Prewett Addograf

This aluminum device consists of two discs sealed together at the rim, with a rotating disc in between. Various numbers are stamped around the rim of the rotating disc. Openings in the outer discs reveal three numbers on either side at one time. One side of the instrument has the numbers from 1 to 20 stamped clockwise around the scalloped rim of the movable disc. The other side of this disc has the numbers from 21 to 40, also stamped clockwise.
At the top of the instrument, three alternate numbers are visible (i.e., 1, 3, 5). Three alternate numbers also are visible on the reverse side (i.e., 35, 37, 39). The sum of two numbers on opposite sides of the disc is always 40 (i.e., 1 and 39). Part of the scalloped edge of the movable disc is exposed at the bottom.
Clay W. Prewett and the Prewett Addograf and System Company (also known as the Prewett System Company) of Los Angeles, California, sold this device. A 1940 brochure describing “The Prewett Addograf and System” indicated that it consisted of not only this instrument but a $10 brochure describing how it worked, a $5 brochure on modern short cuts in multiplication, division, interest, fractions, and mixed numbers; and a $5 multiplication chart. The entire system could be purchased for $15. It was not returnable.
Currently not on view
Object Name
date made
Prewett System Company
Physical Description
aluminum (overall material)
overall: .5 cm x 7.5 cm x 7 cm; 3/16 in x 2 15/16 in x 2 3/4 in
place made
United States: California, Los Angeles
ID Number
catalog number
accession number
Science & Mathematics
See more items in
Medicine and Science: Mathematics
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of L. Leland Locke

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