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Robert Millikan: Biography, Atomic Theory & Oil Drop Experiment

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  • 0:00 Who Was Robert Millikan?
  • 1:20 The Oil Drop Experiment
  • 3:49 Controversy and the…
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about the life and achievements of American physicist Robert Millikan. His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory.

Who Was Robert Millikan?

When you think of famous physicists, Robert Millikan may not be the first one that comes to mind. However, his accomplishments - in particular in the area of electric charge and atomic theory - are numerous. Born and raised in the American Midwest by his minister father, Robert Andrews Millikan attended Oberlin College in Ohio as an undergraduate and upon graduation he enrolled at Columbia University. He holds the distinction of being Columbia's first physics Ph.D. recipient.

He spent the majority of his professional career as professor at the University of Chicago and laboratory director at CalTech, teaching and conducting research in the areas of electric charges, the photoelectric effect, and extraterrestrial radiation. He briefly served in the United States Army Signal Corps during World War I, working on meteorological devices. Millikan was married with three sons, all of whom pursued careers in sciences. His religious upbringing led him to give many lectures and write papers over the years detailing his belief that religion and science should be in harmony with one another, as opposed to the opposition and divisiveness the two subjects tend to lend themselves to. Robert Millikan passed away from a heart attack at the age of 85.

The Oil Drop Experiment

The oil drop experiment was perhaps the most famous scientific work of Robert Millikan's career. While at the University of Chicago, he worked with one of his graduate students, Harvey Fletcher, to attempt to measure the charge of an electron. Up until this point in time, around 1913, the world's view of what's going on inside of an atom was primitive by today's standards. Earlier experiments by J.J. Thomson had shown that atoms contain some type of negatively charged particle, and that these particles were of a mass significantly smaller than that of a hydrogen atom. Exactly what the magnitude of that negative charge was remained unknown. It also wasn't completely accepted yet that subatomic particles (particles smaller than an atom, that are contained within an atom) even existed, so the whole field of atomic theory was really coming of age in the early part of the 20th century. It was the goal of Millikan and Fletcher to quantify the charge of an electron, and they more or less succeeded, as you will see.

The oil drop experiment was conducted using this apparatus.
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The experiment itself was somewhat simple in design, when you compare it to modern day particle accelerators and such. It is called the oil drop experiment because it involved just that - dropping oil between electrodes (electrically charged plates). The force of those droplets passing through the electric field was measured, as was electrical field between the electrodes, and in measuring and calculating those quantities, they were then able to determine the charge on a single electron. After numerous trials, and standardizing their measurements since not all oil droplets were exactly the same size, Millikan and Fletcher concluded that the charge of an electron, or the elementary charge (e), was equal to 1.5924(17) x 10^-19 C. Coulombs (C) are the SI unit of electric charge, e is used to designate electron charge in lieu of writing out the entire numerical value each time it is used, and the final two parenthetical digits represent the accepted uncertainty in the final decimal places. Not only were the pair able to conclusively quantify an electron's charge, e, but in doing so were able to prove that electrical charges are quantized(single, discrete values), as opposed to continuously changing values as was the conventional thought at the time. It was truly a groundbreaking experiment.

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