The May 2012 edition of IEEE's Spectrum
magazine has an interesting article discussing the frustrating effort to establish a new method for determining the standard kilogram. For 125 years, the master standard has been a platinum-iridium alloy cylinder about the size of a plumb. It is stored in a vault, under three concentric Bell jars. Every three years, the master standard is cleaned, and every decade or so, the standard is compared to other country's identical standards to determine whether they all still measure the same mass. The problem is that aside from the need to maintain a physical block, the cumbersome process of comparative measurement introduces perturbations and errors in the results. The master standard has apparently lost 55 micrograms since the first measurements, and nobody knows where they went. That is not a lot of mass compared to a kilogram, but with the precision of measurements made by laboratory and even commercial instruments and products, an uncertainty in the eighth decimal point can be a serious handicap.
The article is only four pages long, but it covers a lot of ground, including the current proposals for the new master standard that does not involve storing a specific mass.The Kilogram, Reinvented
- Kirt Blattenberger
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