Something funny is going on with the kilogram. I blame the French. (Of course, don't Americans always blame the French?) Anyhoo, the Frogs are in charge of the Sacred Kilogram of Destiny - or whatever. And they screwed it up, of course.
About every 50 years, the national prototypes are returned to the headquarters of the International Bureau of Weights and Measures in Sevres, France, to be compared with the International Prototype. During the first major comparison about 1950, scientists noticed discrepancies between the average masses of Le Grand K and its copies. They were concerned but could not discern a trend.Personally, I think it is second-hand cholesterol. My theory is I gain weight just by passing a Long John Silver's, and this could solve the mystery of the fluctuating kilogram.
Science was already grappling with inconsistencies in other units and was trying to replace the pieces of metal and other artifacts that delineated the old world. The meter, for example, was changed in 1960 from two scratches on a platinum-iridium bar to a certain number of wavelengths of light emitted from a particular kind of krypton. In 1983, it was changed again to the distance traveled by light in a specific fraction of a second.
At the last major kilogram comparison done in and around 1990, some copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms. The United States' No. 20 was 18 micrograms heavier.
There was no way to tell which was changing: Le Grand K, its copies or both.
Perhaps the platinum in the cylinders was sopping up mercury from the atmosphere. Maybe dissolved gas was escaping from the cylinders. One idea was that cleaning the cylinders with distilled water and ether had altered their weights.
"Nobody has a really good idea why," said Davis of the International Bureau of Weights and Measures. "It's all speculation." [More]
Of course, all this could have been avoided by using God-fearing ounces and lbs.
[via 3 quarks]