Snapshots from Physics History

Miss Mitchell's Comet

Maria Mitchell
NOAA Central Library
Maria Mitchell

Maria Mitchell, the first female professional astronomer in the United States, was also the first to discover and chart the orbit of a new comet, which became known as “Miss Mitchell’s Comet.”

As a young woman, Mitchell worked briefly as a schoolteacher, then as a librarian at the Nantucket Atheneum, while still continuing her astronomical observations. Every chance she got, if the night was clear, Mitchell would go to the roof of the house to “sweep the heavens,” using the family's 2-inch reflecting telescope.

On the evening of October 1, 1847, Mitchell noticed a small blurry streak, invisible to the naked eye, but clear in the telescope, and she guessed at once that it might be a comet. She recorded the object's position, and continued to observe it to be sure it was a comet. On October 3, Mitchell's father sent off a letter to Cambridge announcing the discovery. This brought Mitchell immediate international fame, and further honors.

Mitchell made many other astronomical observations during her career, including observations of sunspots, comets, nebulae, stars, solar eclipses, and the moons of Saturn and Jupiter. She died on June 28, 1889. The Maria Mitchell Observatory on Nantucket is named after her, as is the Mitchell crater on the moon.

Millikan’s oil drop experiment

Millikan's appatus
Millikan’s apparatus

Robert Millikan’s famous oil drop experiment, reported in August 1913, elegantly measured the fundamental unit of electric charge. The experiment has been called one of the most beautiful in physics history.

J.J. Thomson had discovered the electron in 1897 and had made rough measurements of the particle’s electrical charge using clouds of water. Millikan improved upon these measurements by trying to determine the charge on individual droplets. But the droplets of water evaporated too quickly for accurate measurement. He asked his graduate student, Harvey Fletcher, to figure out how to do the experiment using some substance that evaporated more slowly.

Fletcher quickly found that he could use droplets of oil, produced with a simple perfume atomizer. The oil droplets are injected into an air-filled chamber and pick up charge from the ionized air. The drops then fall or rise under the combined influence of gravity, viscosity of the air, and an electric field, which the experimenter can adjust. The experimenter could watch the drops through a specially designed telescope, and time how fast a drop falls or rises. After repeatedly timing the rise and fall of a drop, Millikan could calculate the charge on the drop.

In 1910 Millikan published the first results from these experiments, which clearly showed that charges on the drops were all integer multiples of a fundamental unit of charge. Millikan then improved on his experiment to collect more data. He published the new, more accurate results in August 1913 in the Physical Review. Millikan won the 1923 Nobel Prize for the work, as well as for his determination of the value of Plank’s constant in 1916.

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