The Chasselon theodolite measures the dip of the Earth magnetic field.
The Brunner-Chasselon workshops manufactured several versions of magnetic theodolites. The model on view is the last one made by the fabricant. It was commercialised mainly in France (magnetic observatories, marine hydrographic service ... ) between 1950 and 1960. The precision obtained for dip measurement is about 15 arc seconds, as in the case of previous models (the 1930 model is presented on one of the pillars of this room).
The apparatus on view, obtained in 1956 with the International Geophysics Year in mind, was then used in the magnetic observatory of Port-aux-Français (Kerguelen islands) from 1957 to 1980.