The oil drop experiment, conducted by physicist Robert Millikan in 1909, was a pivotal scientific procedure that measured the electric charge of a single electron. In the experiment, tiny oil droplets were sprayed into a chamber, where they could be observed falling between two electrically charged plates. By adjusting the voltage across the plates, Millikan was able to balance the gravitational and electric forces acting on the droplets, causing them to hover motionless. Through careful observation and calculation, Millikan determined that the electric charge on each droplet was always a multiple of a fundamental value, which he identified as the charge of a single electron. This experiment provided strong evidence for the quantization of electric charge and was a cornerstone in the development of atomic physics.