Weight of the World
The kilogram is an international standard of weight, but what we use as our official standard is a physical prototype against which all weights in the US are compared against. Since handling by human hands can contaminate the sample, thereby throwing off all measurements of weight, this shiny metal cylinder is sealed beneath a bell jar and locked behind three heavy doors 60 feet under the National Institue of Standards and Technology.
The American prototype is one out of four dozen similar standards of weight, and even those are compared to a singular prototype with the highest authority, the international prototype kilogram. Also known as the Le Grand K, it is protected in a vault outside Paris under no less than three
bell jars, and was forged by Britsh metallurgist George Matthey from an alloy of nine-tenths platinum and one-tenth iridium. The official kilogram is described as being "equal to the mass of the international prototype." Very few have actually laid eyes on it, and its vault can be unlocked with only three keys. Every 40 years or so, protocol dictates that the prototype be washed in alcohol, dried with a chamois cloth, given a steam bath, and then allowed to air dry before it is reweighed against the national-class prototypes. The system has worked for decades, except for one problem, Le Grand K is losing weight. Read this fascinating article on the alternatives to the old system, which tries to tie the kilogram to a natural constant, rather than a physical one. And in case you were wondering, I'm wholly on the side of Team Avogadro ;)http://www.wired.com/magazine/2011/09/ff_kilogram/all/1