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Magnetic Compass

Updated on April 18, 2010

In its most primitive form, a compass was a piece of magnetic ore (lodestone) attached to a piece of floating wood. The compass was first mentioned in writing by Alexander Neckam (1157-1217), an English monk, but the magnetism of lodestone was known to the ancient Greeks. The magnetic compass, which has been used by mariners since the Vikings, is convenient to steer by and is usable when the more accurate method of celestial navigation is not possible due to an overcast sky.

Photo by Pawel Kryj
Photo by Pawel Kryj

Points of the Compass

Since the 12th century, the compass rose has been subdivided by repeated bisection into compass points named in the order: north, north by east, north-northeast, northeast by north, northeast, northeast by east, east-northeast, east by north, east, and so forth. The 11V4° interval between the 32 named points was about the accuracy with which an old mariner's compass could be read. As accuracy improved, it became customary to give directions in degrees from north or south; for example,' northeast became north 45° east, and southeast became south 45° east. Since the 1920's it has become universal practice to indicate direction, called azimuth, by a single number representing degrees measured clockwise from the north.

Construction

In its simplest form, a compass consists of a magnetized steel needle balanced to pivot above a circular dial; the dial indicates correct directions when it is rotated so that its north direction coincides with the north-seeking end of the needle, which is usually colored for identification.

In most modern compasses, such as a mariner's compass, the compass needle is attached out of sight on the bottom side of the dial; the entire dial, then called the card, is balanced and pivoted on a jewel bearing in a nonmagnetic case filled with oil to damp oscillations. A compass often is equipped with a sighting device, or alidade, which permits measurement of direction, or magnetic bearing, to a distant object.

Land Compasses

Scouts, explorers, soldiers, and hikers use a small pocket compass by holding it level in the hand. The most accurate land compass is mounted on a tripod; when also provided with a sighting telescope for measuring elevation angles, the instrument is called a transit or theodolite, and is used in surveying.

Marine Compasses

A ship rolls and pitches while at sea, and thus it is necessary to make a marine compass self-leveling. The card of a marine compass usually is pivoted in a round-bottom case or bowl that is weighted at the bottom and mounted in a hollow pedestal (binnacle ) by using two concentric rings (gimbals) pivoted in perpendicular directions. The glass top of the bowl is marked with the lubber line, which points along the ship's keel and thus indicates the magnetic heading of the ship.

Deviation

The simplest compass error, deviation, is caused by the presence of steel or current-carrying wires that produce a local magnetic field. This causes the compass needle to deflect from the direction of magnetic north by an angle known as compass error or deviation. It is necessary from time to time to swing a ship or airplane into accurately known directions and to make the compass error as small as possible by moving a set of adjustable compensating magnets inside the compass case. Because it is rarely possible to reduce error to zero in all headings, the remaining errors at various headings are written on a compass correction card and posted next to the compass.

Declination or Variation

The earth's magnetic field is far from simple. Its vertical component is zero only very near the equator; elsewhere it points downward at a steep angle, called magnetic inclination or dip. At polar latitudes greater than 65° north or south, its horizontal component, is too weak and erratic for navigational purposes; elsewhere, it points east or west of true north, or geographic north, by an angle called magnetic variation or declination. In the United States, this angle ranges from about 30° east to about 15° west; its local value changes slowly with time.

Aircraft Compasses

In the simplest type of aircraft compass the card is a cylinder, rather than a disk, with degree markings on the outside surface. The card is pivoted in an oil-filled case behind a vertical glass window marked with a vertical lubber line. To minimize compass error, the compass usually is mounted as far as possible from electrical instruments in the cockpit.

It is not necessary to mount an aircraft magnetic compass in gimbals because in all normal maneuvers the bank of an airplane (like a bicycle or motorcycle) is coordinated with its turning radius, and so the gravational force and the centrifugal force on any object add up to a resultant force that always remains perpendicular to the cabin floor. However, these acceleration forces are another source of compass errors.

Turning Errors

A compass needle balanced to swing horizontally before being magnetized would point downward at a steep angle after it was magnetized. Since a compass is intended to indicate only the horizontal component of the earth's field, its card must be weighted on the north or south direction, depending on whether it is to be used in the Southern Hemisphere or Northern Hemisphere, respectively. When statically balanced in this way, the compass card becomes dynamically unbalanced and will start spinning when the compass is subjected to acceleration forces. These acceleration or turning errors make an aircraft magnetic compass nearly useless as a flight instrument because it will give a correct indication only after the plane has been flying straight and level for several minutes. It has therefore been the practice, since the early days of all-weather flight, to use a gyroscopic compass for steering. In level flight the magnetic compass is used to correct drift errors of the gyroscopic compass caused by bearing friction.

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