Le Grand K: The Perfect Kilogram

The world’s most perfect weight isn’t so perfect anymore. And that'd got scientists scared.

(Image credit: Greg L)

 Hidden in a vault outside Paris, vacuum-sealed under three bell jars, sits a palm-sized metal cylinder known as the International Prototype Kilogram, or “Le Grand K.” Forged in 1879 from an alloy of platinum and iridium, it was hailed as the “perfect” kilogram—the gold standard by which other kilograms would be judged.

Although it’s arguably the world’s most famous weight, Le Grand K doesn’t get out much. Since hydrocarbons on fingertips or moisture in the air could contaminate its pristine surface, it goes untouched for decades, under triple lock and key at the International Bureau of Weights and Measures. Every 40 years, however, it makes an appearance.

The weight is ushered from its chamber, washed with alcohol, polished, and weighed against 80 official replicas hand-delivered from laboratories around the world. Today, whenever scientists need to verify something is precisely one kilogram, they turn to one of these replicas, over which Le Grand K reigns supreme.

This system sounds absurd, but not too long ago, lots of units relied on similar methods. The kilogram was just one of seven standards of measurement established by the French Academy of Sciences in 1791, all based on physical prototypes. These benchmarks caught on worldwide because standardization was sorely needed. At the time, some 250,000 different units of weights and measures existed in France alone, which meant that the only constant was complete chaos.

While basing measurements on tangible benchmarks was an improvement, using physical standards wasn’t without its flaws. For one, they have a nasty habit of changing. In Le Grand K’s case, it’s been losing weight. At its most recent weigh-in in 1988, it was found to be 0.05 milligrams—about the weight of a grain of sand—lighter than its underling replicas. Experts aren’t sure where this weight went, but some theorize that the replicas have been handled more often, which could subtly add weight. Others postulate Le Grand K’s alloy is “outgassing,” which means air is gradually escaping the metal.

Whatever the reason for Le Grand K’s gradual wasting away, it’s got scientists scrambling for a more reliable standard. Some argue that this is long overdue, since all other units of measurement are already defined by fundamental constants of nature that can be reproduced anywhere anytime (provided you’ve got some sophisticated lab equipment). The meter, for example, used to be defined by a metal rod stored alongside Le Grand K. But in 1983, it was redefined as the distance light travels in a vacuum during 1/299,792,458 of a second.

Standardizing the kilogram has been trickier, though. Australian scientists are polishing a one-kilogram sphere of silicon, hoping that they’ll be able to count the number of atoms it contains to create a more accurate standard. American physicists at the National Institute of Standards and Technology (NIST) are attempting to redefine a kilogram in terms of the amount of voltage required to levitate a weight. But so far, neither approach can match Le Grand K’s accuracy.

Why should we care whether a kilogram in a vault is “perfect” or not? Because it’s bad news when your standard is no longer standardized. While no one’s worried whether a single kilogram of apples is a hair lighter or heavier at the produce stand, a small discrepancy can become a gargantuan one if you’re dealing with, say, a whole tanker of wheat. The kilogram is also used as a building block in other measurements. The joule, for instance, is the amount of energy required to move a one-kilogram weight one meter. The candela, a measure of the brightness of light, is measured in joules per second.

These links mean that if the kilogram is flawed, so are the joule and candela, which could eventually cause problems in an array of industries, particularly in technology. As microchips process more information at higher speeds, even tiny deviations will lead to catastrophes. Le Grand K’s unreliability “will start to be noticeable in the next decade or two in the electronics industry,” warns NIST physicist Richard Steiner. If your next smartphone is buggy, you’ll know which hunk of metal to blame.

So scientists continue to chase the perfect kilogram. “Maybe we have all been looking for too high-tech an answer,” says Stuart Davidson of England’s National Physical Laboratory. “There could be something really obvious out there we’ve missed.” The NPL’s website encourages others to give it a shot: Any better ideas on a postcard please. Until then, Le Grand K will remain king—short of true perfection, but as perfect as it gets.

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coverThe above article by Judy Dutton is reprinted with permission from the July-August 2012 issue of mental_floss magazine.

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Yes, by defining Planck's constant as a fixed value, the kilogram can be defined in terms of the meter, second and Placnk's constant. There is a Wikipedia article that covers some of this, although doesn't seem to discuss much about exactly how such definitions would be used to calibrate mass measurements. Once upon a time, I had seen a magazine article discussing that aspect in much more detail, which included the discussing the use of electrical units which are easy to calibrate to determine a calibration for via something like the Watt balance.
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Not so much dust, as they use it under pretty clean conditions and store it under a triple set of concentric bell jars. But it does adsorb gases from the air onto the surface, and there is a known rate at which this happens after cleaning. It means that the cleaning has to be done a consistently a certain number of days before it is used. Ask anyone who works with high ultra high vacuum stuff about how much junk from air, especially water, will stick to a metal surface (I think you can get 0.1-1 ug / cm^2 on a metal surface easily, and the kilogram prototype has a surface area of like 75 cm^2).
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They don't assume which particular one is losing or gaining mass in the absolute sense, and acknowledge it is possible all of the prototypes are drifting in one direction. They can measure some drift directly by calibrating a scale, and the Wikipedia article goes into a little detail about the drift.
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