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 towards the magnetic north pole (magnetic north) while the south end points towards the magnetic south pole (magnetic south). Most modern compasses are fluid-filled and may include a protractor. Additional features include map and roamer scales, luminous markings, and various sighting mechanism. There are many different types of compasses. Examples include thumb compasses, gyrocompasses, and solid-state 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 are also available. They contain small, magnetic field sensors that provide data for an integral microprocessor. Compass types include fluxgate compasses, radio compasses, hand compasses, and wrist compasses. 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). As their name suggests, hand compasses are designed to be held in the hand. Similarly, wrist compasses are designed to worn on the user’s wrist. Several types of specialty compasses are available. 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.