The word electron first named a unit of charge, not a particle
In 1874 George Johnstone Stoney proposed using a single basic lump of electric charge as a unit, and he later suggested calling it the electron. The particle had not yet been discovered. When it was, it took the name, and the unit lost it, surviving only in the electronvolt, a measure of energy still used everywhere in physics.
That basic lump, written e, is the size of the charge on one electron. Since 20 May 2019 it has been fixed by definition at exactly 1.602176634×10−19 coulombs, making it one of the seven constants on which the International System of Units is built. Before then it was something to be measured, and the measurements have a long history.
Early estimates leaned on electrolysis: divide the charge carried by a mole of electrons by the number of electrons in a mole. Johann Loschmidt's 1865 estimate of that number gave e only to the right order of magnitude, and Stoney's own 1874 figure came out 20 times too small. Max Planck did far better in 1901, reaching about 3 per cent accuracy by way of his theory of blackbody radiation. In 1898 J. J. Thomson tried counting ions by watching water droplets condense around them.
The breakthrough came in 1909 with Robert Millikan and Harvey Fletcher's oil drop experiment. By balancing tiny drops between gravity, air resistance and an electric field, they could work out each drop's charge, and every value proved to be a whole multiple of one small amount. Their result landed within 0.6 per cent of today's figure.
Nobody knows why charge comes in fixed steps. The Standard Model offers no explanation, and Paul Dirac showed in 1931 that magnetic monopoles, if they exist, would force it. Quarks, proposed in the 1960s, carry thirds of e but never appear alone; they bundle into particles such as protons with whole-number charges. In 1982 Robert Laughlin explained a quantum effect in materials through quasiparticles carrying fractional charge, though these are not true elementary particles.
Source: Elementary charge
