Seven SI fundamental quantities and accuracy
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It turns out there are actually seven. This is an article I just got, in March was when I first saw it. I was down at the National Institute of Standards and Technology, because I was on the review committee for manufacturing. That review committee now includes the group that takes care of the reference standards. If the whole world wants to agree on some standard, and I'm going to buy something that's going to be one inch long, well, how precise is that going to be? It's going to be one inch plus or minus one-thousandth, plus or minus 50 millionths. How precise does it have to be to one inch, and under what conditions?