Frederick Post Model 1795 Blackboard Compass

In the years following the Civil War, American mathematics teachers began to use oversized compasses like this one to draw circles on a chalkboard. This example was sold by the Frederick Post Company of Chicago. It consists of two maple arms, each about sixteen inches (41 centimeters) long, which are held together by a wing nut at one end. At the other end are a rubber tip and a piece of chalk.
Makers often sold such instruments as part of a set that also included a straight edge, a protractor, a T square, and a triangle. After passage of the National Defense Education Act in 1958, such instruments could be purchased by secondary schools with subsidies from the federal government. This particular object was used in mathematics teaching at Montgomery College in Takoma Park, Maryland.
Blackboard dividers that are different from this instrument are advertised as model number 1781 in the Frederick Post Company's 1930 and 1936 catalogs. The instrument is not shown in the 1949–1950 catalog.
Currently not on view
date made
ca 1950
Frederick Post Company
place made
United States: Illinois, Chicago
Physical Description
wood (overall material)
chalk (overall material)
rubber (overall material)
metal (overall material)
overall: 4 cm x 4 cm x 43.2 cm; 1 9/16 in x 1 9/16 in x 17 in
ID Number
catalog number
accession number
Credit Line
Gift of L. Thomas and Margaret G. Aldrich
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Dividers and Compasses
Data Source
National Museum of American History, Kenneth E. Behring Center


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