 Electric charge is quantized in units of the elementary charge, formula_1, where a proton has a charge of formula_2 and an electron has a charge of formula_3.The SI unit of charge is the coulomb.The elementary charge formula_1 was first measured by Robert Millikan in his oil drop experiment in which the electric force on the particle is set to exactly counter the gravitational force that pulls it down, and the terminal velocity of this particle can be used to calculate its charge.