Elementary charge

The elementary charge, 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 as well as some other systems of natural units.

It has a measured value of approximately $1.602 C$, according to the NIST posted CODATA value for e. See the 2006 Committee on Data for Science and Technology (CODATA) list of physical constants: CODATA report, TABLE XLVIII for uncertainty in e. In the centimetre gram second system of units, the value is $4.803$ 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, have fractional electric charges (in units of $1/3$&thinsp;e and $2/3$&thinsp;e so that now the term elementary charge referring to the charge on an electron is no longer strictly correct; this is irrelevant, however, in practical terms, since quarks are not detected except in groupings that have charges that are integer multiples of e. 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.