Where International Standard Units Come From, Part Five: The Controversial Kilogram
Danish National Kilogram Prototype Photo courtesy of the Danish National Metrology Institute All this week, the origin and continued preservation of five of our favorite standard units of measure This week, Sam Kean takes a look at some ridiculously precise standards -- the meter, the second, and other international standard units -- and the role that elements have played in defining, redefining, and re-redefining them over the ages. The kilogram really sticks in the craw of metrologists. Six of the seven fundamental units of the metric system have "operational" definitions-you can define them purely in words, by describing a physical process that produces something of exactly one meter, or whatever. But the kilogram has resisted all attempts to define it that way. It's like a feeling everyone knows and shares but cannot quite articulate. Instead, the kilogram is the last metric standard still bound to a human artifact. That...