The result was obtained by Paul Sophus Epstein in 1924. His result was used for. high-precision measurements of the charge on the electron in the oil drop experiment performed by Robert A. Millikan, as cited by Millikan in his 1930 review paper on the subject. For the early work on that experiment, the drag was assumed to follow Stokes' law. However, for droplets substantially below the submicron scale, the drag approaches Epstein drag instead of Stokes drag, since the mean free path of air species (atoms and molecules) is roughly of order of a tenth of a micron.