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 in a box not strictly hold for the measured momentum ; the energy eigenstate formula_38 is not a momentum eigenstate, and, in fact, not even a superposition of two momentum eigenstates, as one might be tempted to imagine from equation () above: peculiarly, it has no well-defined momentum before measurement! In classical physics, the particle can be detected anywhere in the box with equal probability. In quantum mechanics, however, the probability density for finding a particle at a given position is derived from the wavefunction as formula_39 For the particle in a box, the probability density for finding the particle at a given