The card system is the most important and at the same time most sensitive part of a magnetic compass. The centre, the so-called float, keeps the card in balance. The float contains a ring magnet which is magnetized in such a way that the printed "north" of the card also points in the direction of the magnetic north of the earth.
The card system is resting with a pin/pivot on a jewel/sapphire. This allows a frictionless bearing of the card system, while the compass fluid dampens the movements. This damping guarantees a low oscillation period and a low swirl error. These parameters are determined for each compass on our test bench before a quality certificate is issued.
With spherical compasses, the round glass dome and the liquid produce an enlargement, so that a 100mm card has an apparent diameter of 120mm.
