I finally know how he did it! Knowing the mass, voltage, and radius, he used: q = mgr / v to find the elementary charge. He averaged out all his findings to get the best result But...if the mass of the drop was known, how would one know how many electrons the oil drop had? For example, if all oil drops had no less than 3.2x10^-19C of charge, how would Millikan assume that e = 1.6x10^-19C? It's just kinda confusing.