Robert A. MillikanRobert A. MillikanThe Man and his contributions

to The Atomic Theory

Robert a. millikan• Robert Andrews Millikan was one of the

most noted American physicists and scientists of his day. He was aware of the importance of the public's understanding of science, to which he wrote Science and Life in 1923. He is most honoured for his measurements of electrical charge, which is carried by an electron. Millikan's contributions to science were with the research of cosmic rays.

Biography• Robert Andrews Millikan was born on

March 22nd, 1868, in Morrison, Illinois. (U.S.A.) as the second son of the Reverend Silas Franklin Millikan and Mary Jane Andrews.

• At the age of 5, Millikan’s family moved to the state of Iowa where he went on to attend the Maquoketa High School.

• He entered Oberlin College (Ohio) in 1886 after working for a short time as a court reporter.

Biography• In 1891, he graduated from Oberlin College and took up a teaching

post in elementary physics for 2 years, where he developed his interest in physics.

• In 1893, he was appointed Fellow in Physics at Columbia University, after obtaining his mastership in physics.

• In 1895, he received his Ph.D. for research on the polarization of light emitted by incandescent light surfaces.

• Millikan spent a year (1895-1896) in Germany, at the Universities of Berlin and Göttingen.

• He became an assistant at the newly established Ryerson Laboratory at the University of Chicago in 1896.

Biography• 1n 1902, Millikan married Greta Erwin Blanchard. They had three

sons: Clark Blanchard, Glenn Allen, and Max Franklin.

• He became professor at Chicago University in 1910- 1921.

• Millikan made numerous momentous discoveries, chiefly in the fields of electricity, optics, and molecular physics.

• His earliest major success was the accurate determination of the charge carried by an electron, using the elegant "Oil-drop method"; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity.

• During World War I, Millikan was Vice-Chairman of the National Research Council, playing a major part in developing anti-submarine and meteorological devices.

Biography• In 1921, he was appointed Director of the Norman Bridge Laboratory of

Physics at the California Institute of Technology, Pasadena; he was also made Chairman of the Executive Council of that institute.

• In 1923, he held honorary doctor's degrees of some twenty-five universities, and was a member or honorary member of many learned institutions in his country and abroad. He has been the recipient of the Comstock Prize of the National Academy of Sciences, of the Edison Medal of the American Institute of Electrical Engineers, of the Hughes Medal of the Royal Society of Great Britain, and of the Nobel Prize for Physics.

• Professor Millikan has been President of the American Physical Society, Vice-President of the American Association for the Advancement of Science, and was the American member of the Committee on Intellectual Cooperation of the League of Nations, and the American representative at the International Congress of Physics, known as the Solvay Congress, at Brussels in 1921.

• He died on the 19th of December, 1953, in San Marino, California after devoting his life to discoveries of science and the universe.

Experiments and discoveries

• Millikan began a series of experiments in 1909 that studied electrical charges, more specifically, electrical charges carried by electrons. His discovery was accomplished by spraying minute drops of oil into a specially built chamber. It is known as the Millikan oil-drop experiment. He also verified Einstein's photoelectric equation and obtained a precise value for Planck's constant.

• In 1923 he won the Nobel prize in physics for the experiment and discovery. The University of Chicago had recently opened Ryerson Laboratory and this is where Millikan stayed until 1921.

Experiments and discoveries

• In 1921, at the Norman Bridge Laboratory of Physics at California Institute of Technology Millikan was appointed its director. CIT became famous, with his guidance, for the contributions it made to science. He carried out research on the phenomena of cosmic rays. He was also the pioneering road-paver for measuring cosmic ray intensity with specially built instrument carrying balloons. Millikan's deep devotion to cosmic-ray research was the foundation for establishing a research team at the California Institute of Technology which made the most basic of these discoveries. The meson*, which is a fundamental atomic particle, was the most important and significant discovery.

• *meson: an extremely unstable and heavy electron within cosmic rays. It is also found in particles with the same electric charge as an electron.

Millikan’s Oil-drop experiment

• In 1990, Robert Millikan determined the charge on a single electron by an experiment called, the Oil Drop Experiment. In order to do this, he put a charge on a tiny drop of oil and measured how strong of an electric field needed to be applied to stop the oil drop from falling. Once he was able to work out the mass of the oil drop and could calculate the gravitational force on one oil drop, he could then determine the electric charge on the drop. By varying the charge on different drops, he noticed that the charge on a single electron was -1.6x10-19

Coulombs (C).

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Millikan’s Oil-drop experiment

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In order to conduct the experiment, the apparatus used is shown below:

The atomizer sprayed a mist of oil droplets into the chamber and some of the tiny droplets fell through the hole on the floor of the top chamber into the bottom chamber.

Millikan’s Oil-drop experiment

The droplets were allowed to fall until they reached terminal velocity.Using the microscope, he measured their terminal velocity and calculated the mass of each oil drop, using the formula:

q x E = m x g

Where:

q- is the charge on the drop

E- is the applied electric field

m- is the mass of a drop

g- is acceleration due to gravity (9.8m/s2)

(the values of E,m and g are known values)

Millikan’s Oil-drop experiment

» Therefore:

He then illuminated the bottom chamber with X-rays, causing the air to become ionized and allowing electrons to attach themselves to the oil drops (this is how he applied a charge to the falling oil drops). He then applied an electric voltage to the top and bottom plates of the chambers, by attaching a battery, which acted on the oil drops. If the voltage was just right, the electromagnetic force would balance off the gravitational force on the drop, causing it to suspend in mid-air.

Millikan’s Oil-drop experiment

This allowed Millikan to determine the charge on a drop. The experiment was the repeated several times using varying strengths of x- rays so that differing numbers of electrons would attach themselves to the oil molecules each time, allowing him to obtain various values for q. He was able to determine the charge, q, on a drop, was always a multiple of -1.6x10-19 C, which was the charge on a single electron.

Animation of the experiment: http://www.youtube.com/watch?v=XMfYHag7Liw

BIBLIOGRPAHY

http://www.light-science.com/millikan.html

http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html

http://www.britannica.com/EBchecked/topic/382902/Robert-Andrews-Millikan

http://www.aip.org/history/gap/Millikan/Millikan.html

http://www25.uua.org/uuhs/duub/articles/robertmillikan.html