This is one of the many measuring instruments that developed out of the cooperation between the engineer Marcel Deprez (1843-1918) and the biophysicist Jacques Arsène d'Arsonval (1885-1940) with the engineer who built them, Jules Adrien Carpentier (1851-1921), all French. The instrument presented here is a mobile coil galvanometer which, of the galvanometers of the same kind, was certainly the most common. It consists of a coil with very thin wires wound around a very light rectangular frame, a permanent horseshoe magnet and a soft iron cylinder placed inside the coil and, with this, in the space between the two arms of the magnet; two brass plates attached to the platform bear the words: 'J. Carpentier, Paris' and 'galvanomètre Deprez d'Arsonval'.
At the top of the coil there is a round mirror for the optical reading of the angles of rotation. The entire system lies on a circular wooden platform (diameter 19 cm) with one fixed foot and two adjustable ones. It is protected by a glass bell (diameter at the base 16.5 cm, height 26 cm). The edge of the bell fits into a groove on the platform. The magnet is attached to the platform with the poles facing upward; the mobile element is suspended at the end of a curved brass column from a conducting elastic suspension wire and a device with screws for adjusting its height. The soft iron cylinder is attached to the same column by means of a sliding brass arm.
On the side of the platform there are two terminals for the outside connection; these in turn are connected to the coil by means of a brass column and the suspension wire at the top and by means of another conducting wire and a small metal plate with a spring which keeps the two wires taut at the bottom.
The presence of the soft iron cylinder greatly increases the intensity of the magnetic field in the gap between the magnet and the cylinder, where the vertical sides of the coil move, thus making the action of the earth's magnetic field negligible. Thus, for sufficiently small rotations with respect to the rest position, the moment to which the coil is subject due to the action of the magnetic field in which it moves is approximately constant and proportional to the intensity of the current flowing through it.
On the other hand, the antagonist moment, caused by the elastic torsion of the suspension wire, is proportional to the angle of rotation. At equilibrium the two moments are equal and the angle of rotation of the coil is approximately proportional to the intensity of the current flowing through it.
Brenni (1984), p, 339
Ragozzino - Schettino (1985), p. 81
Roiti (1913), Vol. II, p. 245