Coradi Adjustable Polar Compensating Planimeter Sold by Eugene Dietzgen (Model 6612)

This instrument has two arms. Made of German silver, the first arm is 9" long, and has a brass and steel tracer point with a support. The arm is evenly divided into tenths and numbered from 5 to 38. (One unit is equivalent to 5mm.) It fits into a brass carriage painted black that has a white plastic measuring wheel, vernier, and registering dial. Screws on the carriage adjust the length of the tracer arm. The carriage is marked: No 20495.

A 9" German silver pole arm fits into a hole in the carriage. Near that end, the arm is evenly divided into tenths and numbered from 30 to 34. The arm fits inside a sliding rectangular tube made of brass painted black (i.e., "bronzed brass"). A cylindrical brass weight is attached to the end of the tube. The tube is marked: G. Coradi, Zürich Switzerland D.P.G.M. Eugene Dietzgen Co. An oblong steel testing plate is marked for 0", 1", 2", and 3". It is also marked: G. Coradi Zürich.

A wooden case is covered with black leather and lined with green velvet. A printed paper chart is pasted inside the case. The chart has columns for Scales, Position of the vernier on the tracer bar, Value of the unit of the vernier on the measuring roller, and Length of the pole-arm for the constant 20,000. The values in the Position and Length columns are handwritten in the same hand that indicates the Coradi firm manufactured this planimeter with serial number 20,495 on January 18, 1913. The date and serial number indicate that this instrument was manufactured before MA.321777. 1977.0112.02 is an instruction manual.

Gottlieb Coradi (1847–1929) began to make wheel and disc polar planimeters in the early 1880s. In 1894, he made the pole arm higher than the tracer arm and connected the arms with a ball joint. This "compensating" planimeter could trace in both the clockwise and counterclockwise directions, preventing errors introduced by planimeters made in the Amsler style. The Eugene Dietzgen Company of Chicago sold Coradi's compensating planimeter from at least 1905 to at least 1931. The version with the adjustable pole arm cost $47.00, which was $11.00 more than the standard version. The model number was changed from 6612 to 1806 between 1910 and 1926. Keuffel & Esser also sold the instrument from 1901 to 1939.

The instrument reached the Smithsonian i 1987.

References: "The Lang-Coradi Planimeter," in Olaus Henrici, "On Planimeters," in Report of the Sixty-fourth Meeting of the British Association for the Advancement of Science (London, 1894), 496–523, on 515; Catalogue & Price List of Eugene Dietzgen Co., 7th ed. (Chicago, 1904), 362; Catalogue & Price List of Eugene Dietzgen Co., 9th ed. (Chicago, 1910), 503; Catalogue & Price List of Eugene Dietzgen Co., 12th ed. (Chicago, 1926), 183; Catalogue & Price List of Eugene Dietzgen Co., 14th ed. (Chicago, 1931), 205; Clark McCoy, "Collection of Pages from K&E Catalogs for the 4240 Family of Polar Planimeters," http://www.mccoys-kecatalogs.com/PlanimeterModels/ke4240family.htm.

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