Otto Toepfer & Sohn Cylinder and Disc Planimeter

This instrument consists of a brass drum and disc (covered with paper numbered in pencil by ones from 1 to 25) mounted on metal stands that are fastened to a wooden base. U-shaped metal bars form a frame on two sides of the base. A series of brass gears beneath the disc are turned by a crank at the side of the instrument. A magnifying glass, measuring dial, and registering dial are attached to the center of the disc.

A rectangular shaft connects the lens and dials to a carriage that moves along a track next to the drum. The carriage has a pointer that moves on the surface of the recording drum, above a metal scale with a vernier. A wooden platform next to the carriage track folds out on two supports to make a surface for writing. A nameplate at the center front of the wooden base is marked: Otto Toepfer & Sohn (/) Werkstätten (/) für wissenschaftslische Instrumente (/) POTSDAM.

A drawing of a curve was placed on the disc and traced with the pointer. The rotations of the disc and shaft were recorded by the drum to provide a measurement of the space enclosed by the curve. For an earlier, similar instrument, see 1986.0633.01.

In 1905 Adolf Schmidt (1860–1944), director of the geomagnetic observatory in Potsdam, Germany, from 1902 to 1928, described his invention of this planimeter. It worked by converting a first-order derivative to linear equations. Schmidt argued that his device was more controlled and accurate over repeated observations than similar instruments. The Potsdam firm of Otto Toepfer & Sohn (1873–1919), which also manufactured Schmidt's other inventions and the instruments used by the observatory, sold the instrument as model 53 for 850 DM in 1907. This example was found in the Museum collections around 1982.

References: H. Podewski, "Geheimrat Professor Dr. Adolf Schmidt," http://www-app3.gfz-potsdam.de/obs/niemegk/en/geschichte/geschichte_obs/aschmidt/adsmi_en.html; J. Bartels, "Adolf Schmidt, 1860–1944," Terrestrial Magnetism and Atmospheric Electricity 51 (1946): 439–447; Adolf Schmidt, "Ein Planimeter zur Bestimmung der mittleren Ordinaten beliebiger Abschnitte von registrierten Kurven," Zeitschrift für Instrumentenkunde 25 (1905): 261–273; Otto Toepfer & Sohn, Erdmagnetische Variations-Instrumente (Potsdam, 1907), 68–69, http://www.astropa.unipa.it/biblioteca/Strumenti/e-catalogues/Toepfer1907/Catalogo.html.

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