This is one of a collection of phototubes from the University of Wisconsin that represents the efforts of the staff of the Department of Astronomy there, led by Joel Stebbins, to develop photoelectric sensor technology for astronomy in the first half of the 20th century. This is an example of an early Kunz cell, named for its creator Jakob Kunz. Kunz worked at the University of Illinois to develop photoemissive surfaces and he developed tubes like this one specifically to meet the unique needs of astronomers. While at the University of Illinois, Stebbins first used selenium photoresistors in conjunction with physicist F.C. Brown in 1907. Beginning in 1912 Stebbins began working with Kunz to develop and apply photoemissive devices to astronomical observation and they were the first to make significant observations with this type of Cell. Kunz Tubes contained a variety of metal coatings and electrode arrangements as each tube was unique and constructed for different properties, but always with the goal of greater sensitivity and signal to noise ratio. This specific tube has a guard ring set into the glass on one end to improve the tubes insulation. Photoemissive devices like those pioneered at Wisconsin did not come into widespread use in astronomy until after the Second World War, but later astronomical detectors combined the concept of Stebbins's early photometers with more advanced commercial photocells to make important observations on large telescopes like the 100 inch reflector at Mount Wilson. The University of Wisconsin donated this set of objects to the Museum in 2017.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.