Between the years 1906 and 1914, the American physicist Robert Andrews Millikan devised an experiment to determine the magnitude of the charge of an electron. This experiment is often referred to as R.A. Milikan’s oil drop experiment. It involved the application of high potential difference between two horizontal metallic plates, which were held apart by an insulating material. When oil was sprayed into a chamber above the plates, it obtained an electric charge and began to rise. After certain calculations, the charge of the electron was found to be -1.6*10-19 Coulombs.
With the charge of the electron and the charge-to-mass ratio of an electron (which was found to be 1.75 * 1011 C/kg by the British physicist JJ Thomson), the mass of an electron was determined as 9.1094 * 10-31 kilograms.