Parts of a Magnetic Compass

A compass is a magnetically sensitive navigational device that responds to the Earth's magnetic field. You can use a compass to follow a map or determine the direction you are heading. A compass is a valuable tool for hikers, climbers or anyone who likes to explore remote areas. Unlike a GPS, a compass does not require a power source, is small, lightweight and very durable.

  1. Base Plate

    • The base plate is the hard surface where all the parts of the compass mount. Most base plates are clear plastic. The base plate is rectangular with rulers on the edges for measuring distances on a map. The base plate may have a magnifying circle for reading small words or features on a map.

    Dial and Housing

    • The dial or ring sits on a fluid-filled housing and is marked in degrees from 0 to 360. You turn the dial to turn the housing and align the compass. The housing contains the magnetic needle.

    Magnetic Needle

    • The needle is a small piece of magnetized metal. One end is painted red to indicate north. The needle sits on a nearly frictionless pivot point that allows it to rotate freely. The fluid in the housing reduces vibrations and keeps the needle steady and accurate. A bubble of air in the housing fluid indicates when the compass is level.

    Direction-of-Travel Arrow and Orienting Lines

    • An arrow marked on the base plate indicates the direction of travel. When you use the compass, point the arrow in the direction you wish to go. Use the parallel orienting lines or meridian lines on the housing and on the base plate under the needle for alignment on a map.

    Options

    • Some compasses have an adjustable declination arrow to correct for any magnetic declination. Magnetic declination is the angle between magnetic north (the direction where the compass needle points) and true north. A clinometer is another option that measures slope. Some compasses have a cover with a mirror on the inside. The mirror shows the compass needle and objects in the distance at the same time. You can also use the mirror for signaling.

    Pocket Compass

    • A pocket compass is round and shaped like a pocket watch. Pocket compasses have a magnetized needle set over a compass rose marked in degrees from 0 to 360. Pocket compasses have limited features and are used mostly for determining direction.