Millikan oil drop experiment (R.A. Millikan)
A popular experiment designed to compute the electronic charge. Drops of oil were carried past a consistent electric field among charged plates. After charging the drop along x-rays, he adjusted the electric field among the plates so that the oil drop was balanced exactly against the force of gravity. After that the charge on the drop would be known. Millikan did this frequently and found that all the charges he calculated came in integer multiples only of a certain smallest value, that is the charge on the electron.