View Single Post
Old 11-14-2010, 08:32 PM
MikeS MikeS is offline
Charter Member
Join Date: Oct 2001
Location: New London, CT
Posts: 3,789
Originally Posted by Dewey Finn View Post
This part ("A team of scientists based in Germany measured the [Avogadro] constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28.") of that article is hilarious. So the standard kilogram is something for which the number of atoms have been counted?
Well, not yet, but that's the general idea behind the new standard. One mole of carbon-12 is defined to be that number of atoms of carbon-12 such that their total mass is precisely 12 grams. We can use precision measurements to find out what the ratio of the mass of an atom of silicon to an atom of carbon is, so we know how many grams a mole of silicon should weigh. What we don't know is precisely how many atoms are in a mole. So if we can manufacture some physical object for which we know the number (and type) of atoms extremely precisely, and we weigh that object relative to our "old kilogram" standard, then that allows use to precisely measure how many atoms (of silicon, say) make up an "old kilogram". Then we define the "new kilogram" to be the weight of that number of atoms of silicon, and the iridium bar is no longer needed.