Robert Millikan

19th Century Scientist

Personal Backround

Millikan was born on March 22, 1868. When Robert was in college he was asked to teach elementary physics. While teaching Robert was asked to teach elementary school physics. While teaching physics he earned his bachelors and masters degrees. Millikan then participated in the war effort in Washington as third vice-chairman, director of research, and executive officer of the National Research Council. After the war Millikan returned to the University of Chicago, where he began several research projects. In 1921, he went to the California Institute of Technology as chairman of its Executive Council and director of the Norman Bridge Laboratory of Physics.

Expiremental designs

  • In Wilson's experiments, air was compressed in a cloud chamber, ionized with x-rays, and then rapidly expanded, causing tiny water droplets to condense on the ions and form a mist. These droplets were allowed to fall, either under the influence of gravity alone or under the influence of gravity plus an electric field. By observing their velocities of fall in the first case, Wilson used Stokes' law to calculate their radii; by observing their velocities in the second case, he could then calculate the magnitude of the charge they carried--which Wilson found to vary between wide limits. The atomicity, or definiteness, of the charge of the electron was therefore still very much in doubt.

Previous Experiments

  • In 1897 physicist J.J Thomson discovered the electron. This led to the begging of knowledge of atomic structure. He showed that charge came in discrete quanta and indirectly measured the charge-to-mass ratio but his method could not measure the charge or mass directly. Thomas then tried to observe the behavior of falling charged water drops but it was inaccurate because the drops evaporated during his experiment so he could only estimate how fast they evaporated in order to interpret the results.

Roberts experiment

  • Robert completed an experiment based on Thomson's earlier attempt. however robert used oil drops instead of water drops. This stopped the evaporation problem basically ended the uncertainty of the experiment. Due to its success the experiment is referred to as Millikan's oil drop experiment, which was extremely accurate for its time.


    During the process of the experiment, oil drops were first sprayed between the plates of a parallel-plate capacitor. A microscopeinstalled in the capacitor then looked over the oil drops. The drops soon reached terminal velocity downwards due to the balance of air resistance and their weight. The drops then became charged in the spraying process, so an electric field provided by the capacitor was used to make the drops rise and not fall. The mass of the drops was determined by measuring their radii, as their density was known.

Millikan's oil drop experiment to determine charge of an electron - Chemistry