In Sevres, France sits a kilogram of platinum. In fact, it is the kilogram, the standard for all kilograms. But, as a standard, it is slipping. Its weight has decreased since it was designated the standard (in 1889) - a decrease of 50 micrograms, "equal to the mass of a smallish grain of sand." Apparently it is the only unit of measurement - at least in the metric system - that continues to be defined in terms of a physical object. By way of contrast, a meter is defined as "the length of the path traveled by light in a vacuum during a time interval of 1 / 299,792,458 of a second." Scientists are now struggling to similarly define the kilogram, perhaps basing it on Planck's Constant (which is hard to define with any certainty).
Only three people have the key to the platinum kilogram in Sevres. What will happen to it when it's replaced?
Stay tuned.
[all from "Missing Micrograms Set a Standard on Edge," New York Times, February 13, 2011, A12.
