NIST: World Metrology Day: Kilogram in the Balance

In terms of the fundamental science, I am finally far more comfortable saying that the Planck constant is, well, constant, than I am saying that a hunk of metal is an invariant of the universe. The IPK has proved to be an amazingly precise hunk of metal, and for over a century it has served humanity astonishingly well as a worldwide standard of mass. …