Dietzgen 1298-D Blackboard Protractor

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.In the 16th and 17th centuries, surveyors and navigators began to use instruments made especially for measuring off angles. These were generally small instruments made of metal and finely divided. In the years following the Civil War, as the number of American high schools grew, so did the number of students studying practical geometry and trigonometry. To teach them, teachers used inexpensive protractors made for use at the blackboard.

This example is made of fiberboard, painted white on the front, and has a wooden handle so that it can be held upright. It is divided along the edge to intervals of 5 degrees. By comparison, most protractors are divided much more finely. The Eugene Dietzgen Co. of Chicago sold blackboard protractors like this one from about 1925.

The 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.

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