty of the Meter in 1875.

"These are our national standard kilograms," explains Kubarych. "That's what we use as the basis of our mass dissemination system in the United States."

These antique artifacts are very important for anyone who needs to do comparisons to make sure their precise measurements aren't out of whack. That includes state measurement labs that test and certify the scales used in stores and pharmaceutical companies, for example, as well the scales used by airplane manufacturers and makers of scientific instruments.

"It pretty much spans most of the sectors of the U.S. industry," says Kubarych.

The nation's official kilograms are so precious that "we don't touch them," says Kubarych.

"We don't even try to touch them with gloved hands."

Instead, the metal artifacts are carefully picked up with a special tool wrapped in filter paper — to avoid leaving any tiny scratches or fingerprints that could throw off the weight.

And that is the big problem with basing a measurement system on the comparison of pieces of metal. If Le Grand K gets heavier or lighter — or absorbs atoms of something from the air — the definition of the kilogram literally changes.

Scientists believe something like this has been happening, because Le Grand K seems to now weigh slightly less than its official copies.

Maybe some teeny fluctuation wouldn't matter to the average person buying, say, coffee by weight at the supermarket. But scientists despair at the idea of trying to understand reality using measurements that are constantly shifting.

Redefining the kilogram means that scientists will "finish something that was begun probably before the French Revolution," notes Stephan Schlamminger of NIST.

Back in those revolutionary times, Enlightenment thinkers hated the world's hodgepodge of measurement systems — each one based on some random object, such as the length of a local duke's foot. Their radical vision was to create something rational and universal, says Schlamminger, noting that the metric system was supposed to be "for all times; for all people."

These originators of the metric system based it on the globe itself, with the meter described as one 10-millionth of the distance from the North Pole to the equator.

But, for a long time, the meter was literally a metal bar in France.

As physics advanced, says Schlamminger, scientists were able to ditch the metal bar and define the meter in terms of the distance that light can travel through a vacuum in about one 300-millionth of a second.

"And this," Schlamminger says, "is the idea of going from an artifact to a fundamental constant."

Now, after researchers spent years creating an elaborate new kind of weighing machine called a Kibble balance, it's finally the kilogram's turn.

In the future, to see whether a hunk of metal really weighs 1 kilogram, it won't have to get flown to France and compared with Le Grand K. It can be evaluated in this type of instrument, using Planck's constant.

Schlamminger and two of his colleagues are so devoted to redefining the kilogram that they recently got tattoos depicting the numbers of Planck's constant.

NIST's Jon Pratt, one of the tattooed researchers, says that precisely measuring Planck's constant was hard, as was persuading the world to switch to a new kilogram based on it.

"There were plenty of committees and fisticuffs and hemming and hawing on this," Pratt says. "So we knew what we were in for. We knew that it was going to be painful. And we all agreed that we'd get inked with the number when we got it done."

If the vote goes as expected, the new definition of the kilogram will go into effect in May.

Meanwhile, Le Grand K will remain in its lonely vault.

"It is an historic artifact that has been under study for 140 years and will retain a bit of metrological interest," says its caretaker, the Bureau International des Poids et Mesures, "even though its mass will no longer define the kilogram."

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

DAVID GREENE, HOST:

So every time you weigh something, whether it's coffee or bananas - I'm not sure what you're weighing - you might be doing it in pounds - well, whatever you're doing, you're unknowingly relying on a global system of measurement that has this long and storied history. That history is about to enter a new chapter. For the first time in more than a century, scientists are about to redefine the kilogram. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: I wanted to get up close and personal with the kilogram, so I went to Maryland to the National Institute of Standards and Technology, our nation's premier agency for the science of measurement. In a building there, there's a massive elevator.

(SOUNDBITE OF BUZZING)

GREENFIELDBOYCE: It takes me down to a basement. I have to put on a hairnet, booties, a white lab coat. Zeina Kubarych unlocks a door to a secure, climate-controlled lab. Only a few keys exist.

ZEINA KUBARYCH: Three people have access to this room.

GREENFIELDBOYCE: Over in a corner, I see small cylinders of a silver-looking metal. It's actually a platinum-iridium alloy - the sexy, futuristic material of the 19th century. That's when these cylinders were forged. Each cylinder is under two old-fashioned glass bell jars.

KUBARYCH: So these are our national standard kilograms.

