[QUOTE=Jinx]
We take for granted that Europe was always metric, but this is not true. When and how did these independent nations get unified under the metric system? And, was it phased in? (It’s amazing they could all agree to do it!) I assume it was Western Europe, but as a bonus, when did Russia and the Iron Block countries accept going metric, if ever?
[/QUOTE]
Well, you probably know the metric system originated in France during the French Revolution. The reason for it there was the reason for it everywhere: when you have multiple measuring systems, it’s a mess when trading with anyone else. France’s measurements changed from town to town, so it was a very big mess.
France went on the metric system in the 1790s, but dropped it due to public opposition. It returned to the system in 1840, by which time it had become standard in Belgium, Luxemborg, and the Netherlands. They had started using when the French invaded, and, after Napoleon fell, the rulers saw the advantage of a single measurement system and kept it. Napoleon had also introduced the system in parts of Italy, which slowly started adopting it as a way to unite the various Italian states. It became required in 1865.
Spain and Portugal started converting in 1852, though under a phase in that wasn’t particularly firm. Still, that led to the spread in the Spanish colonies and Brazil.
I’m not sure of the date when it was adopted in Russia, but since it was a product of the French Revolution, the Bolsheviks were delighted to use it.
Note that these were all individual decisions. No international body required it, but the individual nations, for various reasons (but usually to regularize trade) adoped it.
If you’re interested, I heartily recommend reading The Measure of All Things: The Seven-Year Odyssey and Hidden Error That Transformed the World by Ken Alder, which describes this and how the meter was finally (and incorrectly) determined.