Sur la mesure de l'intensité du champ magnétique terrestre et la distribution du magnétisme dans les aimants

The measurement of the horizontal component, H, of the geomagnetic field represents an important part of geomagneticians work from 1830 to the end of the 19th century. This measurement remained difficult at the required accuracy of 10−9T (1 nT or 1γ), till the advent of proton and optical pumping magnetometers in geomagnetic observatories, around 1955. The measurement of H consisted in determining the mutual torque between two cylindrical magnets, by measuring either oscillation periods or angular deviations. Poisson was the first, indisputably, to propose the general principle of the measurement, in 1825. But Gauss, some years later (1832), successfully operated his famous oscillation-deviation method, described since then in all the elementary manuals of physics. The computation of the mutual torque depends on the tri-dimensional distribution of the magnetization in each one of the magnets. In the absence of an accurate-enough knowledge of these distributions, Poisson, Gauss, and their successors showed how to make the computations with a minimum number of hypotheses on them. Nevertheless physicists, in particular French ones, worked on the 'distribution of magnetism' in magnets; the first of them was Coulomb. We come back to this question through experiments conducted in the magnetic observatory of Chambon-la-Forêt, France. To cite this article: B. Leprêtre, J.-L. Le Mouël, C. R. Geoscience 337 (2005).