Robert Milikan won the 1927 Nobel Prize in Physics "for his work on the elementary charge of electricity and on the photoelectric effect." Perhaps Milikan’s most famous experiment is the oil drop experiment in which he had charged oil drops suspended in space by an electric field. When the electric field is turned off the charged oil drops fall due to gravity. By careful measurements he was able to determine the amount of charge of the electron. He also made precise measurements of Einstein’s photoelectric effect and Planck’s constant that related energy and frequency.