The magnetic needle will thus align itself along the direction of the magnetic field that results by rotating by an angle the tangent of which, as can easily be seen by graphically composing the vectors representing the two fields, is given by the ratio between the intensity of the magnetic field created by the coil and that of the horizontal component of the earth's magnetic field. Since the magnetic field created by the coil is proportional to the intensity of the current to be measured with a constant given by the ratio between the number of turns and their diameter (SI), the intensity of the current flowing through the instrument is proportional to the tangent of the angle of deviation with a proportionality constant that depends on the local horizontal component of the earth's magnetic field, on the number of turns and their diameter.
It is an absolute measuring instrument: knowing the diameter of the coil, the number of its turns and the value of the local horizontal component of the earth's magnetic field, by measuring the angle of deviation the value of the intensity of the current is determined. ...continues