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The things I do manage to remember bear an inverse relationship to any usefulness: Avogadro’s number, the Fibonacci sequence, the smell of Chanel No. 5.

Avogadro’s number had been estimated but, like the speed of light, never precisely measured and agreed upon.

The ampere, the kelvin and the mole will also be redefined based on their relationships to the charge on the electron, Boltzmann’s constant and Avogadro’s number, respectively.

Another counts the atoms in two spheres of silicon-28 to derive a value for Avogadro’s number, which is converted to Planck’s constant.

An international coalition of metrological laboratories known as the Avogadro Project has produced silicon spheres of near-perfect purity and crystal structure, each weighing precisely one kilogram.