Compass has a half-life of only about 12 years, so a compass that contains 120 mCi of tritium when new will contain only 60 when it is 12 years old, 30 when it is 24 years old, and so on. Consequently, the illumination of the display will fade. Mariners' compasses can have two or more magnets permanently attached to a compass card, which moves freely on a pivot. A "lubber line", which can be a marking on the compass bowl or a small fixed needle, indicates the ship's heading on the compass card. Traditionally the card is divided into thirty-two points
Qibla compass forms. The indicator usually comprises a round brass box with a hinged lid and an inset magnetic compass. A list of important Islamic places with their longitudes, latitudes, is inscribed in Arabic on all sides of the box. The compass has a blued steel needle with an open circle to indicate North. It is surmounted by a brass pyramidal pivot and a glass plate covers all. A brass ring over the rim of the compass carries a degree circle numbered in 'abjad' numerals and the cardinal points are marked. The folding triangular gnomon is supported by a decorative open-work motif.