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Compass

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For other uses, see Compass (disambiguation).

Image:Compass in a wooden frame.jpg A compass (or mariner's compass) is a navigational instrument for finding directions on the earth. It consists of a magnetized pointer free to align itself accurately with Earth's magnetic field, which is of great assistance in navigation. The cardinal points are north, south, east and west. A compass can be used in conjunction with a clock and a sextant to provide a very accurate navigation capability. This device greatly improved maritime trade by making travel safer and more efficient. An early form of compass appears in China in the 11th century. The familiar mariner's compass was invented in Europe around 1300.

More technically, a compass is a magnetic device using a needle to indicate the direction of the magnetic north of a planet's magnetosphere. Any instrument with a magnetized bar or needle turning freely upon a pivot and pointing in a northerly and southerly direction can be considered a compass. A compass dial is a small pocket compass with a sundial. A variation compass is a specific instrument of a delicate type of construction. It is used by observing variations of the needle. A gyrocompass or astrocompass can also be used to ascertain True north.

Contents

[edit] History of the navigational compass

[edit] Pre-history

Prior to the introduction of the compass, direction at sea was primarily determined by the position of celestial bodies. Navigation was supplemented in some places by the use of soundings. Difficulties arose where the sea was too deep for soundings and conditions were continually overcast or foggy. Thus the compass was not of the same utility everywhere. For example, the Arabs could generally rely on clear skies in navigating the Persian Gulf and the Indian Ocean (as well as the predictable nature of the monsoons). This may explain in part their relatively late adoption of the compass. Mariners in the relatively shallow Baltic made extensive use of soundings.

[edit] Developments in China

Due to the place of its first known appearance, most scholars credit at present the invention of the compass to China. Since there has been frequently confusion as to when a compass was introduced for the first time, it may be appropiate to list the important events leading up to its invention in chronological order:

  • The earliest Chinese literary reference to magnetism lies in a 4th century BC book called Book of the Devil Valley Master (鬼谷子): "The lodestone makes iron come or it attracts it."<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.175</ref>
  • The first mention of the magnetic attraction of a needle is to be found in a Chinese work composed between 20 and 100 AD (Louen-heng): "A lodestone attracts a needle."<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.176</ref> In 1948, the scholar Wang Tchen-touo tentatively constructed a 'compass' in the form of south-indicating spoon on the basis of this text. However, it should be noted that "there is no explicit mention of a magnet in the Louen-heng" and that "beforehand it needs to assume some hypotheses to arrive at such a conclusion".<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.180</ref>
  • The earliest reference to a magnetic device as a direction finder is recorded in a Song dynasty book dated to 1040-44. Here we find a description of an iron "south-pointing fish" floating in a bowl of water, aligning itself to the south. The device is recommended as a means of orientation "in the obscurity of the night." There is, however, no mention of a use for navigation, nor how the fish was magnetized.<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.181</ref>
  • The first incontestable reference to a magnetized needle in Chinese literature appears as late as 1086.<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.182f.</ref> The Dream Pool Essay written by Song Dynasty scholar Shen Kua contained a detailed description of how geomancers magnetized a needle by rubbing its tip with lodestone, and hung the magnetic needle with one single strain of silk with a bit of wax attached to the center of the needle. Shen Kua pointed out that a needle prepared this way sometimes pointed south, sometimes north.
  • The earliest recorded actual use of a magnetized needle for navigational purposes then is to be found in Zhu Yu's book Pingzhou Table Talks (Pingzhou Ke Tan) of AD 1117: The navigator knows the geography, he watches the stars at night, watches the sun at day; when it is dark and cloudy, he watches the compass.

Thus, the first clear instance of a magnetic direction finder, a compass, appeared ca. 1044. However, it should be pointed out that the compass remained known to the Chinese only in the form of a magnetic needle floating in a bowl of water<ref>Barbara M. Kreutz, “Mediterranean Contributions to the Medieval Mariner's Compass,” Technology and Culture, Vol. 14, No. 3. (Jul., 1973), p.373</ref>. The true mariner's compass used a pivoting needle in a dry box, and was invented in Europe no later than 1300.<ref>Frederic C. Lane, “The Economic Meaning of the Invention of the Compass,” The American Historical Review, Vol. 68, No. 3. (Apr., 1963), p.615ff.</ref> It was then introduced in China via the sea lanes in the late 16th or early 17th century, according to a contemporary Chinese source by "the Japanese".<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.193f.</ref>

A pilot's compass handbook titled Shun Feng Xiang Song (Fair Winds for Escort) in the Oxford Bodleian Library contains great details about the use of compass in navigation.

[edit] Question of Diffusion

Navigational mariner's compass
There is much debate on what happened to the compass after its first appearance with the Chinese. Different theories include:

The latter two are supported by evidence of the earlier mentioning of the compass in European works rather than Arabic. The first European mention of a magnetized needle and its use among sailors occurs in Alexander Neckam's De naturis rerum (On the Natures of Things), probably written in Paris in 1190.<ref>Barbara M. Kreutz, “Mediterranean Contributions to the Medieval Mariner's Compass,” Technology and Culture, Vol. 14, No. 3. (Jul., 1973), p.368</ref> Other evidence for this includes the Arabic word for "Compass" (al-konbas), possibly being a derivation of the old Italian word for compass.

