Robert Andrews Millikan
Abstract
Robert Andrews Millikan (1868–1953) was an American experimental physicist renowned for his precise measurement of the elementary electronic charge and his experimental verification of Einstein's photoelectric equation. Born in Morrison, Illinois, he pursued his education at Oberlin College and Columbia University, later conducting seminal research at the University of Chicago and serving as chairman of the executive council of the California Institute of Technology. Awarded the Nobel Prize in Physics in 1923, Millikan's meticulous methodologies established enduring benchmarks for experimental physics.
Early Life and Education
After working for a short time as a court reporter, Millikan entered Oberlin College in Ohio in 1886. During his undergraduate course his favorite subjects were Greek and mathematics. In 1893, after obtaining his mastership in physics, he was appointed Fellow in Physics at Columbia University, where he subsequently 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 an 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, which 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 \(1.5924(17) \times 10^{-19}\text{ C}\), 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 stands as 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, culminating in his receiving the Nobel Prize in Physics in 1923.