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The Most Famous Successful Experiment
In a recent article titled The Most Famous Failed Experiment, we saw how Albert Michelson and Edward Morley in 1887 failed to prove the existence of ether and left scientists without a transmitting medium for light. But failure and success sometimes can co-exist. The Michelson-Morley experiment opened the path to Einstein’s relativity theory which became a cornerstone of modern physics.
We shall take a more positive outlook today and describe one of the most successful experiments of the twentieth century. The oil drop experiment performed by Robert Millikan and Harvey Fletcher in 1909 to measure the charge of the electron was a stunning success in its scientific outcome and yet very controversial in its circumstances.
I won’t dwell on the scientific details, which most of you have learned in high school, but we can just briefly review the experimental concept. The apparatus was a closed chamber with two horizontal metal plates connected to an external battery. Oil drops were sprayed into the space between the metal plates. With the use of X-rays, the oil drops were given an electric charge. With the battery turned off there was no electric field, and the oil drops would fall freely because of gravity. When the battery was turned on, the electric field would work against gravity and the oil drops would be suspended in mid-air. By equating the force of gravity with the electrostatic force, Millikan and Fletcher were able to calculate the electric charge of individual oil drops.
The experimental set up was ingenious both in its simplicity and its scientific substance. Millikan and Fletcher made numerous measurements with oil drops of various sizes and found that the charged oil drops had an electric charge which was an integer multiple of a minimum value. The results were stunning. It turned out that electric charges are quantized, just like Planck’s energy quanta.
Millikan received the Nobel Prize in Physics in 1923, for his work on the elementary…
