What is Millikan’s oil drop experiment

Some fine oil droplets were allowed to be sprayed into the chamber by the atomizer. The air in chamber is subjected to ionization by X-rays. The electrons produced by the ionization of air attach themselves to the oil drops. When sufficient amount of electric field is applied which can just balance the gravitational force acting on the oil drop, the oil drop remains suspended in air. From this experiment Millikan observed that the electrons produce 1.59 × 10^—19 coulombs and the charge on each drop was always an integral multiple of that value. On the basis of the observation, he concluded that the 1.59 × 10^—19 coulombs is the smallest possible charge and considered that value as the charge of electron