Dip Circle

This type of unusually small dip circle was designed by Emile Brunner in Paris in the 1880s and widely used by the French Magnetic Survey. A key feature is the vertical circle that is movable, located inside the box, and equipped with two small concave mirrors. The center of curvature of these mirrors is in the plane of the needle, and so the mirrors give real and inverted images of the points of the needle. To take a reading, the viewer rotates the circle until these two images are brought into contact with the points themselves. The vertical is graduated to 30 minutes and read by vernier to single minutes. The horizontal circle is silvered and graduated.

This example is marked "Chasselon B Paris No. 40" and ";USC&GS NO 24." It was made by Chasselon around 1900. It belonged to the U.S. Coast and Geodetic Survey, which found impurities in the brass of which it was constructed.

Ref: E. Mascart and J. Joubert, A Treatise on Electricity and Magnetism, English transl. By E. Atkinson, (London, 1888), vol. II, pp. 591-593.

E. Mascart, Traité de Magnétism Terrestre (Paris, 1900), pp. 168 and 214.

Daniel Hazard, "Results of Magnetic Dip and Intensity Observations," Report of the Superintendent of the United States Coast and Geodetic Survey Showing the Progress of the Work from July 1, 1901, to June 30, 1902, Appendix No. 6, p. 350.

Currently not on view
Date made
ca 1900
Place Made
France: Île-de-France, Paris
Physical Description
metal (overall material)
verticle circle: 3 in; 7.62 cm
overall; dip circle: 6 1/4 in x 4 5/8 in x 4 3/8 in; 15.875 cm x 11.7475 cm x 11.1125 cm
ID Number
accession number
catalog number
Credit Line
U.S. Department of Commerce, Coast & Geodetic Survey
Science & Scientific Instruments
Science & Scientific Instruments
See more items in
Medicine and Science: Physical Sciences
Measuring & Mapping
Science & Mathematics
Data Source
National Museum of American History