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.

As a unit

In some natural unit systems, such as the system of atomic units, e functions as the unit of electric charge. The use of elementary charge as a unit was promoted by George Johnstone Stoney in 1874 for the first system of natural units, called Stoney units. Later, he proposed the name electron for this unit. At the time, the particle we now call the electron was not yet discovered and the conceptual distinction between the particle electron and the unit of charge electron was still blurred. Later, the name electron was assigned to the particle and the unit of charge e lost its name. However, the unit of energy electronvolt (eV) is a remnant of the fact that the elementary charge was once called electron.