In the Arab world, the earliest reference comes in The Book of the Merchants' Treasure, written by one Baylak al-Kibjaki in Cairo about 1282.<ref>Barbara M. Kreutz, “Mediterranean Contributions to the Medieval Mariner's Compass,” Technology and Culture, Vol. 14, No. 3. (Jul., 1973), p.369</ref> Since the author describes having witnessed the use of a compass on a ship trip some forty years earlier, some scholars are inclined to antedate its first appearance accordingly. There is also a slightly earlier non-Mediterranean Muslim reference to an iron fish-like compass in a Persian talebook from 1232.<ref>Barbara M. Kreutz, “Mediterranean Contributions to the Medieval Mariner's Compass,” Technology and Culture, Vol. 14, No. 3. (Jul., 1973), p.370</ref>

[edit] Question of independent European invention

There have been various arguments put forward whether the European compass was an independent invention or not:

Arguments pro independent invention:

  • The navigational needle in Europe points invariably north, whereas always south in China.
  • The European compass showed from the beginning sixteen basic divisions, not twenty-four as in China.
  • The apparent failure of the Arabs to function as possible intermediaries between East and West due to the earlier recorded appearance of the compass in Europe (1190) than in the Muslim world (1232, 1242, or 1282).
  • The fact that the European compass rather soon developed from the magnetized needle (1190) into the dry compass (by 1300) may indicate that the prior invention of the needle-and-bowl device was also done independently.

Arguments contra independent invention:

  • The temporal priority of the Chinese navigational compass (1117) as opposed to the European (1190).
  • The common shape of the early compass as a magnetized needle floating in a bowl of water.

[edit] Impact in the Mediterranean

In the Mediterranean the practice from ancient times had been to curtail sea travel between October and April, due in part to the lack of dependable clear skies during the Mediterranean winter (and much of the sea is too deep for soundings). With improvements in dead reckoning methods, and the development of better charts, this changed during the second half of the 13th century. By around 1290 the sailing season could start in late January or February, and end in December. The additional few months were of considerable economic importance; it enabled Venetian convoys, for instance, to make two round trips a year to the eastern Mediterranean, instead of one.

Around the time Europeans learned of the compass, traffic between the Mediterranean and northern Europe increased, and one factor may be that the compass made traversal of the Bay of Biscay safer and easier.

[edit] Construction of a simple compass

A magnetic rod is required when constructing a compass. This can be created by aligning an iron or steel rod with Earth's magnetic field and then tempering or striking it. However, this method produces only a weak magnet so other methods are preferred. This magnetised rod (or magnetic needle) is then placed on a low friction surface to allow it to freely pivot to align itself with the magnetic field. It is then labeled so the user can distinguish the north-pointing from the south-pointing end; in modern convention the north end is typically marked in some way, often by being painted red.

Flavio Gioja (fl. 1302), an Italian marine pilot, is sometimes credited with perfecting the sailor's compass by suspending its needle over a fleur-de-lis design, which pointed north. He also enclosed the needle in a little box with a glass cover.

[edit] Modern navigational compasses

Image:Liquid filled compass.jpg Modern navigational compasses hold a magnetized needle inside a fluid-filled capsule; the fluid causes the needle to stop quickly rather than oscillate back and forth around magnetic north. The needle may be mounted on a "card" with various markings, which rotates with it. Other features common on modern handheld compasses are a baseplate with rulings for measuring distances on maps, a rotating bezel, for measuring bearings of distant objects or setting the bearing of travel, and a sighting mirror that lets the user see both the compass needle and a distant object at the same time, or a lens that permits reading the bearing off of the compass card with only a slight glance down from the sights (see photo). Further, some modern compasses include an inclinometer for measuring gradients and are adjustable to account for varying magnetic declination, a serious accuracy issue.

Near the poles, the magnetic force is partly vertical, which can put so much tilting force on the needle that it binds, and doesn't swing to the correct position. (If the needle is heavily counterbalanced, for use near the poles, and is then used near the equator, it may also bind.)

Almost identical to the photo is a military lensatic compass that uses electromagnetic induction, by the swinging needle, of eddy currents in an imperfectly conductive, nearby metal part for damping, rather than liquid. A needle lock, to reduce wear when not in use, is operated by the folding action of the rear sight/lens holder. While needle locks have been common on dry, handheld compasses, they can prove desirable on liquid filled, handheld compasses under heavy use, but are not sufficiently feasible to provide.

Mariner's compasses can have two or more magnetic needles permanently attached to a compass card. These move freely on a pivot. A lubber line, which can be a marking on the compass bowl or a small fixed needle indicates the ship's heading on the compass card.

