Robert Millikan made numerous discoveries in the fields of optics, electricity and physics. One of his most important discoveries was the determination of the charge of an electron. His calculations were instrumental in proving that electrons are fundamental, or indivisible, particles.
Millikan is also known for discovering cosmic rays, which are forms of ionizing radiation from outer space. The study of cosmic rays is essential in determining the composition and acceleration of the sun and other formations in space. Millikan was appointed to the National Research Council during World War I and made important contributions to the development of anti-submarine devices.