GREENFIELDBOYCE: They are exact copies of the official kilogram held in France - a hunk of metal that is kept in a vault outside of Paris. Its weight is the definition of a kilogram - a little over two pounds. This piece of metal is simply called the kilogram or, affectionately, Le Grand K. After signing the Treaty of the Meter in 1875, the U.S. got these two copies of it.

KUBARYCH: That's what we use as the basis of our mass dissemination system in the United States.

GREENFIELDBOYCE: Let's say you're in the supermarket and the cashier weighs your bag of apples. You only know the scale is accurate because the state measurement lab checked it using a certified mass. How does the state lab know that its certified masses are the correct weight? Because they sent them here to Maryland to be compared to our nation's kilograms. Who else turns to this lab? Pharmaceutical companies, airplane manufacturers, makers of scientific instruments, anybody who needs to make sure their precise measurements aren't out of whack.

KUBARYCH: It pretty much spans most of the sectors of the U.S. industry.

GREENFIELDBOYCE: Our national kilograms occasionally get flown to France to be compared to Le Grand K. I would say they get the white glove treatment, but...

KUBARYCH: We don't touch them. We don't even try to touch them with gloved hands.

GREENFIELDBOYCE: Kubarych says the metal artifacts are carefully picked up with a special tool wrapped in filter paper. A tiny scratch in the metal or a fingerprint could throw off the weight. And that is the big problem with basing your entire system on comparing pieces of metal. If Le Grand K gets heavier or lighter, if it absorbs atoms of something from the air, then the definition of the kilogram literally changes. And that, theoretically, affects every single measurement of mass in the world that is based on it. Scientists believe this has been happening. Le Grand K seems to now weigh slightly less than its official copies - a difference that's like a tiny fraction of the weight of a snowflake. Still, for folks who are really into this stuff, this is disturbing. Maybe some teeny fluctuation won't matter to you when you buy your coffee, but in the world of science, it's a disaster to try to understand reality using measurements that are constantly shifting. So on November 16, there will be a vote. The General Conference on Weights and Measures will meet in Versailles. This international group is expected to redefine the kilogram. Kubarych is going just to watch.

KUBARYCH: It's fantastic. It's great. It's history in the making. We are changing a mass realization system that we've had for 129 years. This is a huge event.

GREENFIELDBOYCE: The new kilogram will be based on fundamental, unchanging constants of nature - the speed of light, the vibrations of a cesium atom used to tick off time and Planck's constant, which is central to quantum physics. Stephan Schlamminger has been working hard at another lab here to make that happen.

STEPHAN SCHLAMMINGER: I think what we do here now is we finish something that has begun probably before French Revolution.

GREENFIELDBOYCE: Way back in those revolutionary times, Enlightenment thinkers hated the world's hodgepodge of measurement systems, each one based on some random object like the local duke's foot. Their radical vision was to create something rational and universal or, as they put it...

SCHLAMMINGER: For all times for all people.

GREENFIELDBOYCE: The result was the metric system. Its measurement of length, the meter, was already redefined decades ago.

SCHLAMMINGER: So the meter, at some point, was also an artifact.

GREENFIELDBOYCE: The meter was literally a metal bar in France. Its length was supposed to be one ten-millionth of the distance from the North Pole to the equator. Schlamminger says as physics advanced, scientists were able to ditch the metal bar and define the meter in terms of unalterable features of the universe.

SCHLAMMINGER: You just defined a meter as the distance that light travels in one over 300-millionth of a second. And this is the idea of going from artifact to a fundamental constant.

GREENFIELDBOYCE: Now, it's finally the kilogram's turn. Researchers spent years creating an elaborate kind of new scale.

SCHLAMMINGER: Let's walk inside here.

GREENFIELDBOYCE: Schlamminger shows me a device the size of a small car with all kinds of shiny metal parts. In the future, to see if a hunk of metal really weighs one kilogram, it can just go in here. Schlamminger and two of his colleagues are so devoted to redefining the kilogram that they recently had the numbers of Planck's constant tattooed on their bodies. Jon Pratt shows me his tattooed arm. He says convincing the world to switch to a new kilogram was really hard.

JON PRATT: There were plenty of committees and fisticuffs and hemming and hawing on this, so we knew what we were in for. We knew that it was going to be painful, and we all agreed we'd get inked with the number when we got it done.

GREENFIELDBOYCE: He expects the vote on Friday to be unanimous, and humanity will finally be free from the unbearable uncertainty of Le Grand K.

Nell Greenfieldboyce, NPR News.

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