Traditionally the card is divided into thirty-two points (known as rhumbs), although modern compasses are marked in degrees rather than cardinal points. The glass-covered box (or bowl) contains a suspended gimbal within a binnacle. This preserves the horizontal position.

Large ships typically rely on a gyrocompass, using the more reliable magnetic compass for back-up. Increasingly electronic fluxgate compasses are used on smaller vessels.

Compasses are available marked in mils - a unit of measurement commonly used by the military.

[edit] Solid state compasses

Small compasses found in clocks, cell phones (e.g. the Nokia 5140i) and other electronic gear are Solid-state electronics usually built out of two or three magnetic field sensors that provide data for a microprocessor. Using trigonometry the correct heading relative to the compass is calculated.

Often, the device is a discrete component which outputs either a digital or analog signal proportional to its orientation. This signal is interpreted by a controller or microprocessor and used either internally, or sent to a display unit. An example implementation, including parts list and circuit schematics, shows one design of such electronics. The sensor uses precision magnetics and highly calibrated internal electronics to measure the response of the device to the Earth's magnetic field. The electrical signal is then processed or digitized.

[edit] Compass correction

Image:MuseeMarine-compas-p1000468.jpg

Main article: Magnetic deviation

A ship's compass must be corrected for errors, called compass deviation, caused by iron and steel in its structure and equipment. The ship is swung, that is rotated about a fixed point while its heading is noted by alignment with fixed points on the shore. A compass deviation card is prepared so that the navigator can convert between compass and magnetic headings. The compass can be corrected in three ways. First the lubber line can be adjusted so that it is aligned with the direction in which the ship travels, then the effects of permanent magnets can be corrected for by small magnets fitted within the case of the compass. The effect of ferromagnetic materials in the compass's environment can be corrected by two iron balls mounted on either side of the compass binacle. The coefficient <math>a_0</math> representing the error in the lubber line, while <math>a_1,b_1</math> the ferromagnetic effects and <math>a_2,b_2</math> the non-ferromagnetic component.

Fluxgate compasses can be calibrated automatically, and can also be programmed with the correct local compass variation so as to indicate the true heading.

[edit] Using a compass

Image:CompassUseMapMarked.jpg Image:CompassUseTargetMarked.jpg The simpliest way of using a compass is just to know that the arrow always points in the same direction, magnetic North, which is also the top of the map. This is enough to protect from walking in a different or even opposite direction than expected. On flat terrain, in a forest or dense fog, such mistakes are not unlikely.

More exact navigation requires a map and knowledge of the current location. Then the edge of the compass is placed so that it connects the current location with the desired destination (some sources recommend to draw the line). The compass scale is then rotated, making "north" marking to point to the actual map's north (normally the top edge of the map). In the given example, the huge mountain is selected as a target, as it was also just visible, making the illustration more obvious.

The hand held compass usually has the additional arrow drawn on the corpus. After orienting the compass so that the north arrow matches the marking on the scale, this arrow is oriented more or less toward the chosen target, in a real world. However for the precise orientation you may need additionally to turn the scale several degrees in a known direction, compensating the magnetic declination. The exact value of the magnetic declination is place dependent and is frequently given on the map itself. If not, any local walker club should know it.

  • Literature [1]

[edit] Compass balancing

Because the Earth's magnetic field varies at different latitudes, compasses are often balanced during manufacture. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings.

Some different compass systems:

Compass with 400 grades division and conversion table
Swiss army compass with mils division
Land surveyor compass with clinometer
Stratum compass after Prof. Clar


[edit] Points of the compass

Main article: Boxing the compass

The mariner's compass card is divided into thirty-two equally spaced points. Four of these - east, west, north, and south - are the cardinal points, and the names of the others are derived from these.

[edit] See also

[edit] Gallery

A simple compass typical to a small yacht
A compass suitable for orienteering in the night


[edit] Notes

<references/>

[edit] Literature

  • Admiralty manual of navigation, Chapter XXV The Magnetic Compass (continued) the analysis and correction of the deviation, His Majesty's Stationary Office, London, 1914.
  • Amir Aczel, The Riddle of the Compass: The Invention that Changed the World, ISBN 0-15-600753-3.
  • Barbara M. Kreutz, “Mediterranean Contributions to the Medieval Mariner's Compass,” Technology and Culture, Vol. 14, No. 3. (Jul., 1973), pp. 367-383
  • Frances and Joseph Gies, Cathedral, Forge, and Waterwheel subtitled "Technology and Invention in the Middle Ages"
  • Frederic C. Lane, “The Economic Meaning of the Invention of the Compass,” The American Historical Review, Vol. 68, No. 3. (Apr., 1963), pp. 605-617.
  • J.E.D. Williams From Sails to Satellites. New York: Oxford University Press, 1992
  • Joseph Needham, Colin A. Ronan: The Shorter Science & Civilisation in China Vol 3 Chapter 1 Magnetism and Electricity.
  • Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), pp. 175-196.

[edit] External links

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