The history of the kilogram dates back to 1799, when Enlightenment thinkers set about re-envisioning a whole host of the micro-institutions that govern everyday life. In the wake of the French Revolution, they reset the calendar to Year 0 and generated a set of interrelated measurement standards that we now know as the metric system. This work continued the efforts of the Consultative Commisioner for Units under Louis XVI, and lives on to this day:
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.
This lonely cylinder is the International Prototype of the Kilogram, known colloquially as Le Grand K, and it is the last remaining physical object to define a unit of measure. It’s a quaint throwback to a time when people compared the ocean’s depth to the span of a man’s outstretched arms and the second to a tiny fraction of a year. Now we fix our rulers to the speed of light and our clocks to a spectral property of cesium. By thus linking measurement to fundamental and unchanging phenomena, scientists have paved the way for GPS satellites, gravity-wave detectors, and many other precision technologies that simply wouldn’t have been possible before.
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.
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