Millikan, Robert

AMERICAN PHYSICIST
1868–1953

Born in Morrison, Illinois, Robert Andrew Millikan was the second son of the Reverend Silas Franklin Millikan and Mary Jane Andrews. When Millikan was seven, his family moved to Maquoketa, Iowa, where he attended high school. In 1886 he entered Oberlin College in Ohio. In 1887 he enrolled in several classics classes there, and because he did quite well in Greek, at the end of his sophomore year, he was asked to teach an introductory-level physics class. He enjoyed teaching physics and accepted a two-year teaching post at Oberlin upon graduation in 1891. It was during this period that he developed an even keener interest in physics.

In 1893 Millikan began his doctoral work at Columbia University, receiving a Ph.D. in 1895. After traveling to Germany, he eventually accepted a faculty position at the University of Chicago. It was as a teacher and textbook author that Millikan first made his mark. He wrote or cowrote a number of elementary physics texts that became the classics in this field. However, while valued activities, they did not lead to his promotion to full professor. Determined to ascend in academic rank, Millikan began his research into the charge on the electron.

At the time, the debate over whether or not atoms were real had almost played out, but the questions surrounding the true nature of the electron were still unanswered. Although the work of the English physicist J. J. Thomson had elucidated the charge-to-mass ratio, determining that the electron had a discrete, fixed charge and mass remained.

Being an experimentalist, Millikan used a tiny, submillimeter drop of oil suspended between capacitor plates to measure the incremental charge on an electron. His reasoned that the oil drop would pick up a charge due to friction as it entered the region between the plates. By ionizing the atmosphere and monitoring the motion of multiple drops, he was able to compare the time that the drop took to fall under the influence of gravity and with the electrical plates off, against the time that it took for the drop to climb under the influence of applied voltage . The interaction of the drop with the electric field always occurred in discrete units, indicating that the electron charge was a single value, and that it was the same value for all different forms of electricity.

Millikan's oil-drop experiment settled the argument and determined accurately (within one part in a thousand) both the charge and, by virtue of the charge-to-mass ratio, the mass of the electron. Both numbers allowed the Danish physicist Niels Bohr to finally calculate Rydberg's constant and provided the first and most important proof of the new atomic theory .

Millikan went on to demonstrate the photoelectron effect, providing a valuable proof of Albert Einstein's equations. His experiments also aided both Einstein and Bohr in their later research efforts. In 1923 he was awarded a Nobel Prize in physics for both his work in determining the charge on the electron and exploring the photoelectric effect.

see also Bohr, Niels; Einstein, Albert; Thomson, Joseph John.

Todd W. Whitcombe

Bibliography

Kargon, R. (1982). The Rise of Robert Millikan. Portrait of a Life in American Science. Ithaca, NY: Cornell University Press.

Millikan, R. A. (1950). The Autobiography of Robert A. Millikan. New York: Ayer Company Publishers.

Internet Resources

Millikan, R. A. (1913). "On the Elementary Electrical Charge and the Avogadro Constant." Physical Review 2:109–143. Available from <http://www.aip.org/history/gap/PDF/millikan.pdf>.

Millikan, R. A. (1924). "The Electron and the Light-Quanta from the Experimental Point of View." Nobel Lecture, May 23, 1924. Available from <http://www.nobel.se/physics/laureates/1923/millikan-lecture.pdf>.

"Robert A. Millikan—Biography." Nobel e-Museum. Available from <http://www.nobel.se>.