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How to Replace the Kilogram, the Last Physical Unit of Measurement

Of course, the whole anthropocentric measurement thing is rather imprecise (Can you imagine designing a microprocessor with cubits?) so, over time, we've come to define units of measurement with various mathematical constants. A meter, for example is...

by Derek Mead
May 3 2012, 3:35pm

We all know about the origins of feet and inches, but did you know that a picul is the weight that one person can carry, or that a masu is a Japanese measurement for how much rice one person needs for one day? There are an incredible number of anthropocentric (human-based) units of measurement — Wikipedia, being Wikipedia, has an excellent list — and, really, they totally make sense. Before people started mass producing tape measures and measuring cups and all that stuff, the best way to describe the dimensions of a room was to say it was four alds) across.

Of course, the whole anthropocentric measurement thing is rather imprecise (Can you imagine designing a microprocessor with cubits?) so, over time, we’ve come to define units of measurement with various mathematical constants. A meter, for example is defined as the distance light travels in a vacuum in 1 ⁄ 299,792,458 of a second. However, even in our super-precise modern world, there’s been one last physical holdout: the kilogram, which is defined as being equal to a single hunk of platinum-iridium alloy that’s hidden away in France.

Think of the ramifications of this: the entire weight component of the metric system is based on one hunk of metal. Weighing things is pretty freaking crucial to the world economy, so if it was lost, we’d be at the mercy of the scales we have now. And sure, they’re all totally accurate and whatnot, but we’d have lost the only reference to what a kilogram would be.

Rachel Courtland, in an excellent feature on the fate of the kilogram on the IEEE’s Spectrum blog, describes the security around the world’s most prized hunk of metal:

Once a year, three officials bearing three separate keys meet at the bottom of a stairwell at the International Bureau of Weights and Measures, in Sèvres, France. There they unlock a vault to check that a plum-size cylinder of platinum iridium alloy is exactly where it should be. Then they close the vault and leave the cylinder to sit alone, under three concentric bell jars, as it has for most of the past 125 years.

With that kind of prodigious security, France’s Le Grand K isn’t likely to be going anywhere. But there’s another, more real issue: What if the reference kilogram is changing? How can we be sure that the kilogram of today is the same as it was decades ago? Courtland describes the issue thusly:

The trouble posed by the master kilogram is apparent in the many friction-filled steps by which it calibrates other masses. Once every few decades, a scientist plucks the cylinder from its perch with chamois-leather-padded pincers, rubs its surface with a cloth soaked in alcohol and ether, and steam-cleans it. Then he puts the prototype in a precise balance that compares it to the bureau's official copies, which are in turn compared to copies kept by member countries. And thus the prototype's mass trickles down to set the standard for the rest of the world.

The system has been far from seamless. When the cylinder was last removed from the vault in 1988, the bureau's metrologists were disappointed to discover that its mass and those of its official copies had drifted apart by as much as 70 micrograms since 1889. That discrepancy is tiny—comparable to the mass of a small grain of sugar—but it confirmed a troubling instability. All that metrologists can say is that the master kilogram seems to have lost as much as 50 µg over the course of a century relative to its siblings. But the actual drift could be up or down, and it might even be a lot more than 50 µg, because the prototype and its metallurgically identical copies could all be changing as an ensemble.

Sure, the fluctuations are small, but when you multiply that by how many kilograms of stuff are weighed every year — a number closer to infinity than I’m willing to count — those changes can add up to massive amounts. More importantly, because no scale is perfect, is the issue of confidence; as we increasingly demand more precision in everything, if people can’t trust the one of the most basic units of measurement, there’s liable to be weight-panic similar to old-time runs on banks.

And there we get to the meat of Courtland’s excellent piece: We need an alternative. Planck’s constant seems to be the leading contender for a mathematical definition of a kilogram, but there are issues in how we define that constant to begin with. It’s a surprisingly dramatic story considering it’s all about measurement, but, really, we’re talking about one of the cornerstones of our economy. Go read the feature, and then answer me one question: after Le Grand K is rendered obsolete by math, what rapper is going to end up with it around his neck?

Follow Derek Mead on Twitter: @drderekmead.

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