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Definition: Device for determining direction parallel to the Earth's surface. Most compasses make use of the Earth's magnetic field (see Geomagnetism); if a bar magnet (see Magnetism) is pivoted at its center so that it is free to rotate horizontally, it will seek to align itself with the horizontal component in its locality of the Earth's magnetic field. A simple compass consists of a magnet so arranged and a compass card marked with the four cardinal points and graduated in degrees. In ship compasses, to compensate for rolling, the card is attached to the magnet and floated or suspended in a liquid, usually alcohol. Aircraft compasses often incorporate a Gyroscope to keep the compass horizontal. The two main errors in all magnetic compasses are variation (the angle between lines of geographic longitude and the local horizontal component of the Earth's magnetic field) and deviation (local, artificial magnetic effects, such as nearby electrical equipment). Both vary with the siting of the compass, and may, with more or less difficulty, be compensated for. (See also Navigation.) A radio compass, used in aircraft, is an automatic radio Direction finder, calibrated with respect to the station to which it is tuned.
Source: Illustrated Dictionary of Science, Andromeda. London: Andromeda, 1988. Credo Reference. Web. 08 August 2012.