This instrument measures slight changes in the horizontal intensity of the earth's magnetic field. It is based on a design introduced by Hermann Ebert, a professor of physics in Munich, and Ebert tested it before it left Edelmann's shop. This example is marked "Physikalisch-mech. Institut. Von PROF. DR. TH. EDELMANN MÜNCHEN." The Department of Terrestrial Magnetism of the Carnegie Institution of Washington bought it for $94.69 in 1906.
Ref: M. Th. Edelmann, "On the Construction of Earth-Magnetic Instruments," <i>Weather Bureau Bulletin</i> 11 (1893): 511-539.