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Dr. Robert A. Millikan: Fundamental Measurements in Electricity and Radiation, 1937

The Full Gamut

Among the sources used to change the drop's charge were: alpha, beta or gamma ray bombardment from radium, ultraviolet illumination and X-ray irradiation—the full gamut of the electromagnetic spectrum.

The experimental facts determined are: there is a charge on an electron, there is a smallest "unit" charge and the charge changes in discrete amounts or "quanta."

Over years of further study the absolute value of the electron in absolute electrostatic units was obtained. They have no dimensions but are simply point charges of electrical force.

Next, the principles of atomic theory emerged with observations that the mass of positively charged electrons is 1,845 times that of negatives, the number of positives and negatives in an atom can be measured, the negatives are distributed through the atom's nucleus and outer regions.