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Roundest Object In the World Created
holy_calamity writes "An international research group has created the most perfect spheres ever made, in a bid to pin down a definition of the kilogram. It should be possible to count exactly the number of atoms in one of the roughly 9cm silicon spheres to define the unit. Currently the kilogram is defined only by a 120-year-old lump of platinum in Paris, but its mass is changing relative to copies held elsewhere. Other SI units have more systematic definitions."
why not just define a kg as 1 Liter of pure H2O at 4deg C?....it is that way anyway.
I think that Gravity Probe B has the most perfect spheres and they are much smaller that the Kilogram sphere.
Kilogram Silicon Spheres
"If you were to blow up our spheres to the size of the Earth, you would see a small ripple in the smoothness of about 12 to 15 mm, and a variation of only 3 to 5 metres in the roundness"
Gravity Probe B Spheres
http://www.nasa.gov/mission_pages/gpb/index.html
"If these ping pong-sized balls of fused quartz and silicon were the size of the Earth, the elevation of the entire surface would vary by no more than 12 feet"
We are Dead Stars looking back Up at the Sky
Is this really the reason why it's a sphere? Crystals don't PRECISELY grow into a sphere do they? Won't they still need to shave or polish it to get it to the exact radius? And then they'll need to calculate the number of atoms using Pi, an irrational number!
Why don't they make it a cube and find a length that is close enough (cubed) to give them the approx. right number of atoms and then make THAT the standard? They'll then have an EXACT number of atoms making up each length. It should be easier to cut or shave off the requisite number of atoms to maintain it, a (perfectly) flat surface seems much easier to maintain than a 3D curved surface. In fact if they make it just a little too small they could probably even ADD to the cube in single atomic layers using vapor deposition!
Obviously brighter minds than mine have thought this through more thoroughly, so really, I'm curious: why is it a sphere?
By the way, maybe this is a good use for the ISS, to keep the 1kg reference MASS somewhere it won't be distorted by gravity, not kept at any particular country for measurement and you can keep it in a high quality vacuum for free! (A little expensive to get to though).
That's actually the reasoning behind the Watt Balance another potential method for defining the kilogram.
Keep in mind that the Joule is a composite SI unit, and is itself dependent on the mass of the kilogram. Unless you can calibrate the energy measurements (exactly what they are trying to do) you end up with a circular definition.