Eastman X-Ray Exposure Rule (1920s)

The might not be the first slide rule developed to perform radiological calculations, but it is one of the earliest. The patent date is 1919, hence my guess that it dates from the 1920s.

Eastman X-Ray Exposure Rule (1920s)

An Eastman catalog from 1926 describes it as follows:

"A Guide to Correct Exposure. As the X-ray approaches universal use, standardization of radiographic technique becomes imperative. The Eastman X-Ray Exposure Rule is a material step in this direction. It is based upon the results of scientific study in the Research laboratories of the Eastman Kodak Company, and both its utility and its essential accuracy have been verified in the practice of prominent roentgenologists."

The slide rule is operated from right to left. First, the right slide is adjusted so that the tube distance (to the plate) is next to the part of the body being X-rayed. Second, the left slide is positioned so that the tube current in milliamperes is adjacent to the minimum spark gap distance (inches) that prevents an arc. Finally, the required exposure time is found on the left side of the rule next to the type (material) of the film or plate being used. The calculations assume that the patient is 150 pounds.

According to Burns (1999), the first radiographic slide rules were produced in Germany (ca. 1905-1909). Shortly thereafter (ca. 1910-1912), a couple of English versions of these were produced. One was described in a catalog of Cavendish Electrical Co. (London) that was printed sometime around 1912. The other appeared in a 1914 catalog published by Schall & Son of London.

Eastman X-Ray Exposure Rule (1920s)
Eastman X-Ray Exposure Rule (1920s)

Note: The original plastic top had aged to a dark brown—sufficiently dark that the text on the top of the slide rule was difficult to read. As such, I replaced it with a clear colorless one.

Price: $2.50

Size: 2.5" x 5"


  • Burns, J.E. Radiographic Exposure Slide Rules. Brit. J. Radiol. 72:48-54. 1999.
  • Eastman Kodak Company. X-ray materials and Accessories. 1926.
  • Eastman Kodak Company. X-rays. 1926.