Felsenthal FAE-21A Artillery Protractor

This clear plastic semicircular protractor is divided by tens and marked by hundreds from 100 to 3,100 mils and from 3,300 to 6,300 mils. Mils are units used in artillery and other military applications; 1,000 mils is equivalent to 56.25°. A pinhole is at the origin point. A second hole with diameter 3mm appears just above the origin point. The bottom edge of the protractor is divided by hundreds and marked by thousands from 5,000 to 0. A vernier appears to the right of the scale. The scale is labeled: 1/25,000 METERS. Superimposed on the scale is a second scale, divided by two hundreds and marked (in red) by two thousands from 0 to 10,000. A vernier appears to the left of that scale, which is labeled (in red): 1/50,000 METERS. The red markings are significantly faded.
The interior of the protractor has a relatively small semicircular opening with a groove parallel to the origin point. Below the opening is a maker's mark: 12 INCH (/) FELSENTHAL INSTRUMENTS CO. (/) CHICAGO (/) FAE-21A. The letters U.S. are printed above the origin point. Donor Ben Rau dated the object to 1950, but the form of the signature indicates the protractor was probably manufactured for the U.S. Army in the 1960s.
This protractor is very similar to 1977.1141.12. See also 1977.1141.01, 1977.1141.02, 1977.1141.03, 1977.1141.05, 1977.1141.08, 1977.1141.09, 1977.1141.10, 1977.1141.11, 1977.1141.18, 1977.1141.19, 1977.1141.20, 1977.1141.21, 1977.1141.22, 1977.1141.23, 1977.1141.24, 1977.1141.30, and 1977.1141.39.
Reference: Copyright Office, Library of Congress, "Books and Pamphlets," Catalog of Copyright Entries, 3rd ser., 16, part 1, no. 2 (1964): 1392.
Currently not on view
Object Name
date attributed by donor
date made
Felsenthal Instrument Co.
Physical Description
plastic (overall material)
overall: .2 cm x 30.5 cm x 16.5 cm; 3/32 in x 12 in x 6 1/2 in
Place Made
United States: Illinois, Chicago
ID Number
accession number
catalog 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 Ben Wharton Rau and Margery Felsenthal Rau

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