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This is What a Kilogram Looks Like

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For years, my only point of reference when visualizing a kilogram was something along these lines:

While I'm sure cocaine dealers are highly exacting in their attention to weights and measures, the internationally-accepted, definitive kilogram is kept in Sevres, France, on the western outskirts of Paris:

The Bureau International des Poids et Measures stores the "international prototype" along with six official copies:

As the BIPM website explains:

The international prototype of the kilogram, an artefact made of platinum-iridium, is kept at the BIPM under the conditions specified by the 1st CGPM in 1889 when it sanctioned the prototype and declared:
    This prototype shall henceforth be considered to be the unit of mass.
The 3rd CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the use of the word "weight", confirmed that:
    The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

The complete declaration appears here.

It follows that the mass of the international prototype of the kilogram is always 1 kilogram exactly, m(grand K) = 1 kg. However, due to the inevitable accumulation of contaminants on surfaces, the international prototype is subject to reversible surface contamination that approaches 1 µg per year in mass. For this reason, the CIPM declared that, pending further research, the reference mass of the international prototype is that immediately after cleaning and washing by a specified method (PV, 1989, 57, 104-105 and PV, 1990, 58, 95-97). The reference mass thus defined is used to calibrate national standards of platinum-iridium alloy (Metrologia, 1994, 31, 317-336). 

Here's a bit of background, courtesy of the Economist:

A kilogram is the mass of a cylindrical lump of platinum-iridium alloy that was cast in 1879 in Hatton Garden, the jewellery district of London, and then dispatched to the International Bureau of Weights and Measures (BIPM) in Sèvres, near Paris. It sits there still, nestled beneath three cheese-domelike bell jars in a safe that can be unlocked (though it rarely is) only by turning a set of three keys, each entrusted to a high-ranking BIPM dignitary.

The kilogram is the last bit of the International System of Units (SI) to be tied explicitly to an artefact. Once, the metre was too. It was the length of another platinum-iridium ingot stored in Sèvres. But the metre has been redefined twice since that ingot was deposited in 1889: first, in 1960, in terms of the wavelength of a particular sort of light; then, in 1983, as the path travelled by light in a vacuum in 1/299,792,458 of a second. Which, of course, raises the question of what a second is. Not, as you might think, a sixtieth of a sixtieth of a twenty-fourth of the period of the Earth’s rotation. No. A second is the duration of 9,192,631,770 periods of a phenomenon called microwave transition in an atom of caesium-133. The remaining four basic SI units, the ampere (electricity), kelvin (temperature), mole (quantity of atoms, molecules or other particles) and candela (light), followed suit—though the ampere, candela and mole are linked to the kilogram, and so indirectly to the Sèvres prototype.

It's all right here, in this drab, institutional-looking French government outpost:


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