The Controversial Kilogram

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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.