Coradi Precision Disc Polar Planimeter

This polar planimeter has a German silver tracer arm that is evenly divided to half-millimeters and numbered from 8 to 66. (One unit is equivalent to 5mm.) The tracer point is bronzed brass and steel and has a support. The length of the tracer arm may be adjusted by thumbscrews on a bronzed brass carriage. A hinged horizontal extension from the carriage holds a white plastic vernier, measuring wheel, and registering dial. The wheel rests on a black-coated aluminum disc faced with white paper. The wheel and disc both rotate when the tracer point is moved. The disc regulates the motion of the wheel, so that it is not affected by folds or other uneven points in the paper with the diagram to be traced.

A bronzed brass holder for the disc is marked: G. Coradi. Zürich. Switzerland (/) No 4012. The holder rests in the middle of a round brass weight that is 6" in diameter. An oblong German silver testing rule is marked for 0", 1", 2", and 3". It is also marked: G. Coradi Zürich. An oblong brass testing rule is marked for inches and for 0, 50, and 100mm. It is also marked: G. Coradi Zürich.

A wooden case covered with black morocco leather is lined with purple 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 Constant. The values in the Position and Constant columns are handwritten in the same hand that indicates the Coradi firm manufactured this planimeter with serial number 4,012 on March 25, 1916.

Gottlieb Coradi (1847–1929) began to make wheel and disc polar planimeters in the early 1880s. The Interstate Commerce Commission transferred this instrument to the Smithsonian in 1962. There is no record of where the ICC purchased the planimeter, but American firms such as Keuffel & Esser (1906–1936 as model 4251) and Dietzgen (by 1926, as model 1808) distributed Coradi's precision disc planimeter. K&E sold it for $95.00 in 1906, for $85.00 in 1909 and, by special order, for $215.00 in 1936. Dietzgen charged $127.00 in 1926. ID number 1977.0112.02 is an instruction manual.

References: Clark McCoy, "Collection of Pages from K&E Catalogs for the 4251 Family of Precision Polar Disc Planimeters," http://www.mccoys-kecatalogs.com/PlanimeterModels/ke4251family.htm; Catalogue of Keuffel & Esser, 32nd ed. (New York, 1906), 336; Catalogue of Keuffel & Esser, 33rd ed. (New York, 1909), 324; Catalogue of Keuffel & Esser, 38th ed. (New York, 1936), 343; Catalogue & Price List of Eugene Dietzgen Co., 12th ed. (Chicago, 1926), 183.

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