Millikan and Fletcher stared the oil drop experiment in 1909 and published the results in 1913. The goal was to find the elementary charge: the smallest possible charge. The experiment was based on the idea that such a charge actually existed—in other words, that electric charge is discrete like a staircase, not continuous like a hill.
Millikan reasoned that all possible charges must be multiples of the elementary charge. If dividing the smallest charge into all the others didn’t give all integer values, it must not be the elementary charge. For his experiment, he used parallel plates and a charged sprayer. He put a small static charge on oil droplets and sprayed them over the plate. The droplets passed through small holes in the top plate. With the correct voltage on the plates, he could make the droplets balance in the air because `|F_"el"| = |F_"g"|`.
In short, the experiment worked and Millikan determined the elementary charge (which turns out to be the charge on one electron) to within ±0.6% of the currently accepted value:
`e = 1.602xx10^-19\ "C"`.
All other charges are multiples of the elementary charge:
`q = Ne`.