郑嘉颖叶问粤语版:Navis.gr - Magnetic Compass

Magnetic
Compass
See also:How toapply the Compass Error
Before thedevelopment of sophisticated electronic and sounddetection systems, navigators calculated directions fromobjects in the sky the sun, the North Star, and the moon.A much more reliable guide for finding direction is amagnetic compass, which works at all times and in mostplaces. When a piece of magnetized iron is placed on asplinter of wood and floated in a bowl of water, the woodwill swing until the iron is pointing north and south.Any other direction can be found.

In China and Europe the magnetized iron found in thelodestone, a naturally occurring magnetic ore, was usedto make a floating compass in the 12th century. Soonafterward it was discovered that an iron or steel needletouched long enough by a lodestone also had the tendencyto align itself in a north-south direction. A smallpocket compass works on the same principle as the firstcrude compass: instead of a lodestone and a woodsplinter, it has a magnetized needle that swings on apivot to indicate north. Larger compasses have two ormore parallel needles attached to the underside of a diskcalled a compass card.

The compass works because the Earthitself is a huge magnet. Its magnetic poles are ovalareas about 1,300 miles (2,100 kilometers) from the geographic North and South poles.Irregular lines of force connect the magnetic poles, andthe compass needle simply aligns itself with these linesof force. In a few places, where lines of force happen tolie along meridians (that is, where magnetic north andtrue north coincide), the compass points to true north.Near the magnetic pole the magnetic compass is uselessbecause there the lines of force are vertical straightdown into the Earth. In otherareas iron ore deposits affect the compass's accuracy.Generally, however, the magnetic compass points a littleeast or west of true north. The angle between true northand magnetic north is called variation or declination. Acompass rose, or graduated circle, is used to measurethis angle on charts.

A compass card usually has direction pointers consistingof 32 points. The four principal, or cardinal, points arenorth, east, south, and west. They are marked N, E, S,and W. Between these lie the intercardinal points, suchas northeast (NE). Further division gives such points asnorth-northeast (NNE). A final division is by points,such as north by east (N by E). Naming all the points ofa compass in their order is called boxing the compass.

Surveyors, navigators, andsimilar technicians need more exact directions they usedegrees. The compass card has 360 degrees marked on it.North is 000° (or 360°); East, 090° ; South, 180° ;and West, 270°.
On ships the magnetic compass is usually carried in astand called a binnacle. It holds a bowl containing thecompass card with its needles mounted on a pivot and hasa provision for illuminating the compass face from below.The bowl is filled with a nonfreezing liquid on which thecard floats to reduce vibrations. On the forward insideedge of the bowl is a vertical line called a lubber'sline. This marks the "dead ahead" of the ship.In steering, the helmsman watches the mark for his courseon the compass card, keeping it always opposite thelubber's line.
A compass aboard a ship isaffected by the magnetic force of the ship itself, whichacts like a huge magnet. The effect of this magnetism onthe compass is called deviation. It is measured by theangle between compass north and magnetic north. Variationand deviation together pull the compass away from truenorth by an amount called compass error.

Navigators remove most of the deviation by compensating the compass. They take the shipto a range where they line it up with markers indicatingthe four cardinal points. Then they "swingship" by pivoting the craft so that the bow pointsin turn to each of the markers. They remove the deviationon each heading by placing counteracting magnets in thebinnacle these magnets serve to cancel the magneticeffects of the metal in the ship.

In an effort to develop anavigational instrument whose accuracy would beunaffected by stray magnetic fields, the gyrocompass, which does not usemagnetism, was developed. Gyrocompasses are often used inmodern navigation systems because they can be set topoint to true north rather than to magnetic north. Todaylarge ships carry both magnetic compasses andgyrocompasses.

Special compasses have also been developed for airplanes. Gyroscopic systems areespecially useful in such applications because, unlikemagnetic compasses, their accuracy is not affected byrapid alterations of course or speed.
The aperiodic compass is a magnetic compass whose needleis extremely stable under most flying conditions for aircraft. The magnesyn compass isa remote-indicating magnetic compass. Readings from itspickup coil are transmitted to repeaters in other partsof the airplane.

