A telescope rigged with a photometer, circa 1910-1915.

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Sometimes a splash of serendipity is needed for an innovative idea to become a reality. This was certainly the case for Joel Stebbins, F.C. Brown and Jakob Kunz when they collaborated to create the photoelectric photometer.

Joel Stebbins was one of the early directors of the University of Illinois Observatory. Part of Stebbins’ work was measuring the brightness of stars. In the 19th century, this was done through visual observations or by using photographs to calculate brightness. Visual observations were the least accurate, and while photographic methods were better, there was still room for improvement.

Stebbins recalled at an awards ceremony his wife would occasionally help him at the observatory. She noted it was a tedious process, and Stebbins commented that one day it might be done using electricity, which could make it faster. He recalled every so often she would ask when the recordings could be done with electricity.

Through some lucky happenstance, a few months later, Stebbins was at a physics open house at the University of Illinois. He observed F.C. Brown’s experiment where a lamp was turned on to illuminate a selenium cell. When the light fell on the cell, it would cause a bell to ring.

Stebbins developed a friendship with the physics professor, and Brown and Stebbins worked to design a selenium cell that could measure a star’s brightness and changes in light in space. This cell led to several discoveries and measurements, including the light curve for the moon, the 1907 lunar eclipse and the magnitude of Halley’s comet.

Stebbins was not completely satisfied with the selenium cell. The cells were not very sensitive, and it could only record bright astronomical changes.

Another physicist on campus, Jakob Kunz, had been working on photovoltaic cells (also known as photoelectric cells). The photoelectric cells’ electrical characteristics, such as voltage, varied when exposed to light and were more sensitive than the selenium cells.

Stebbins worked with Kunz to improve what he had started with Brown. Kunz and Stebbins’ photoelectric photometer became the most accurate instrument in photometry.

The cells Kunz created were more sensitive than commercially-made photoelectric cells and could detect fainter stars than the selenium cells.

This new technology allowed for measuring the diameter and masses of stars, as well as discovering new stars previously unable to be detected.

The Illinois Distributed Museum has online content about the innovations that have come from the University of Illinois, as well as self-guided tours of campus where you can view objects and buildings related to these innovations.

The Illinois Distributed Museum is a project under the direction of the University of Illinois Archives. See more at

A little inspiration from a spouse, a chance meeting shortly afterward and continuous collaborations resulted in another innovative technology and new understandings of the universe from the University of Illinois.

Kristen Wilson is the Illinois Distributed Museum coordinator at the University of Illinois Archives, in the University of Illinois Library. She can be reached at

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