Wikipedia Reference

The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron.

This is a fundamental physical constant and the unit of electric charge in the system of atomic units as well as some other systems of natural units.

It has a value of 1.602 176 487(40) × 10^-19 C, according to the NIST Reference on Constants, Units and uncertainty [1]. In the centimetre gram second system of units, the value is approximately 4.803 × 10^-10 statcoulombs.

Since it was first measured in Robert Millikan's famous oil-drop experiment in 1909, the elementary charge has been considered indivisible. Quarks, first posited in the 1960s, are believed to have fractional electric charges (in units of e/3 and 2e/3), but only to exist in particles with an integer charge. They have never been detected singly, and for this reason as well as historical ones, they are not considered the elementary charges. In 1982 Robert Laughlin tried to explain the fractional quantum Hall effect by predicting the existence of fractionally charged quasiparticles. In 1995, the fractional charge of Laughlin quasiparticles was measured directly in a quantum antidot electrometer at Stony Brook University, New York. In 1997, two groups of physicists at the Weizmann Institute of Science in Rehovot, Israel, and at the Commissariat à l'Énergie Atomique laboratory near Paris, claimed to have detected such quasiparticles carrying an electric current.

References

1. NIST Fundamental physical Constants.