Elementary charge

The elementary charge, usually denoted by e, is the magnitude of the charge on an electron and a fundamental physical constant defined to be exactly e = 1.602176634×10−19 C. The elementary charge is a measure of the coupling strength of the electromagnetic force and one of the seven defining constants of the International System of Units (SI). Before being set to a fixed value as part of the 2019 revision of the SI, the elementary charge was viewed as a quantity to be determined experimentally. Robert A. Millikan and Harvey Fletcher's oil drop experiment first directly measured the magnitude of the elementary charge in 1909, differing from the modern accepted value by just 0.6%. Under assumptions of the then-disputed atomic theory, the elementary charge had also been indirectly inferred to ~3% accuracy from blackbody spectra by Max Planck in 1901 and (through the Faraday constant) at order-of-magnitude accuracy by Johann Loschmidt's measurement of the Avogadro constant in 1865.