Robert Millikan's primary contribution to atomic theory came as a result of his oil-drop experiment, which measured an electron's charge. Previously, J.J. Thomson had suggested the hypothesis that an electron's mass was at least 1,000 times less than the mass of the tiniest atom. Millikan's contribution to this hypothesis was to specify the size of the electron's charge.
Millikan's oil-drop experiment involved spraying oil from a perfume atomizer into a sample chamber. Some of the droplets traveled through a pinhole into a space between two plates, one of which had a negative electric charge and the other of which had a positive charge. This central chamber was then ionized with X-rays. Particles that failed to catch electrons followed gravity to the bottom plate, while particles that managed to catch at least one electron floated up to the positive plate or fell at a slower rate of speed. Millikan reached the conclusion that each drop had a charge that was a multiple of 1.59 x 10^(-19) Coulombs.
Later, Millikan also verified Albert Einstein's photoelectric equation and developed the first photoelectric calculation of Planck's constant h. Later studies involved hot-spark spectroscopy of the various elements, pushing the ultraviolet spectrum far past the known lowest limit, and his motion law for a particle falling earthward after coming into the atmosphere, which led to his radiation studies.