Millikan suspended oil droplets between two electric plates and determined their charges.
He used an apparatus like that below:
Oil droplets from a fine mist fell through a hole in the upper floor. From their terminal velocity, he could calculate the mass of each drop.
Millikan then used x-rays to ionize the air in the chamber. Electrons attached themselves to the oil drops.
He adjusted the voltage between two plates above and below the chamber, so that the drop would hang suspended in mid-air.
Millikan calculated the mass and the force of gravity on one drop and calculated the charge on a drop. The charge was always a multiple of -1.6 x 10⁻¹⁹ C. He proposed that this was the charge on an electron.