History of Chemistry

The Millikan Oil Drop Experiment

The Millikan Oil Drop Experiment

In this article, you will learn all you need to know (and more) about Robert A. Millikan’s Oil Drop Experiment. If you like this article, check out our other articles!

Who was Robert A. Millikan?

Robert A. Millikan was born on the 22nd of March, 1868 in Illinois, (U.S.A.). Growing up, Millikan spent most of his childhood living in a rural town called Morrison. Then, in 1875, his family relocated to Maquoketa, Iowa where Millikan started attending Maquoketa high school. Millikan excelled in his learning and decided to further his studies by attending Oberlin College in Ohio. During this time, Millikan started teaching a physics class and decided to pursue the subject as a career. He later obtained his Ph.D. in physics from Columbia University.

Robert. A. Millikan

After graduating from Columbia, Millikan traveled to the universities of Berlin and Göttingen. There, he furthered his knowledge within his field before returning to the United States to be an assistant at Chicago University’s Ryerson Laboratory. During his time there, Millikan authored (and co-authored) several physics textbooks. Eventually, in 1907, a research project of Millikan’s led to the development of the Oil Drop Experiment.

The Oil Drop Experiment

The Experiment

Devised by Robert A. Millikan and Harvey Fletcher, the Millikan Oil Drop Experiment is conducted in a chamber and is a method of measuring the electric charge of a single electron.

To elaborate, this chamber contains an atomizer, a microscope, a light source, and two parallel metal plates. These metal plates obtain a negative and a positive charge when an electric current would pass through them.

Experiment Chamber

The Procedure

First, the atomizer was to release a fine mist of oil that would drift within the chamber. While drifting, the droplets of oil would make their way into the bottom half of the chamber (between the metal plates) due to a gravitational pull. Here, the oil droplets would be ionized into being negatively charged. Thereafter, while these negatively charged droplets are being pulled down by gravity, the external power-dial would be used to add a charge to the two metal plates (above and below the droplets). Specifically speaking, the top plate would cultivate a positive charge, and a negative charge would be cultivated on the bottom plate.

This creates a situation in which the oppositely charged (positive) metal plate is pulling the negatively charged droplet upwards, while gravity is pulling the droplet downwards. Or in other words, the electrostatic and gravitational forces are now controlling the direction in which the droplet is flowing. Now, if the electrostatic force is greater, then the droplet would rise towards the positively charged plate. Likewise, if the gravitational force is greater than the electrostatic force, then the droplet would be pulled down.

Observations and Conclusion

The purpose of this experiment was to balance these two electrostatic and gravitational forces – which would cause the droplets to halt midair. By doing this, the droplet’s mass, gravitational force, and electrostatic force could be measured, revealing the charge of the electron. Furthermore, by doing these final calculations, Millikan was able to reveal that the charge of an electron would be multiples of 1.602×10−19 Coulombs.

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