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
Beam compass a beam compass. Their relatively small size makes them easy to store or transport. They consist of two separate metal pieces (approx. 2.5" × 5" × 1/2") that are usually connected by a piece of wood, The wood timber is not included in the purchase of the trammel points. It can be ripped on a table saw. A lumber yard or woodworking store should have a piece readily available to fit the opening also, metal, or pipe. They work like a scratch awl. As for any compass, there are two uses. The beam compass is used to scribe a circle,