Which experiment led to the measurement of the charge on an electron?

The experiment that led to the measurement of the charge on an electron is the "Oil-drop experiment." In this experiment, Robert Millikan observed oil droplets falling between two charged plates and was able to determine the charge on an electron. He applied an electric field to the charged plates to slow down the motion of the droplets and then calculated the charge on each droplet by measuring the strength of the electric field required to keep the droplets suspended in mid-air. By using this information, he was able to determine the charge on an individual electron.