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In that case, metrologists were adapting to fit the needs of chemists, who wanted a way to express SI units on the scale of Avogadro’s number—the 6 × 1023 units in a mole, a measure of the quantity of substances.

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.

In effect, Dr. Bettin had devised an experiment that would precisely measure a constant known as Avogadro’s number, which for many years dictated that one mole of a substance contains 6.022 × 10^23 particles such as electrons, atoms, ions, or molecules.

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.