The Friday Fillip: An Indifferent Foot

Remember school physics and the whole business of learning the difference between mass and weight? Weight is what you gain as you get older and what the Beatles were singing about; mass is, well, a much heavier concept by far, given that it remains constant no matter where you are — here, on the moon, or in deep space.

And constancy is the thing. At least it is if you’re to swap measurements with others or carry measurements over time. Think only of the Chancellor’s foot, as John Selden did four hundred years ago:

. . . what an uncertain measure would this be? One Chancellor has a long foot, another a short foot, a third an indifferent foot . . .

So, fed up with arpents, minots, perches, lieues and, yes, pieds, not to mention three-footed Chancellors, the revolutionary French brought reason to bear and went metric. With respect to mass, the unit was the gram — and the source of constancy was water, thought to be (on Earth) everywhere and always the same: a gram was the mass of one cubic centimetre of water. You’ve spotted the flaw? So had the French: what’s a centimetre, then? Things were stopped from going circular or regressing into the channel by the decision to define a metre as “la mesure de longueur égale à la dix-millionième partie de l’arc du méridien terrestre compris entre le pôle boréal et l’équateur” (one ten-millionth of the distance between the North Pole and the Equator). Easy peasy. [All of this is spelled out in Le décret relatif aux poids et aux mesures, 18 germinal an 3 (7 avril 1795).]

Surveying expeditions thither and yon notwithstanding, this basis proved problematic. And as a means of measuring mass in a practical way, it was difficult to use. So as early as 1799 a prototype object was created the mass of which was deemed to be one kilogram. This object and the definition behind it eventually gave birth to a more . . . exquisite object, which still to this day is what a kilogram is for all the world’s purposes. The object — the International Prototype Kilogram (or, as it’s familiarly known, Le Grand K) — is held in France, with “exact” copies held at various crucial spots around the world, one of which is Canada’s National Research Council. The copies are checked by comparing them with the original from time to time, to ensure . . . the constancy we started with.

Only it’s not working perfectly enough for current scientific purposes. Occasional checks against all the copies have shown that Le Grand K varies in mass ever so slightly over time, perhaps accumulating molecules from the air, perhaps for other reasons.

So there’s a move afoot to dump the kilogram — or, rather, to find a purer basis for the measurement of mass and then to create a practical kilogram in these new, more fundamental terms. Not that any of this will affect the amount of bacon you bring home from the butchers, but there’s something appealing about absolutes, about fixed things — about constancy. So I thought you might be interested in listening to University of Nottingham prof Michael Merrifield talk about where things might be going mass-wise, as yet another element in our culture moves into the abstract:

[PS: This involves something called Planck’s Constant, which is, as they say, another story. But there’s a pretty good explanation of it (i.e. one that non-physicists can understand) over on the PBS website, in case you’re intrigued.]

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