The elementary charge (symbol e) 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.
It has a value of 1.602 176 462(63) × 10-19 C.
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, were believed to have fractional electric charges (in units of e/3), but only to exist in particles with an integer charge. They have never been detected singly. In 1982 Robert Laughlin tried to explain the fractional quantum Hall (FQH) effect by predicting the existence of fractionally charged quasiparticles. In 1997, two groups of physicists at the Weizmann Institute of Science in Rehovot, Israel, and at the CEA laboratory near Paris, claimed to have detected such quasiparticles carrying an electric current.