Magnetometers are instruments that measure the intensity of a magnetic field and the magnetic moment of a bar magnet by means of the deviation or oscillation of a magnetic needle of a size small enough to allow us to consider the field uniform throughout the region it occupies.
Magnetometers have been used mostly in the study of the Earth's magnetic field and its variations and, basically speaking, they use the methods proposed by Karl Friedrich Gauss (1777-1855) who is considered the inventor, some time about 1827. The specimen exhibited here, which is the single-fibre type and which is equipped with a large number of accessories, is without any instructions for its use, which certainly must have been rather complex. It is entirely made of burnished brass, with the exception of the four vertical walls of the case and the sleeve that protects the fibre from which the magnetic needle is suspended, which are made of glass, and the magnetic needle itself.
Essentially, it consists of:
1) a horizontal circle (diameter 22 cm) graduated from 0 to 360°, with divisions in ten minutes, and equipped with a double alidade with verniers allowing readings to 20 seconds
2) a vertical circle (diameter 14.5 cm) divided into four quadrants, each graduated from 0 to 90° with divisions of twenty minutes and equipped with a vernier allowing the reading to one minute.
The instrument is equipped with an air bubble level, it is supported by a sturdy tripod with levelling screws and can rotate around a vertical axis.
The case, with a rectangular base (18 x 18 cm), is 20 cm in height and its sides can be opened completely, while only the upper half of the back can be opened (upwards). It can be held open by a square hook.
In the centre of the cover the glass sheath protecting the fibre from which the magnetic needle is suspended is held in place by a ring.

Daguin (1878) T. I, p. 511
Jamin (1887), T. III, p. 2
Murani (1906), Vol. I, p. 355
Privat Deschanel (1869), p. 791

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