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 instrument was used by Margaret G. Aldrich (1918-2007), who taught at Montgomery College from 1957 to 1984, chairing of the math department on the Takoma Park campus for many years. She had an undergraduate degree in mathematics and an M.A. in psychology, both from the University of Minnesota.

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.

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