Compass(redirected from a pair of compasses)
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1 In mathematics, an instrument for making circles and measuring distances. Frequently called a pair of compasses, it consists of two metal legs with one end of each attached to a pivot to form a V-shaped device. The free ends are pointed; a pen or pencil may be substituted for one of the points.
2 In navigation, an instrument for determining direction. The mariner's compass consists of a magnetic needle freely suspended so that in the earth's magnetic field it turns until aligned with the magnetic north and south poles. Declination is the angle between the magnetic needle and the geographical meridian. Use of the compass by the early Chinese is probably legendary. The first known reference in European literature dates from the 12th cent. Another more accurate form of navigational compass is the gyrocompass. It consists essentially of a rapidly spinning, electrically driven rotor, suspended in such a way that its axis automatically points along the geographical meridian. The gyrocompass is unaffected by magnetic influences. This compass came into wide use in warships and aircraft during the Second World War. See gyroscopegyroscope
, symmetrical mass, usually a wheel, mounted so that it can spin about an axis in any direction. When spinning, the gyroscope has special properties. Many spinning objects exhibit some of these properties; the rotation of the earth about its axis gives it the
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a direction-finding instrument. According to principle of operation, a distinction is made among magnetic compasses, which use the property of straight permanent magnets (magnetic pointers) or coils with electric current of assuming a north-south orientation along a magnetic meridian upon interaction with the earth’s magnetic field; gyrocompasses, which use the property of a rapidly spinning gyroscope rotor of maintaining a constant direction of its axis of rotation, which, under certain conditions, remains in the plane of a geographic meridian; celestial compasses, in which a special device (a direction finder) continuously tracks the location of some celestial body (for example, the sun), making possible determination of the direction of the geographic meridian if the geographic coordinates of the location are known; and radio compasses, which are radio receiving devices that automatically lock in the direction toward a radio beacon. Compasses are used in maritime and aircraft navigation (to determine the course of a vessel or aircraft and to find the direction toward a reference point), in overland travel, in military affairs, and in geodesy, topography, and mining.
The oldest and most widely known type of compass is the magnetic type. Permanent magnets were used in China more than 2,000 years ago to determine the north-south direction. In Europe the compass appeared not later than the 12th century; it consisted of a magnetic pointer attached to a cork floating in a water-filled vessel. During the early part of the 14th century the compass was improved: the magnetic pointer was placed on a point located in the center of a paper circle (a compass rose), which was graduated for convenient reading. The circle was subdivided first into 16 and later 32 equal sectors.
In the 16th century a gimbal suspension was introduced to reduce mechanical vibrations (for example, during rolling and pitching at sea). In the 17th century marine compasses were equipped with a direction finder—a rotating rule with sights at both ends—which facilitated accurate reading of directions or bearings. The improved magnetic compass became the main navigational instrument for determining the course of a ship. The reading accuracy of currently used ship’s magnetic compasses in medium latitudes and in the absence of pitch and roll can attain 0.3°˗0.5°.
An aircraft magnetic compass has the same basic parts as a ship’s compass, but the aircraft design takes into account some operational conditions peculiar to this type of service, such as possible strong vibrations and accelerations.
Among the disadvantages of a magnetic compass are the need to introduce reading corrections to compensate for the noncoincidence of the magnetic and geographic meridians (consideration of magnetic declination) and for deviation. Near the earth’s magnetic poles and strong magnetic anomalies the accuracy of the readings of a magnetic compass is drastically reduced; in such localities other types of compasses must be used. However, none of the types mentioned above can provide an accurate course reading in all areas of the earth, under any weather conditions, and for any state of the magnetosphere and radio interference level. For this reason it is customary in marine and air travel and military affairs to use a combination of several types of compasses; consolidated course-finding systems are based on such combined use.
REFERENCESAnderson, E. Printsipy navigatsii. Moscow, 1968. (Translated from English.)
Seleznev, V. P. Navigatsionnye ustroistva. Moscow, 1961.
(in Russian, krontsirkul’ ), a drawing instrument for constructing circles 2–80 mm in diameter. There are drawing compasses with a screw joint between the measuring legs, as well as compasses that have a vertically mounted support leg and a movable measuring leg, whose position is fixed by a screw (the balerinka, or “little ballerina”).
(in Russian, tsirkul’), an instrument used to draw circles or arcs, to measure the lengths of line segments, to transfer dimensions, or to scale measured dimensions up or down.
The following main types of compasses are distinguished: dividers, drawing compasses, bow compasses, beam compasses, and proportional compasses (see Figure 1). Dividers, which do not have a pen or pencil point, are used to measure or transfer linear dimensions. Drawing compasses are employed to draw circles with diameters of up to 300 mm; bow compasses, circles with diameters of 2 to 80 mm; and beam compasses, circles with diameters of more than 300 mm. Proportional compasses make it possible to change the scale of measured dimensions.
Based on the evidence of drawn circles that have been preserved, the ancient Babylonians and Assyrians used compasses. In France, an iron compass was discovered in a Gallic barrow dating from the first century of the Common Era. Many bronze compasses used by the Romans during the first century of the Common Era were found in the ruins of Pompeii; they represent all the modern types of compasses. In addition to simple compasses, the Roman instruments include proportional compasses for enlarging or reducing the scale of measured dimensions and calipers for measuring the inside and outside diameters of objects.
In ancient Rus’, a compass-drawn ornament consisting of tiny perfect circles was widely used on articles made of bone. A compasslike steel cutter for scribing the ornament was found during archaeological excavations in Novgorod.
What does it mean when you dream about a compass?
Guidance. Finding one’s way out of situation in which one feels lost. We also talk about our “moral compass,” which guides us in making ethical choices.