elementary charge

This is the one entry in the charge family that is a constant of nature rather than a human convenience: every free particle ever observed carries a whole-number multiple of it. One coulomb is about $6.24 \times 10^{18}$ elementary charges, so even a modest 20 mA LED current is a parade of roughly $1.2 \times 10^{17}$ electrons per second. Chemists meet the same number multiplied by Avogadro's constant as the Faraday constant, about 96,485 coulombs per mole of electrons, the bridge the site's Faraday electrolysis solver uses to turn a current and a time into grams of metal.

Its history runs in both directions from here. Robert Millikan pinned the value down between 1909 and 1913 by watching charged oil drops hover in an electric field, and his result came out about 0.6 percent low because he used a slightly wrong figure for the viscosity of air. Later experimenters crept toward the true value in small steps rather than correcting the error outright, each hesitant to stray far from the number before them, an episode Richard Feynman retold in 1974 as a standing warning about experimenter bias.

Then, on 20 May 2019, the measuring stopped. The revised SI fixed $e$ at exactly $1.602176634 \times 10^{-19}$ coulombs, with no uncertainty and no further digits ever to be found, and the ampere is now defined from it rather than the other way around. A century of refinement ended not with a final measurement but with a decree, and an experiment that once measured the elementary charge now calibrates the apparatus instead.