[r9etb]

Making them spherical gives them a predictable volume. Since they know the isotope used and its density, knowing the exact volume tells you the exact mass of the sphere.

Not quite. They're also ensuring that the mass of the sphere is exactly 1 kg.

As the article said, the idea is to be able to define the kilogram in terms of atoms. Since they know the volume and the mass, they would know the density. And because they're using a near-perfect crystal, they can translate the density to the number of atoms in the sphere.

At that point, the definition of "kilogram" becomes "the mass of N atoms of silicon-28."

[ApplegateRanch]

Not quite. They're also ensuring that the mass of the sphere is exactly 1 kg.

Thought problem/experiment:

Perfect spheres, of X atoms diameter = 1 KG mass + an excess of atoms such that removing the excess uniformly from the surface leaves the sphere massing 1 KG - 1/2 the number of atoms in the outer most layer needed to make the sphere 1 kg.

To grind, or not to grind? Grind ONE and leave one, then use both in tandem?