Once the instrument has been levelled, all that is required to perform the measurement is to place the magnetic needle on the plane of the magnetic meridian.
The operation is quite simple: by turning the instrument around its vertical axis, one observes that the inclination miscellaneouss and that in a certain position it reaches the highest value of 90 degrees; this takes place only when just the vertical component of the Earth's magnetic field acts on the needle, since the horizontal component is directed along the rotational axis of the needle.
At that point the needle lies on the vertical plane perpendicular to the plane of the magnetic meridian and, with a 90-degree rotation of the instrument, it is brought to lie on the plane of the magnetic meridian. In that position, at one end of the magnetic needle it is possible to read the value of inclination on the vertical circle.
In practice, the procedure is more complex if it is necessary to eliminate systematic errors caused by the imperfect coincidence of the magnetic axis of the dipole with that of the geometric axis and by the fact that the axis of rotation may not pass through the centre of gravity of the dipole or through the centre of the graduated circle.