Originally Posted by melgross
Heh heh. Don't forget that mass and weight are two different things.
If we really want to be accurate, the only place where the gram weighs a gram, accurate to just so many decimal places, is where is was determined in the first place. Move it a few miles, and the gravity is different. It will seem the same, because everything else will change along with it, but it really won't be. strain gauge scales will be able to tell, but most conventional scales won't.
That's why using atomic mass is more accurate. Eliminates the gravity field.
Definitely, I kind of figured they (BIPM, NIST, others) weren't weighing the artifacts (as these Kilogram prototypes are referred to as) due to just the effects (plus a WHOLE bunch of others!) you mentioned. I did find several very insightful PDF's by BIPM/NIST and you may be surprised at how they measure the mass while avoiding these effects.
They use a beam balance, which in principle is just like the ones we all used in high school!
Of course these beam balances are precision electro-mechanical-optical devices able to achieve accuracies of up to 1 part in a trillion (10^12), which to my surprise is still about 2 orders of magnitude higher than other current methods (atomic, etcetera). But if all goes according to plan, by 2011 or so they will be able to ditch these artifacts for other methods based on physically known constants.
To use the beam balance they use the the primary (plus several copies) and do weigh-offs with newly fabricated ones (i. e. when the balance is level the copy is good to go). Supersets of 1 Kg are fairly straight forward (in fact NIST makes them as large as 28,000 Kg!). Subsets are where they get a little more clever, for instance they step down to 1000+500+200+200+100 g unknowns and 2 1000 g knowns using a 6 step permutation sequence. And a similar procedure is used to obtain even smaller sizes, until you end up with a set of weights like the set of SS weights we all used in high school!
BTW, a metric tonne is just 1000 Kg of pure water which just happens to fit into a 1 meter cube, thus the original relationship between the Kilogram and Meter, and why rho = 1 g/cc.
PS - I know this is off topic, but I just couldn't resist, it's a subject area I've dealt with for over 30 years now (geometric, kinematic, and dynamic similitude AKA Froude scaling). I hope you'll excuse the interruption!