Robert Millikan was a physicist who discovered the elementary charge of an electron using the oil-drop experiment. He charged droplets of oil between two electrodes and balanced the gravitational force with the upward forces, using mechanical equilibrium to determine the charge, which he found within one percent of the current accepted value. He also designed an experiment to verify Einstein’s equation for the photoelectric effect and found Einstein’s prediction about the linearity between energy and frequency to be correct. In 1923 Millikan was awarded the Nobel Prize in Physics "for his work on the elementary charge of electricity and on the photoelectric effect."