American physicist Robert Millikan conducted the "Oil Drop Experiment" to observe and measure the electrical charge carried by an electron. He placed very small drops of oil between two charged horizontal parallel plates, which applied a gravitational, electrical and drag force on the oil drops. Millikan set the force of gravity and that of the electric field to be the same. Through altering the force of the electric field, when the droplets were shot out of a nozzle their paths would be changed accordingly. The electric field either caused the drops to travel and longer or shorter path. Causes of experimental error included charge, size and density of the oil. Since the drops varied in size, the value kept changing. Due to this, Millikan adjusted his experimental setup several times in order to minimize experimental error. Millikan adjusted the pressure, kept temperature constant and minimized the drag. Millikan applied the elementary constant of charge to Neil Bohr's theoretical formula for the hydrogen spectrum and accurately found the Rydberg constant. Millikan's work with this experiment was the first and most compelling proof of Bohr's quantum theory of the atom.
How does it relate to this work on electrostatics?
In his experiment, Millikan was able to find the charge carried by an elctron. This shows us exactly how much charge, or how many electrons for that matter, are necessary to cause certain electrical currents in atoms. Before knowing the charge of the electron, scientists did not know the relationship between force exerted by an atom and the charge that the atom possessed. Millikan allowed scientists to answer that question through his oil drop experiment.