Compasses are navigational instruments that determine direction relative to Earth’s magnetic poles. Mechanical versions of these magnetically-sensitive devices usually contain a face with the cardinal points of north, south, east, and west. A magnetized pointer on the north end is free to align itself with Earth’s magnetic field. The north end of the compass points toward the magnetic north pole (magnetic north) while the south end points toward the magnetic south pole (magnetic south).
Most modern compasses are fluid-filled and may include a protractor.
There are many different types of compasses.
- Thumb compasses are used mainly in orienteering, map reading, and navigational training. Consequently, these devices have few markings.
- Gyrocompasses are compasses that are similar to gyroscopes. These non-magnetic devices use an electrically-powered, spinning wheel along with frictional forces to find true north. Unlike magnetic compasses, gyroscopes are not affected by the ferrous metal in a ship’s hull, making them well-suited for maritime applications.
- Solid-state compasses contain small, magnetic field sensors that provide data for an integral microprocessor.
- A fluxgate compass is a simple electromagnetic device that consists of two wires wrapped around a core of highly-permeable magnetic materials. Flux gate compasses provide readings in an electronic format that can be digitized, transmitted, and displayed remotely.
- Radio compasses or radio direction finders (RDF) are designed to find the direction to a radio source. They are sometimes called automatic direction finders (ADFs).
Optical or prismatic hand-bearing compasses are used by surveyors, spelunkers, foresters, and geologists. They use a liquid-damped capsule and floating, magnetized dial with an integrated optical sight. The sight may be direct or lensatic and is usually illuminated. By using the optical sight or prism, navigators can take extremely accurate readings, often to a fraction of a degree. Most optical compasses and prismatic compasses are designed for demanding outdoor applications. They feature jeweled bearings, sturdy magnetic needles, and tripod mounting for extra accuracy.
Additional features include map and roamer scales, luminous markings, and various sighting mechanisms.
- Roamer scales are marks on the base plate that are graduated for different scales of the map.
- Luminous marks allow the user to navigate in the dark.
- Sighting mechanisms could include a mirror, rubber feet, bezel, and magnifying glass.