The old kilogram was a lot more fun

My coffee shop stopped taking cash the other week. It came as a shock, providing as it once did an opportuni...

Posted: Nov 30, 2018 2:33 PM
Updated: Nov 30, 2018 2:33 PM

My coffee shop stopped taking cash the other week. It came as a shock, providing as it once did an opportunity to offload pockets of change or the satisfaction of throwing a dollar bill into the tip jar. The green stuff was tangible and knowable. You could hold it, feel it and smell it (although my advice is that it's best not to). Not like the new computer system, with its plastic, its bytes and its invisible cloud.

That's roughly what ran through my head when I heard that the kilogram -- the unit of mass in the International System of Units (SI) -- had this month been redefined into mathematical abstraction.


Until now, if you wanted to touch, hold or drop the official kilogram, which is equivalent to 2.2 pounds, on your toe then it was a simple matter of visiting a tightly guarded vault inside the Louis XIV Pavillon de Breteuil, outside Paris, where "Le Grand K" was kept in elegant repose, beneath a triple layer of glass covers to protect it from picking up even a molecule of contaminants.

But, tragically, the metal cylinder was not immune to the inevitabilities of its old age. Time had stripped it of about 50 micrograms of its original mass and, in doing so, stripped it of its identity. Le Grand K is no longer a kilogram.

Before the kilogram came into being, the unit of mass was the "grave," equivalent to the mass of a liter of water at freezing point. But after French revolutionaries toppled the aristocracy in 1789, the new French government decided to revolutionize weights and measures, replacing the grave with the much-loved gram. Soon after, a mass artifact, a thousand times the mass of a gram and a much more practical size for a base measurement, was created.

Either way, an ingot of platinum-iridium or a block of ice, the unit was real, touchable and knowable. It could be placed on a scale and physically balanced against any object.

This is how measurements once worked. Units of weight were determined by the weight of grains. The pied du roi or "king's foot" was used for 1,000 years as a unit of length in France, coming in at about 32.5 cm, a fraction longer than the foot being used in England at the time and that we have come to know today. Just right for Gallic pride.

It is difficult not to miss that sort of folk history when considering the new kilogram. Out goes Le Grand K and in comes the Kibble balance, a complicated bit of kit that measures the electromagnetic force required to offset the weight of a specific mass. It's a heck of a lot more difficult to explain than a lump of metal in France, but suffice to say that it balances mechanical force on the one hand with electromagnetic force on the other.

The quantity that the Kibble balance spits out is then expressed as a function of one of the universe's fundamental constants, the Planck Constant.

It is beautiful, universal and, yes, constant. It won't change with temperature or microscopic deposits. And it is democratic. There need be no weights and measures authority with white gloves and disapproving looks, no keeper of the metal ingot. Anyone (with sufficient expertise) can build a Kibble balance. All pluses.

But simple, it is not. The kilogram follows the other SI units that have gone from (relatively) straightforward to mind-numbingly complex. The meter, once a ten-millionth of the distance from Equator to North Pole, is now the length travelled by light in 1/299,792,458th of a second inside a vacuum.

Where's the poetry in that? Where's the history that connects mass with the Romans, the French Revolution and the very human requirement to occasionally scrub Le Grand K clean of deposits, as if the kilogram were a baby to be protected from the world? What does the Planck constant even feel like?

So, let's stick with the old ways or switch altogether from the metric system to America's Imperial system. If feet and inches were good enough to put a man on the moon, they're good enough for me. Just don't try to tell me that the American pound is measured as 0.453592 of one of your newfangled kilogram. It will always be about the weight of a pint of a milk.

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