Crookes tube (X-ray generator)

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Crookes tube (X-ray generator)


Mineralogy; Physics; X-ray research


Physical Description: The body of the Crookes tube instrument consists of a copper cube, in the center of a metal stand. The stand consists of a long tube with three legs protruding from the base. There are two additional tubes that extend beyond the cube's surface. When looking at the instrument from the front, extending to the right is a glass cylinder that has copper tube extending the length of it on the inside. Less than halfway down the copper tube inside the glass cylinder is a metal ring that has holes along its circumference. Adjacent to the glass tube, pointing towards the viewer when looking at the instrument from the
front, is a metal tube. On the upper half of the instrument above the cube, there is a metal tube that narrows in diameter towards its top. The are several places that rubber vacuum tubes would attach to create a vacuum, two at the end of the glass cylinder, two at the end of the tube protruding from the top, and one directly attached to the main cube.

Functional Description: To operate the Crookes tube a voltage is applied between the metal electrodes at either end of the glass tube. The electric field produced by the application of a voltage causes the gas particles in the tube to accelerate and collide with other gas molecules. If the energy of the collision is high enough an electron will be forced off the gas molecule and a positive ion will form. This process of ionization will continue to occur as a chain reaction until most of the gas molecules in the tube have been ionized. The positive ions are then attracted to the negative cathode, and when the ions collide with the metal the electrons are removed from the surface. The voltage being applied to the tube causes the electrons to accelerate as they all move towards the positively charged anode at the other end of the tube. Due to the increased speed the electrons collide with the wall behind the anode which causes them to become excited. As the electrons become excited and return to their original energy level, x-rays are produced. The x-rays will then exit through the opening at the end of the metal tube which sits perpendicular to the glass tube. A sample can then be placed a certain distance away from the opening and when the x-rays hit the sample an image will be produced on a screen behind the sample.


Nicole Bliven, Shelby McGuire, Zach Nelson, Joseph Aldape, and Will Christian


c. 1940-1950




Physical object


United States of America

Physical Dimensions

Total height: 20.4cm
Stand to tip Including glass tube length: 27.5cm
Glass tube length: 20.4cm
Glass tube circumference: 9.7cm
Outward facing tube diam.: 2.3cm
Outward facing tube length from stand: 10.6cm


Copper, glass, rubber


Reynolds M. Denning, Houghton, Michigan

History of the Object

Not much is known regarding this particular Crookes tube, other than that it was built by Reynolds M. Denning in the mid 1940s. It is thought that the tube was constructed at the Michigan College of Mining and Technology (now Michigan Technological University), though even this isn’t known for sure. At the time that Denning designed and built his Crookes tube, this type of appliance was somewhat antiquated, with other, more efficient, X-ray and electron beam generators readily available. This would suggest that he constructed this particular device for his own enjoyment rather than for any research purpose. This would be consistent with his fascination with different instruments. Denning was known to design and construct many apparatuses, both for professional reasons and as a hobby. Denning’s research interests did, however, include crystallography, optics, and various physics topics, which may have inspired him to create a Crookes tube.

Denning was born on September 3rd, 1916 in Fitchburg, MA to William Wallace and Emma McConnell Denning. His father was a research electrical engineer, and young “Rey” received much of his first knowledge from him. However, Rey’s father died in 1932, and him and his mother moved to Rochester, NY. It was here that Denning got his first taste of academia when he performed physics experiments with the faculty at University of Rochester while still in high school.

After high school, Denning obtained his BS in geology in 1939 from the Michigan College of Mining and Technology. From here, he briefly worked with the Army Corps of Engineers in Arkansas before accepting a teaching fellowship in mineralogy through Stanford University. In 1942, Denning married Helen Green, then very quickly after, traveled to South America. Denning worked in Bolivia during World War II, researching mineralogy and geology regarding the mining of tin. He afterward returned to Houghton to complete his M.S. degree, writing his thesis on sandstone. This dissertation was titled "The Petrology of the Jacobsville Sandstone, Lake
Superior." Denning then continued his Ph.D. (which he began during his time at Stanford) at the University of Michigan. Denning wrote his doctoral thesis on the hardness of diamond and was granted his Ph.D. in 1953. In 1956, Denning became an associate professor at the University of Michigan where he continued research on diamonds, mixing in various aspects of optics. Denning was granted his full professorship in Minerology, and taught at the University of Michigan until his death in 1967.


A. E. Seaman Mineral Museum, Houghton, Michigan, catalog number: DM3323


Reynolds M Denning Memorial.” University of Michigan Faculty History Project. 2011, accessed 2/25/17.

Cloke, Paul L., and Edward H. Poindexter. "Memorial of Reynolds McConnell Denning." Mineralogical Society of America. March & April 1967. Accessed March 20, 2017.

Spiegel, Peter K. "The first clinical X-ray made in America--100 years." AJR. American Journal of Roentgenology 164, no. 1 (1995): 241-243.

Crookes, William. "The Bakerian Lecture: On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules." Philosophical Transactions of the Royal Society of London. Vol. 170. 135-164. 1878. Accessed March 20, 2017.

Dictionary of Scientific Biography, Edited by Charles Coulston Gillispie. Vol. 3. 474-481. NY: Charles Scribner's Sons, 1971.


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Nicole Bliven, Shelby McGuire, Zach Nelson, Joseph Aldape, and Will Christian, “Crookes tube (X-ray generator),” Michigan Tech Inventory of Historic Scientific Instruments, accessed April 17, 2021,