A thin rectangular magnetic needle (length 15 cm; width 9 mm), sharpened at both ends, kept in a rectangular brass box with a glass cover. at the centre of the box on the inside there is a small vertical pin on which the needle is suspended by means of a agate pinhead; in the rotations, the ends of the needle, which act as indexes, move over circular scales graduated from 0 to 30 degrees.
If we know the geographic meridional plane of a place, this simple instrument allows us to measure, although not precisely, the magnetic declination of that place (which is to say the angle formed by the local magnetic meridional plane and the geographic meridional plane). It is sufficient to place the instrument in such a way that the line passing through the zeros of the two scales lies on the geographic meridional plane and the magnetic declination is measured by the angle by which the needle is rotated in its position of equilibrium.


Bibliografia:
Daguin (1878),T. III, p. 19
Drion - Fernet (1883), p. 418
Ganot (1864), p. 486


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