Coradi Model 30 Rolling Sphere Planimeter

This planimeter can measure larger diagrams than planimeters with a fixed arm, and it is designed to eliminate errors introduced by irregularities in the paper. It moves on two German silver rollers. The roller on the left rotates a steel wheel that in turn rotates an axle that turns the measuring wheel and registering dial. The measuring wheel has a vernier. All three parts are made of white plastic. The ten-inch rectangular German silver tracer arm is attached to a carriage below the measuring wheel and between the rollers. It has a brass tracer with steel point and support. The length of the arm is adjustable, and it is evenly divided to 0.5mm and numbered from 10 to 54. An extension for the tracer arm adds eight inches to its length and is numbered from 55 to 88.

Above the roller on the right is marked: G. Coradi, Zürich (/) Switzerland (/) No 1550. An oblong German silver testing rule is marked for 0", 1", 2", 3", and 4". It is also marked: G. Coradi. A fitted wooden case is covered with black leather and lined with purple velvet. The brush that goes in the corner of the case is missing. A paper printed calibration chart glued inside the lid has columns in French 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 are handwritten, and the Constant column is crossed out. A paragraph explains how to store the instrument. The date on the chart indicates the Coradi firm made serial number 1,550 on November 2, 1901. A separate card has the chart values translated into English. According to a 1915 catalog, Coradi sold this size of planimeter as model 30.

The top of the case is marked: MU 3412. The key for the case is tied to the handle with string. A metal circle on the bottom of the case is marked: WEBCO. The Zurich workshop of Gottlieb Coradi (1847–1929) made a variety of planimeters beginning in the early 1880s, with the rolling sphere form debuting around 1900. The Department of Civil Engineering at the University of Missouri-Columbia donated this example in 1972. Although when and where the department acquired it is not known, American firms such as Keuffel & Esser and Dietzgen distributed Coradi's precision disc planimeter. K&E sold this size before 1901 as model 1106 and from 1901 to 1936 as model 4260, charging $82.50 in 1900. Dietzgen only sold a larger version that had twelve-inch and eight-inch tracer arm pieces. Compare to 1977.0112.01. A later instruction manual is 1977.0112.02. For other objects from the Univ. of Missouri's donation, see MA.333663 and MA.333636.

References: J. W. Beardsley, "Description and Theory of Coradi's Rolling Ball Planimeter," Journal of the Association of Engineering Societies 28 (1902): 67–77; J. Y. Wheatley, The Polar Planimeter and Its Use in Engineering Calculations (New York: Keuffel & Esser, 1903), chapter 10, http://www.leinweb.com/snackbar/planimtr/wheatley/s10-6.htm; Clark McCoy, "Collection of Pages from K&E Catalogs for the 4260 Family of Precision Rolling Planimeters," http://www.mccoys-kecatalogs.com/PlanimeterModels/ke4260family.htm; Catalogue of Keuffel & Esser, 30th ed. (New York, 1900), 308; Catalogue & Price List of Eugene Dietzgen Co., 7th ed. (Chicago, 1904), 363; Mathematical-Mechanical Institute of G. Coradi, Catalogue of Mathematical Precision Instruments (Zurich, 1915), 13–17.

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