Elementary charge is . Robert A. Millikan's oil drop experiment first 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
Particle decay to further decay. The term is typically distinct from radioactive decay, in which an unstable atomic nucleus is transformed into a lighter nucleus accompanied by the emission of particles or radiation, although the two are conceptually similar and are often described using the same terminology. Particle decay is a Poisson process, and hence the probability that a particle survives for time "t" before decaying is given by an exponential distribution whose time constant depends on the particle's velocity: All data is from the Particle Data Group. Note that this section uses natural units, where formula_4 The lifetime of a particle