Robert Millikan

Robert Andrews Millikan was a highly influential American experimental physicist.

He is best known for his precise measurements of the elementary charge of electricity. In 1923, he received the Nobel Prize in Physics for this work and for his studies on the photoelectric effect. Beyond his research, Millikan was a major leader in scientific education. He served as the Chairman of the Executive Council at the California Institute of Technology, known as Caltech. Under his leadership from 1921 to 1945, Caltech became a leading research institution in the United States.

Millikan's most famous achievement was the oil drop experiment conducted in 1909.

Before this, scientists like J. J. Thomson knew the charge-to-mass ratio of an electron. However, the specific values for the charge and mass of a single electron remained unknown. Working with his graduate student, Harvey Fletcher, Millikan used tiny droplets of oil to solve this mystery. He suspended these charged oil droplets between two metal electrodes. By applying an electric field, he could exert a force on the drops to counteract gravity. By measuring the force required to hold the drops in place, he could determine their charge.

This experiment provided a vital discovery about how electricity works. Millikan observed that the charge on every droplet was an integer multiple of a single, common value. This value, which he measured as 1.592 × 10⁻¹⁹ coulombs, is the elementary charge of a single electron. While modern measurements place the charge slightly higher at 1.602 × 10⁻¹⁹ coulombs, Millikan's result was remarkably accurate. His error was likely caused by using an inaccurate value for the viscosity of air. This experiment proved that electric charge is quantized, meaning it exists in discrete, set amounts rather than as a continuous flow.

Millikan also performed extensive research on the photoelectric effect. This is the process where light hitting a metal surface causes it to emit electrons. In 1905, Albert Einstein proposed that light behaves as a stream of particles called photons. Millikan was initially skeptical of this theory because light was widely viewed as a wave. He spent a decade building a "machine shop in vacuo" to create extremely clean metal surfaces for testing. His results, published in 1914, confirmed Einstein's mathematical predictions with great precision.

Although he resisted Einstein's interpretation for years, Millikan eventually accepted the photon theory of light in his 1950 autobiography.

Later in his career, Millikan focused his attention on cosmic rays. He actually coined the term "cosmic rays" to describe these high-energy phenomena. During the 1930s, he entered a famous scientific debate with Arthur Compton. Millikan believed these rays were high-energy photons that helped create new atoms in space. Compton argued that they were actually charged particles. Eventually, observations showed that cosmic rays are deflected by the Earth's magnetic field. This proved that Compton was correct and that the rays consist of charged particles.

Millikan's academic journey began in his home state of Illinois. He attended Oberlin College, earning both a B.A. and an M.A. before receiving his Ph.D. from Columbia University in 1895. His early career included a year of study at the Universities of Berlin and Göttingen in Germany. He later became a professor at the University of Chicago in 1910. In 1917, George Ellery Hale encouraged him to move to Pasadena, California. This move led to his long tenure at Caltech, where he directed the Norman Bridge Laboratory of Physics.

Throughout his life, Millikan contributed to many different areas of society. He coauthored several influential physics textbooks that used conceptual questions to help students learn. During World War I, he served as Vice Chairman of the National Research Council. He helped develop tools for meteorology and anti-submarine warfare. Following the 1933 Long Beach earthquake, he chaired a committee to improve building codes for earthquake protection. Millikan died in 1953 at the age of 85. His life reflected a deep commitment to both the rigorous pursuit of physical truths and the advancement of scientific institutions.