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Old 11-15-2010, 11:16 PM
RadicalPi RadicalPi is offline
Join Date: May 2009
Location: Santa Barbara
Posts: 1,394
Originally Posted by Simplicio View Post
The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I'm not really sure why they haven't taken the obvious step of just using that as the definition. Perhaps water doesn't have as fixed a density as I'd think at a set temperature?
I believe that part of the reason that they didn't do this was that the volume of water depends a bit on the pressure of the air above it. So, any definition that doesn't take pressure into account is going to be less accurate than the platinum-iridium cylinder. And any definition that does take it into account is going to be circular since pressure is force per unit of area, and force is mass times acceleration. So you'd need to have mass defined before you can define mass.

Someone please correct me if I'm wrong.

Last edited by RadicalPi; 11-15-2010 at 11:18 PM.