Mass Effect; Measurement  

Oleh:

[s.n]

Jenis: Article from Bulletin/Magazine
Dalam koleksi: The Economist (http://search.proquest.com/) vol. 406 no. 8818 (Jan. 2013), page 66.
Topik: Standards; Weights & Measures; Alloys; Cleaning; Measurement

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Perpustakaan Pusat (Semanggi) Nomor Panggil: EE29.75 Non-tandon: 1 (dapat dipinjam: 0) Tandon: tidak ada

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In Sevres, on the outskirts of Paris, sealed beneath a triplet of bell jars in the laboratories of the Bureau International des Poids et Mesures, sits a small cylinder of platinum-iridium alloy. Put it on a scale, and it would weigh 1 kilogram--which is appropriate, since this lump of metal is not just a kilogram but The Kilogram, the ultimate reference standard for the way the Syst e me internationale d'unites (SI or, as it is better known, the metric system) measures mass. The kilogram is unique in that it is defined by reference to a lump of crude, man-made stuff. Besides aesthetic niggles, that fact leads to an important practical problem. The international prototype kilogram (IPK), the technical name for the cylinder in Sevres, does not in fact keep a constant mass. Over the years pollutants from the air settle on its surface, causing its mass to rise. Attempts to clean it then cause its mass to fall. As a result, what science understands by a kilogram has varied, but has done so in a way that is, by definition, unmeasurable. The cleaning that attempts to keep the kilogram constant is done by a member of staff at the bureau, using a combination of steam and a chamois-leather cloth soaked in ethanol and ether. But given the sensitivity of modern instruments, which can detect gains or losses of the order of billionths of a kilogram, it must be done properly.

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