In 1909 Millikan performed the Oil Drop Experiment A closed chamber with transparent sides is fitted with two parallel metal plates, which acquire a positive or negative charge when an electric charge is applied.
At the start of the experiment an atomizer sprays a fine mist of oil droplets into the upper portion of the chamber. Under the influence of gravity and air resistance, some of the oil droplets fall through a small hole at the top of the metal plate.
When the space between the metal plates is ionized by radiation, electrons from the air attach themselves to falling oil droplets, causing them to acquire a negative charge. The amount of voltage needed to suspend a droplet is used along with its mass to determine the overall electric charge on the droplet.