Novotni Weight Slide Rule

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This black Bakelite instrument is held together with metal screws. It calculates the weight of metal required by a blacksmith, foundry worker, shopkeeper, or similar worker, given the dimensions and shape of a metal part to be produced. The top of the base has a logarithmic scale that gives the thickness of the part in inches. There are two slides. The upper slide has a logarithmic scale of widths (in inches) along its upper edge and a logarithmic scale of lengths (in inches or feet) along its lower edge.
The upper edge of the lower slide has two scales for possible metals, one for lengths measured in inches, one for lengths in feet. The metals are copper, brass, steel, cast iron, lead, aluminum, and water. The lower edge indicates the shape of the piece. The bottom of the base has a logarithmic scale that gives the weight in pounds.
The center top of the base is marked: WEIGHT SLIDE RULE. The bottom left corner is marked: Novotni (/) Slide Rule (/) Media, Pa. The bottom right corner is marked: Patented (/) Others Pending.
John L. Novotni (1893–1966), a blacksmith of Czech descent, is listed in the 1920 census as working in a foundry in Delaware County, Penn. In the 1910s, he worked at an automobile and wagon repair shop in Oaklyn, N.J., with Andrew W. Kelly and John Hornyak, who received a patent for this design in 1917 and sold it as the Kelkay Weight Slide Rule by 1920. By 1922 Kelly and Hornyak and Novotni were selling other celluloid rules, such as the Paper Box Board Rule. By 1929 Novotni was selling slide rules under the name of the Novotni Slide Rule Company and was having the rules made of Bakelite. In 1937, he moved to Media, Pa. According to Novotni's daughter, Evelyn Novotni Bond, steel companies continued to purchase the Weight Slide Rule until 1992.
References: Andrew W. Kelly and John Hornyak, "Slide Rule" (U.S. Patent 1,210,488 issued January 2, 1917); "The Kelkay Weight Slide Rule," American Blacksmith 19, no. 11 (August 1920): 290; Directions for the use of the Novotni Weight Slide Rule (Media, Pa., about 1920),; "Firm Tailors Slide Rules to Specific Tasks," Steel 155 (1964); accession file.
Currently not on view
date made
1937-ca 1959
Novotni Slide Rule
place made
United States: Pennsylvania, Media
Physical Description
plastic (overall material)
metal (part material)
overall: 6.8 cm x 30.3 cm x .6 cm; 2 11/16 in x 11 15/16 in x 1/4 in
ID Number
accession number
catalog number
Credit Line
Gift of Michael Konshak
Rule, Calculating
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Slide Rules
Data Source
National Museum of American History


I just got one of these at a hamfest. The 2 slides allow for 2 multiplications to get the volume, then the tick marks convert to weight based on type of metal or material. Clever!

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