Starting in 1908, while a professor at the University of Chicago, Millikan worked on an oil-drop experiment in which he measured the charge on a single electron. J. J. Thomson had already discovered the charge-to-mass ratio of the electron. However, the actual charge and mass values were unknown. Therefore, if one of these two values were to be discovered, the other could easily be calculated. Millikan and his then graduate student Harvey Fletcher used the oil-drop experiment to measure the charge of the electron (as well as the electron mass, and Avogadro constant, since their relation to the electron charge was known).
Professor Millikan took sole credit, in return for Harvey Fletcher claiming full authorship on a related result for his dissertation. Millikan went on to win the 1923 Nobel Prize for Physics, in part for this work, and Fletcher kept the agreement a secret until his death. After a publication on his first results in 1910,  contradictory observations by Felix Ehrenhaft started a controversy between the two physicists. After improving his setup, Millikan published his seminal study in 1913.
The elementary charge is one of the fundamental physical constants, and accurate knowledge of its value is of great importance. His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet could be determined. Repeating the experiment for many droplets, Millikan showed that the results could be explained as integer multiples of a common value (1.592 × 10 coulomb), which is the charge of a single electron. That this is somewhat lower than the modern value of 1.602 176 53(14) x 10 coulomb is probably due to Millikan's use of an inaccurate value for the viscosity of air.
Although at the time of Millikans oil-drop experiments it was becoming clear that there exist such things as subatomic particles, not everyone was convinced. Experimenting with cathode rays in 1897, J. J. Thomson had discovered negatively charged corpuscles', as he called them, with a charge-to-mass ratio 1840 times that of a hydrogen ion. Similar results had been found by George FitzGerald and Walter Kaufmann. Most of what was then known about electricity and magnetism could be explained on the basis that charge is a continuous variable. This in much the same way that many of the properties of light can be explained by treating it as a continuous wave rather than as a stream of photons.
The beauty of the oil-drop experiment is that as well as allowing quite accurate determination of the fundamental unit of charge, Millikans apparatus also provided a hands on demonstration that charge is actually quantized. General Electric Companys Charles Steinmetz, who had previously thought that charge is a continuous variable, became convinced otherwise after working with Millikan's apparatus.