The Chasselon magnetic theodolite measures the declination D and the horizontal component H of the Earth magnetic field.
The instrument on view, which was made in France by Chasselon in 1930, was used in Port Alfred magnetic observatory (Crozet archipelago) between 1974 and 1981.
The Chasselon theodolite, which was made from non-magnetic materials, resembles an ordinary theodolite to which a magnetic cage with a chimney has been added. A cylindrical magnet, suspended from a silk thread and placed inside the magnetic cage, allows us to find the trace of the magnetic meridian by viewing its extremities with the help of a microscope attached to the glass. The geographic meridian is determined through an astronomical viewing using the glass. The precision obtained for measuring the declination is about 15 arc seconds.
Magnetic supports placed laterally allow for measurement of the intensity of the horizontal component H using Gauss-Lamont's oscillation-deviation method.