4.12 - Millikan's Oil Drop Experiment

​The man who measured properties of the electron was Robert Millikan (1868-1953). He taught himself physics while a student at Oberlin College in Ohio since there was nobody on the faculty to instruct him in this field.  Millikan completed postgraduate research training in the U.S. and in Germany.  His studies on the properties of the electron proved to be of great value in many areas of physics and chemistry.

​Millikan carried out a series of experiments between 1908 and 1917 that allowed him to determine the charge of a single electron, famously know as the oil drop experiment.

He sprayed tiny drops of oil into a chamber. In his first experiment, he simply measured how fast the drops fell under the force of gravity.  He could then calculate the mass of the individual drops.

​Then he sprayed oil drops and applied an electrical charge to them by shining X-rays up through the bottom of the apparatus.  The X-rays ionized the air, causing electrons to attach to the oil drops. The oil drops picked up static charge and were suspended between two charged plates. Millikan was able to observe the motion of the oil drops with a microscope and found that the drops lined up in a specific way between the plates, based on the number of electric charges they had acquired.

​By adjusting the voltage between the plates, Millikan could balance the gravitational force pulling the droplets down with the electric force pulling them up, enabling him to calculate the charge on each droplet, which was always a multiple of a fundamental unit, the charge of an electron.

• ​Millikan used the information to calculate the charge of an electron. He determined the charge to be 1.5924 × 10^-19 C, where C stands for coulomb, which is one ampere*second.

• Today the accepted value of the charge of an electron is 1.602176487 × 10^-19 C. Millikan's experimental value proved very accurate; it is within 1% of the currently accepted value.

• Millikan later used the information from his oil drop experiment to calculate the mass of an electron. The accepted value today is 9.10938215 ×10^-31 kg.

• The incredibly small mass of the electron was found to be about 1/1840 the mass of a hydrogen atom.