Schall & Son X-Ray Exposure Rule (ca. 1912)
This beauty is the oldest slide rule in the ORAU collection. I am basing the estimated date of manufacture (ca. 1912) on a description of this device in the paper "Radiographic exposure slide rules" (Burns 1999). There is nothing on the slide rule itself to indicate the maker or the date of manufacture. It is identified as a Schall & Son slide rule because it was first described in a 1914 Schall & Son catalog of electrical equipment (unfortunately, I have not managed to obtain a copy of this catalog). Burns notes that it "is unlikely to date back earlier than about 1910 because the range of exposure times on the scale (0.25-120 s) is appropriate to the use of an early induction coil... rather than an electrostatic generator which would have needed longer exposures." Induction coils were in use well before 1910, but I agree that this slide rule is unlikely to be older than 1910.
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 were produced. One was described in a catalog of Cavendish Electrical Co. (London) that was printed sometime around 1912. The other, this one, appeared in the aforementioned Schall & Son catalog.
Size: 2.5" x 7"
Burns, J.E. Radiographic Exposure Slide Rules. Brit. J. Radiol. 72:48-54. 1999.