Elementary charge

The term elementary charge, e refers to the magnitude of the charge on an electron or proton. It was first measured by Robert Millikan in 1909 using his famous oil drop experiment. e is regarded as a fundamental physical constant and the current internationally agreed upon value from CODATA stands at:

$e=(1.602\;176\;487\pm 0.000\;000\;040)\times 10^{-19}{\textrm {C}}$

Historically the name comes from a period of time before quarks were discovered when e was thought to be the smallest electric charge existing in nature. The discovery of quarks in 1974 showed for the first time that this was not correct and currently the smallest known electric charge is $e/3$ which down-like quarks possess. The term elementary charge when referring to an electron charge is therefore technically incorrect. However a property of quarks called confinement means that at low energies quarks are bound into colourless, multiple quark states which have charges that are integer multiples of e and only at high energies, such as top quark decay, do the fractional charges become evident. Thus e is still the smallest electrical charge that will be typically encountered outside the field of particle physics. Some current literature continues to refer to e as the elementary charge although it is now often referred to as the magnitude of the electron charge.