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.