The elementary charge, usually denoted by e or sometimes qe, is the electric charge carried by a single proton, or equivalently, the magnitude of the electric charge carried by a single electron, which has charge −e. This elementary charge is a fundamental physical constant. To avoid confusion over its sign , e is sometimes called the elementary positive charge. This charge has a measured value of approximately 1.6021766208(98)×10−19 C (coulombs). When the 2019 redefinition of SI base units takes effect on 20 May 2019 , its value will be exactly 1.602176634×10−19 C by definition of the coulomb. In the centimetre–gram– second system of units (CGS), it is 4.80320425(10)×10−10 statcoulombs. Robert A. Millikan's oil drop experimentfirst measured the magnitude of the elementary charge in 1909.
In some natural unit systems, such as the system of atomic units, e functions as the unit of electric charge, that is e is equal to 1 e in those unit systems. The use of elementary charge as a unit was promoted by George Johnstone Stoney in 1874 for the first system of natural units, called Stoney units. Later, he proposed the name electron for this unit. At the time , the particle we now call the electron was not yet discovered and the difference between the particle electron and the unit of charge electron was still blurred.