Patent Model for Simplex Slide Rule Clasp Invented by Rudolph C. Smith

This ten-inch, linear one-sided slide rule has scales on the base labeled 3 and 2. On one side of the slide, scales are labeled 4 and 1. On the other side are scales labeled S and T and a scale of equal parts, which divides each inch into 50 increments. The 4, 1, and 2 scales are identical, divided logarithmically from 1 to 10 twice (like the A and B scales on a Mannheim slide rule). The 3 scale is graduated logarithmically once from 1 to 10 (like the C and D scales on a Mannheim rule, although the numbers on this scale are marked with superscript 2s; i.e. 22).
Under the slide is a scale of centimeters numbered from 27 to 51 and divided to millimeters. The upper edge of the instrument is beveled and has a scale of inches divided to 32nds of an inch. The front edge has a scale of centimeters numbered from 1 to 25 and divided to millimeters.
There is also a brass clasp (detached at present) that holds three paper strips underneath the instrument, so that they may be pulled or fanned out for reference. Smith submitted this model when he patented this clasp in 1887. The strips contain 39 sets of formulas and conversion factors useful to civil engineers, including the weight and strength of materials and the power of engines and pumps. Smith copyrighted these strips in 1884 and 1886. See also his pamphlet, Smith's Slide Rule Formulæ ([New York, 1884]).
Rudolph Charles Smith of Yonkers, N.Y., received more than twenty patents for elevator components and slide rules from the 1880s to 1912. He apparently worked for Otis Brothers & Co., since many of the patents were assigned to that firm or to National Company of Illinois, which merged with Otis Brothers and other firms in 1898 to form the Otis Elevator Company. Elisha Otis started the company in 1853 as Union Elevator Works to sell his safety elevator. His sons, Norton and Charles, adopted the Otis Brothers name in 1864.
According to an order form dated January 1889 held by the Smithsonian Institution Libraries, Smith's Patent Calculator sold in four forms. One had attached slips with formulae of interest to civil engineers, a second had formulae for marine engineers, a third had formulae for mechanical engineers, and a fourth was intended for "assayers, chemists, scientists, students." The instrument cost 50 cents with one set of attachments or 75 cents with all four sets. This example appears to be the instrument for civil engineers. An unsigned review of Florian Cajori's 1909 history of the slide rule criticized Cajori for ignoring Smith's contributions to popularizing the slide rule and educating Americans in its use.
References: Rudolph C. Smith, "Attachment for Calculating Scales" (U.S. Patent 357,346 issued February 8, 1887), "Slide-Rule for Logarithmic Calcuations" (U.S. Patent 450,640 issued April 21, 1891), "Calculating-Scale" (U.S. Patent 592,067 issued October 19, 1897), "Calculating-Scale" (U.S. Patent 746,888 issued December 15, 1903), "Calculating-Scale" (U.S. Patent 1,014,344 issued January 9, 1912); Annual Report of the Commissioner of Patents for the Year 1891 (Washington, D.C.: Government Printing Office, 1892), 339; Practical Applications of Smith's Electro-Calculator (New York, 1894); R. C. Smith, The International Book of Shorthand Computation ([New York, 1900]); "Elevator Trust Sued," New York Times, March 8, 1906; review of A History of the Logarithmic Slide Rule and Allied Instruments by Florian Cajori, Mines and Minerals 30, no. 12 (July 1910): 740.
Currently not on view
Object Name
slide rule
date made
Smith, Rudolph C.
Physical Description
wood (overall material)
paper (part material)
brass (part material)
overall: 26 cm x 2.8 cm x .8 cm; 10 1/4 in x 1 3/32 in x 5/16 in
ID Number
catalog number
accession number
Rule, Calculating
Patent Models
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Slide Rules
Data Source
National Museum of American History, Kenneth E. Behring Center


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