The term elementary charge, designated e, refers to the magnitude of the electrical charge both on an electron and on a 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 ± 0.000 000 040) x 10-19 coulombs.
As the proton carries a positive charge, the magnitude of its charge is designated +e, whereas the magnitude of the charge on the electron, which carries a negative charge, is designated -e.
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 (⅓ e), possessed by down quarks. The term elementary charge when referring to an electron charge is therefore scientifically a misnomer. 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. Current literature continues to refer to e as the elementary charge although it is now also often—and more properly—referred to as the magnitude of the electron charge.
Strictly, at the hierarchical subatomic level of nucleons (protons and neutrons) and electrons, and particles emitted by the nucleus in radioactive processes (e.g., positrons), electrical charge comes in whole units of e, the electron (or proton) charge. Positively charged atoms or compounds carry one or more protons than the number of electrons, and so carry one or more units of +e. Likewise negatively charged atoms or compounds carry one or more electrons than the number of protons, so carry one or more unit of -e. Neutrons, as their name suggests, carry no charge, and therefore are electrically neutral.