The elementary charge, conventionally denoted (e), is a fundamental physical constant specifying the magnitude of the electric charge carried by an electron. An electron has charge (-e), whereas a proton has charge (+e). The constant itself is positive. In the International System of Units (SI), its value is exactly (1.602,176,634\times10^{-19}) coulombs. It connects microscopic electrical phenomena with macroscopic measurements and provides the basis for defining the ampere. (nist.gov)
Value, units, and sign
The defining numerical relation is
[ e=1.602,176,634\times10^{-19}\ \mathrm{C}, ]
where C denotes the coulomb, the SI unit of charge. Because this value is fixed by definition, it has no measurement uncertainty. This does not mean that an experiment measuring charge or realizing an electrical unit is free of uncertainty; it means that the unit system assigns an exact numerical value to the constant. (physics.nist.gov)
The distinction between (e) and the electron’s signed charge is important. Expressions such as “electron charge” sometimes refer to its magnitude, but the electron’s actual charge is negative. Charges are frequently reported in units of (e): a value of (+1) in such a table means (+e), not one coulomb. One coulomb corresponds to approximately (6.2415\times10^{18}) elementary charges, illustrating the enormous difference between microscopic and everyday electrical scales. (nist.gov)
Charge quantization and particle structure
For ordinary bodies whose net charge results from gaining or losing electrons, charge occurs in integer multiples of (e):
[ Q=ne,\qquad n\in\mathbb Z. ]
Removing an electron makes a body’s charge more positive by (e); adding one makes it more negative by the same amount. Millikan’s observations of individual charged droplets provided direct experimental evidence for this discrete structure: different droplet charges and changes in charge were consistent with a common elementary increment. (nobelprize.org)
The word “elementary” does not imply that every constituent particle has charge (0) or (\pm e). In the Standard Model, quarks have fractional charges: up, charm, and top quarks carry (+2e/3), while down, strange, and bottom quarks carry (-e/3). Their antiparticles have opposite charges. Quarks are subject to color confinement and are not observed as freely isolated particles; they form composite hadrons. (opendata.cern.ch)
A proton’s two up quarks and one down quark give a total charge (+e). A neutron, containing one up and two down quarks, has net charge zero. Charged leptons, including electrons, carry charge (-e), while their antiparticles carry (+e). The positron is therefore a positively charged counterpart of the electron. These charge assignments distinguish the elementary charge as a conventional reference magnitude from a universal smallest charge for every particle constituent. (hst-archive.web.cern.ch)
Experimental determination
Before its numerical value became an SI definition, the elementary charge was determined experimentally. Robert A. Millikan’s oil-drop experiment examined tiny charged droplets moving between electrically charged plates. Their motion under gravity and an electric field allowed their charges to be inferred. Measurements of the same droplet after changes in its charge were especially useful for identifying the common charge increment. (nobelprize.org)
The analysis required accounting for the droplet’s mass, buoyancy, and resistance to motion through air. Measurements of falling and rising speeds connected the droplet’s mechanical behavior with the electrical force acting on it. Determining the air’s viscosity and correcting the drag law for very small droplets were essential parts of obtaining an accurate result, rather than merely observing that charge changed in steps. (nobelprize.org)
Millikan received the 1923 Nobel Prize in Physics for his work on the elementary charge and the photoelectric effect. His Nobel lecture described both the experimental arrangements and the evidence for the atomic nature of electricity. (nobelprize.org)
Role in the SI
The revised SI, effective May 20, 2019, made (e) one of its seven defining constants. The ampere is defined by fixing the elementary charge’s numerical value when expressed in coulombs, with
[ 1\ \mathrm{C}=1\ \mathrm{A,s}. ]
Consequently, a current of one ampere corresponds to a net transfer of approximately (6.2415\times10^{18}) elementary charges per second. The definition establishes a reference for electrical measurements without prescribing one particular instrument or experimental technique. (nist.gov)
One realization approach uses devices that transfer individual electrons at a controlled frequency. Ideally, transferring one electron during each cycle produces a current of magnitude (I=ef), where (f) is the cycle frequency. Missing or additional transfers and other practical imperfections contribute uncertainty. Such devices connect the macroscopic unit of current directly with discrete charge transport. (nist.gov)
Related constants and electrical standards
The electronvolt links charge with energy. It is the energy gained by an electron crossing an electric potential difference of one volt:
[ 1\ \mathrm{eV}=1.602,176,634\times10^{-19}\ \mathrm{J}. ]
The electronvolt is therefore an energy unit, not a unit of charge. (wwwcompass.cern.ch)
In precision metrology, (e) combines with the Planck constant (h). The Josephson effect supplies the relation (K_J=2e/h) underlying quantum voltage standards, while the quantum Hall effect supplies the resistance scale (R_K=h/e^2). Because (e) and (h) have exact SI values, these combinations are exact constants, although practical standards retain experimental uncertainties. Quantum electrical standards thus provide reproducible links between microscopic physics and measurements of voltage, resistance, and current. (nist.gov)