Photoelectric cell photometry

Lantern slide 'Potassium Photoelectric cell'

Photolectric cell

It was not until December 1912 before Kunz and another physicists, W. F. Schulz were ready to place a photometer with a potassium hydride photo emissive cell at the focus of the 12-inch refractor. They successfully measured the current produced by the light of Capella falling on the cell in December and the light of Acturus in April 1913.  At this time, Stebbins was on sabbatical in Europe. While there he discovered two other groups had also been working with photoelectric cells, paralleling the work at Illinois.  He visited the Berlin-Babelsburg Observatory where Paul Guthnick had been developing a photometer since 1912.  He used the photometer, attached to the 12-inch Zeiss-Respold refractor, to study Beta Cephei in 1913. Edgar Meyer and Hans Rosenberg, working independently of Guthnick, were also applying photoelectric cells at a private observatory near Tübingen Germany. Stebbins attended the meeting of the Astronomische Gesellshaft in 1913 where Rosenberg spoke on his work. Rosenberg continued to work with photocells until 1925 while Guthnick used them until his death in 1947.

Photograph 'Photoelectric photometer'

The photocells were mounted in a light-tight wood box attached to the drawtube of the telescope by four support rods. The photocells could be changed if desired, but most of the work done after 1916 used the first quartz photocell, numbered QK99. Inside of the cell box was the grounding key that controlled the current to the cell. The switch was thrown by pulling a cord.  Light from the star could be viewed by using a movable prism. A glass plate covered the entrance pupil. Different neutral shade glasses could be placed in the light path to reduce the intensity of the light and different diagrams also added. The box was continually pumped with dry air to prevent moisture from damaging the cells.  The air bubbled through two sulfuric acid washes bottles, located in the east closet next to the Equatorial room, and then was brought over to the telescope. 


 From the bottom of the cell box hung a gimbal arrangement that maintained the electrometer in a vertical position. Weights hung under the electrometer to help maintain position. A fine wire from the negative terminal of the cell was carried through the joint on amber bushings to the electrometer.  Deflections, due to the current produced by the photocell, were measured by the electrometer.  This instrument served two functions:  first accumulating the charge until the rising voltage was large enough to be measured and then the means to accurately measure the voltage.  The first electrometer was by Edelmann of Munich, but it was replaced by a special electrometer built by William Gaertner of Chicago.   Batteries located in the east closet provided power.

Lantern slide "U of I Eclipse station 1918, Rock Springs, Wyoming"

Eclipse expedition.

Stebbins and Kunz traveled to Rock Springs, Wyoming to observe the solar eclipse on 9 June 1918.  They arrived a week early to find an observing site and to arrange for a small shelter to be built there.  They also visited the Mt. Wilson and Yerkes expeditions stations nearby.  The study the eclipse, they used two cells mounted in wooden boxes at the end of long 4-inch tubes.  This was secured to an old 4-inch telescope mount.  The nearby hut held the galvanometer and standard lamp to compare brightness to.  Eclipse day began as a beautiful sunny day, but by first contact, clouds started to roll in.  Finally, two minutes before totality, the area of the sky around the cleared and they succeed in measuring the brightness of the solar corona.

Lantern slide 'Photoelectric photometer on 12-inch telescope (1925)'

Filter and shades used with the photoelectric photometer.

With most of the development on the photometer completed, Stebbins continued his search for eclipsing binary stars in 1916.  The photometer was rebuilt in 1919 by Dr. Elmer Dershem. The program entailed checking known spectroscopic binaries for variation in light due to eclipses of the stars.  His method of observation was to compare the suspect star to two or three nearby stars of the same spectral type. Analyses of the binaries orbit, derived from the spectrum, helped Stebbins determine when to study the star.  Numerous measurements of 45 suspect stars over a six-year period were made.  Stebbins continued this work until his departure from Illinois in 1922. When he moved to Washburn Observatory at the University of Wisconsin, he was able to transfer most of his photoelectric equipment with him.

Photoelectric photometer filters
National Historic Landmark plaque
Enter the photoelectric cell
Photoelectric cell photometry