Millikan

Robert Millikan calculated the charge of an electron with his famous oil-drop experiment in 1910, which took advantage of the fact that droplets of oil can carry an electric charge on their surfaces. He took a closed transparent chamber with two parallel horizontal metal plates, one passing through the middle of the chamber and one on the bottom. The upper plate had a small hole in it, and the plates were connected by an electric current, giving them a charge. Millikan sprayed tiny droplets of oil into the chamber's upper half; these floated downward, with some falling through the hole in the upper plate. Their mass could be calculated by measuring how fast they fell. Millikan then ionized the air in the lower half by beaming x-rays at it, which stripped electrons from the air molecules; the electrons attached themselves to the droplets, giving them a negative charge. By changing the voltage between the two plates, which changed the electric differential between them, he could modulate the rate of the droplets' fall. If the voltage equaled the known gravitational force acting on a droplet, the droplet remained stationary. This voltage, together with the droplet's mass, he then used to calculate the droplet's charge. Millikan found through many experiments that the charge was always a whole-number multiple of a particular quantity, which he deduced was the charge of a single electron (1.602 × 10 ⁻¹⁹ coulombs). For this discovery, he was awarded the Nobel Prize for physics in 1923.