His “Oil Drop” Experiment Measured the Charge of the Electron.
Introduction
Robert Andrews Millikan (1868–1953) was an American physicist known for his work on measuring the elementary electronic charge and his studies of the photoelectric effect.
Early Life and Education
After working for a short time as a court reporter, he entered Oberlin College (Ohio) in 1886. During his undergraduate course his favourite subjects were Greek and mathematics. In 1893, after obtaining his mastership in physics, he was appointed Fellow in Physics at Columbia University. He afterwards received his Ph.D.
On the instigation of his professors, Millikan spent a year (1895-1896) in Germany, at the Universities of Berlin and Göttingen. He returned at the invitation of A. A. Michelson, to become assistant at the newly established Ryerson Laboratory at the University of Chicago (1896).
Millikan’s experimental skills and meticulous approach led to groundbreaking discoveries in physics.
Contributions
Millikan’s key contributions include his “Oil Drop” experiment, conducted with Harvey Fletcher at the California Institute of Technology in 1909, measured the elementary electric charge (the charge of the electron).
- The experiment involved observing tiny charged oil droplets between two horizontal metal electrodes.
- First, the terminal velocity of a droplet was measured with no applied electric field.
- At terminal velocity, the drag force equals the gravitational force. These forces depend on the radius in different ways, allowing the droplet’s radius, mass, and gravitational force to be determined using the known oil density.
- Then, an adjustable voltage was applied to create an electric field, and the voltage was adjusted until the drops were suspended in mechanical equilibrium, indicating balanced electrical and gravitational forces.
- Using the known electric field, Millikan and Fletcher calculated the charge on the oil droplet.
- By repeating the experiment with many droplets, they confirmed that the charges were all small integer multiples of a base value, found to be 1.5924(17)×10-19 C, within 1% of the currently accepted value.
- They proposed this as the (negative of the) charge of a single electron.
Vision
Millikan’s oil drop experiment provided a precise and direct measurement of the fundamental unit of electric charge. His work solidified the understanding of the quantized nature of electricity.
Legacy
Millikan’s oil drop experiment is a cornerstone of modern physics, demonstrating the discrete nature of electric charge. His work had a profound impact on the development of atomic and subatomic physics. He won the Nobel Prize in Physics in 1923.