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
Surface charge Surface charge Surface charge is the electrical potential difference between the inner and outer surface of the dispersed phase in a colloid. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field. Surface charge causes a particle to emit an electric field, which causes particle repulsion and attraction, affecting many colloidal properties. Surface charge practically always appears on the particle surface when it is placed into a fluid. Most fluids contain ions, positive (cations) and negative (anions). These ions interact with the object surface.