The declination variometer enables us to measure variations in the declination of the Earth magnetic field.
Designed in Denmark by D. La Cour from 1926, over one hundred La Cour magnetographs were produced by the Danish Meteorological Institute from 1926 onwards and were installed in French observatories in Terre Adélie and in Kerguelen in 1957 for International Geophysical Year.
A magnet with a horizontal magnetic axis is suspended from a quartz thread. This small magnet (12mm long, 1mm thick) is free to move and has a vertical mirror. If we manage to put the magnet in place without its doing one or more complete rotations, the quartz thread will be almost completely straight, and the magnet will therefore be controlled by the magnetic meridian. Thus, the magnet's rotations measure the variations in the declination D.