World's Roundest Object!

The most spherical thing in the world contributes to the resolution of measurement's oldest conundrum, the kilogram's definition.
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What a kilogram used to be has changed. Literally. Its original term, the "grave," was suggested in 1793, but like its inventor Lavoisier, it perished in the French Revolution. This is how the story of the strangest SI unit starts. The only base unit still defined by a physical artifact, the international prototype kilogram, or IPK, is the kilogram. It is the only base unit with a prefix in its name.

But this definition's flaw has been clear for some time. When compared to 40 identical cylinders produced at the same time, the IPK doesn't appear to keep its mass. Therefore, it is intended to do away with the kilogram's reliance on a tangible thing. The Avogadro Project and the Watt Balance are the two main strategies under consideration to accomplish this goal.

The Avogadro project seeks to redefine Avogadro's constant (now defined by the kilogram, or the number of atoms in 12 g of carbon-12), and reverse the relationship such that Avogadro's constant accurately specifies the kilogram. The most ideal sphere on Earth had to be made in order to use this technique. It is composed of a single silicon crystal with 28 atoms. The diameter may be precisely measured, allowing the volume to be determined. Since the silicon atom spacing is well understood, it is possible to calculate the precise number of atoms in a sphere. This makes it possible to calculate Avogadro's constant with extreme precision.

We would especially like to acknowledge the assistance of Katie Green, Dr. David Farrant, the CSIRO, and the National Measurment Institute. Nessy Hill was also credited for filming and analyzing early iterations of this video.

Whether or not this is actually the roundest thing ever made is up for discussion. The rotors of the Gravity Probe-B are similarly spherical and have very tight tolerances, so they might even be rounder.

Kevin McLeod is a musician (incompetech.com). Decision, Dance of the Dead, and Scissors.