Both the gyro flux gate compass and the gyrosyn compassare remote-indicating, gyrostabilized compasses. For itsindications, the obsolete Earth-inductor compass usedcurrent generated in a coil revolving in the Earth's magnetic field.

The astrocompass is an astronomical instrument by whichthe air or sea navigator findsthe true heading by sighting a celestial body. A form ofastrocompass is the sun compass, which utilizes theshadow of a pin.

Local Magnetic Anomalies

In various parts of the world, magnetic ores on orjust below the seabed may give rise to local magneticanomalies resulting in the temporary deflection of themagnetic compass needle when a ship passes over them. Theareas of disturbance are usually small unless there aremany anomalies close together. The amount of thedeflection will depend on the depth of water and thestrength of the magnetic force generated by the magneticores. However, the magnetic force will seldom be strongenough to deflect the compass needle in depths greaterthan about 1500 m. Similarly, a ship would have to bewithin 8 cables of a nearby land mass containing magneticores for a deflection of the needle to occur.

Deflections may also be due to wrecks lying on the bottomin moderate depths, but investigations have proved that,while deflections of unpredictable amount may be expectedwhen very close to such wrecks, it is unlikely thatdeflections in excess of 7° will be experienced, norshould the disturbance be felt beyond a distance of 250m.

Greater deflections may be experienced when in closequarters with a ship carrying a large cargo such as ironore, which readily reacts to induced magnetism.

Power cables carrying direct current can cause deflectionof the compass needle. The amount of the deflectiondepends on the magnitude of the electric current and theangle the cable makes with the magnetic meridian. Smallvessels with an auto-pilotdependent upon a magnetic sensor may experience steeringdifficulties if crossing such a cable.

The Effect of Magneticand Ionospheric Storms on the Compass Needle

Disturbances on the sun may cause disturbances of themagnetic compass needle and interference with radiocommunications.
At the time of an intense solar flare or eruption, aflash of ultra-violet light and a stream of chargedparticles are emitted from the sun.

The flash of ultra-violet light takes only 8 minutes toreach the Earth, where it produces great ionisation(electrification) at abnormally low layers of the upperatmosphere. Short radio waves which travel round theEarth by being reflected from a higher layer of the upperatmosphere cannot penetrate this barrier of ionisationand a radio 'fade-out' is experienced. Long radio waveshowever may be reflected more strongly from the base ofthe lower layer of ionisation. Since these short rangeradio fade-outs and long wave enhancements are caused bythe effects of ultra-violet light from the sun, they areconfined to the sunlit side of the Earth and are almostsimultaneous with the flare, lasting on the average forabout 20 minutes.

The stream of charged particles, travelling much moreslowly than light, arrives at the Earth, if it issuitably directed, at from 1 to about 3 days after itleaves the sun; it visibly signals its arrival byproducing a bright and active aurora.It too causes great ionisation in the upper atmosphere,which is much more prolonged than that caused by theultra-violet light. There is again deterioration in shortwave radio communications, which may be a complete'black-out' in higher latitudes. At this time currents ofthe order of a million amperes may circulate in the upperatmosphere. The magnetic field of the fluctuatingcurrents is appreciable at the Earth's surface and maydeflect a compass needle noticeably from its normalposition. The effects on these so-called magnetic andionospheric storms, which may persist with varyingintensity for several days, are usually greatest inhigher latitudes. Radio 'black-outs' and simultaneousdeviations of the magnetic compass needle by severaldegrees are not uncommon in and near auroral zones. Whena great aurora is seen inabnormally low latitudes, it is invariably accompanied bya magnetic and ionospheric storm. Unlike the fade-outwhich occurs only on the sunlit side of Earth, theinterference with radio communications which accompaniesan aurora and magnetic storm may occur by day and atnight.

All these effects occur most frequently and in mostintense forms at the time of sunspot maximum; maxima arelikely to occur in 2001-02.

Increases in solar activity could affect the reliabilityof GPS and other satellite systems; for further detailssee the relevant AdmiraltyList of Radio Signals. See theALRSPublications

Charting and describing

Local magnetic anomalies are depicted by a specialsymbol on Admiralty Chartsand are mentioned in Sailing Directions. Theamount and direction of the deflection of the compassneedle is also given, if known.