Editing Compass (section)

== Design ==
[[File:walkers compass arp.jpg|thumb|upright|A liquid-filled protractor or orienteering compass with lanyard]]

Modern compasses usually use a magnetized needle or dial inside a capsule completely filled with a liquid (lamp oil, mineral oil, white spirits, purified kerosene, or ethyl alcohol are common). While older designs commonly incorporated a flexible rubber diaphragm or airspace inside the capsule to allow for volume changes caused by temperature or altitude, some modern liquid compasses use smaller housings and/or flexible capsule materials to accomplish the same result.<ref>''Gear Review: Kasper & Richter Alpin Compass'', OceanMountainSky.Com</ref> The liquid inside the capsule serves to damp the movement of the needle, reducing oscillation time and increasing stability. Key points on the compass, including the north end of the needle are often marked with [[phosphorescent paint|phosphorescent]], [[photoluminescent]], or self-luminous materials<ref>Nemoto & Co. Ltd., [http://www.nemoto.co.jp/en/products/luminova/luminova.html Article] {{webarchive|url=https://web.archive.org/web/20081205120711/http://www.nemoto.co.jp/en/products/luminova/luminova.html |date=2008-12-05 }}: In addition to ordinary phosphorescent luminous paint ([[zinc sulfide]]), brighter photoluminescent coatings which include radioactive [[isotope]]s such as [[Strontium-90]], usually in the form of [[strontium aluminate]], or [[tritium]], which is a radioactive isotope of [[hydrogen]] are now being used on modern compasses. Tritium has the advantage that its radiation has such low energy that it cannot penetrate a compass housing.</ref> to enable the compass to be read at night or in poor light. As the compass fill liquid is noncompressible under pressure, many ordinary liquid-filled compasses will operate accurately underwater to considerable depths.

Many modern compasses incorporate a baseplate and [[protractor]] tool, and are referred to variously as "[[orienteering]]", "baseplate", "map compass" or "protractor" designs. This type of compass uses a separate magnetized needle inside a rotating capsule, an orienting "box" or gate for aligning the needle with magnetic north, a transparent base containing map orienting lines, and a bezel (outer dial) marked in degrees or other units of angular measurement.<ref name=des110>[[#Johnson|Johnson]], p. 110</ref> The capsule is mounted in a transparent baseplate containing a ''direction-of-travel'' (DOT) indicator for use in taking bearings directly from a map.<ref name=des110 />

[[File:Cammenga-lensatic-compass-model-27.jpg|thumb|left|Cammenga air filled lensatic compass]]

Other features found on modern orienteering compasses are map and [[Romer (tool)|romer]] scales for measuring distances and plotting positions on maps, luminous markings on the face or bezels, various [[sighting compass|sighting mechanisms]] (mirror, prism, etc.) for taking bearings of distant objects with greater precision, gimbal-mounted, "global" needles for use in differing hemispheres, special rare-earth magnets to stabilize compass needles, adjustable declination for obtaining instant true bearings without resorting to arithmetic, and devices such as [[inclinometer]]s for measuring gradients.<ref name=des110111>[[#Johnson|Johnson]], pp. 110–111</ref> The sport of orienteering has also resulted in the development of models with extremely fast-settling and stable needles utilizing rare-earth magnets for optimal use with a [[topographic map]], a land navigation technique known as ''terrain association''.<ref>Kjernsmo, Kjetil, ''[http://www.learn-orienteering.org/old/buying.html How to use a Compass]'', retrieved 8 April 2012 {{Webarchive|url=https://web.archive.org/web/20200302170651/https://www.learn-orienteering.org/old/buying.html |date=2 March 2020 }}</ref> Many marine compasses designed for use on boats with constantly shifting angles use dampening fluids such as [[isopar M]] or [[isopar L]] to limit the rapid fluctuation and direction of the needle.<ref>{{cite web|url=https://eastmarineasia.com/products/ritchie-compass-fluid|title=Ritchie Compass Fluid|work=EastMarineAsia.com}}</ref>

The military forces of a few nations, notably the United States Army, continue to issue field compasses with magnetized compass dials or cards instead of needles. A magnetic card compass is usually equipped with an optical, lensatic, or [[prismatic sight]], which allows the user to read the bearing or azimuth off the compass card while simultaneously aligning the compass with the objective (see photo). Magnetic card compass designs normally require a separate protractor tool in order to take bearings directly from a map.<ref name=des112>[[#Johnson|Johnson]], p. 112</ref><ref>U.S. Army, ''Map Reading and Land Navigation'', FM 21–26, Headquarters, Dept. of the Army, Washington, D.C. (7 May 1993), ch. 11, pp. 1–3: Any 'floating card' type compass with a straightedge or centerline axis can be used to read a map bearing by orienting the map to magnetic north using a drawn magnetic azimuth, but the process is far simpler with a protractor compass.</ref>

The U.S. M-1950 military lensatic compass does not use a liquid-filled capsule as a damping mechanism, but rather [[electromagnetic induction]] to control oscillation of its magnetized card. A "deep-well" design is used to allow the compass to be used globally with a card tilt of up to 8 degrees without impairing accuracy.<ref>''[http://landnavigation.org/Documents/Compass%20Mil%20Specs.pdf Article MIL-PRF-10436N]'', rev. 31 October 2003, Washington, D.C.: U.S. Dept. of Defense</ref> As induction forces provide less damping than fluid-filled designs, a needle lock is fitted to the compass to reduce wear, operated by the folding action of the rear sight/lens holder. The use of air-filled induction compasses has declined over the years, as they may become inoperative or inaccurate in freezing temperatures or extremely humid environments due to condensation or water ingress.<ref>Kearny, Cresson H., ''Jungle Snafus&nbsp;... And Remedies'', Oregon Institute Press (1996), {{ISBN|1-884067-10-7}}, pp. 164–170: In 1989, one U.S. Army jungle infantry instructor reported that about 20% of the issue lensatic compasses in his company used in a single jungle exercise in [[Panama]] were ruined within three weeks by rain and humidity.</ref>

Some military compasses, like the U.S. M-1950 ([[Cammenga]] 3H) military lensatic compass, the [[Silva compass|Silva 4b ''Militaire'']], and the [[Suunto]] M-5N(T) contain the radioactive material [[tritium]] ({{nuclide|hydrogen|3}}) and a combination of phosphors.<ref>Ministry of Defence, ''Manual of Map Reading and Land Navigation'', HMSO Army Code 70947 (1988), {{ISBN|0-11-772611-7|978-0-11-772611-6}}, ch. 8, sec. 26, pp. 6–7; ch. 12, sec. 39, p. 4</ref> The U.S. M-1950 equipped with self-luminous lighting contains 120 mCi (millicuries) of tritium. The purpose of the tritium and phosphors is to provide [[Illumination (lighting)|illumination]] for the compass, via [[radioluminescence|radioluminescent]] [[tritium illumination]], which does not require the compass to be "recharged" by sunlight or artificial light.<ref>{{cite web|url=https://www.orau.org/health-physics-museum/collection/radioluminescent/military-compass.html |title=Military Compass |publisher=Orau.org |access-date=2021-11-03}}</ref> However, tritium has a [[half-life]] of only about 12 years,<ref>[[CRC Handbook of Chemistry and Physics]]. p. B247</ref> so a compass that contains 120&nbsp;mCi of tritium when new will contain only 60 when it is 12 years old, 30 when it is 24 years old, and so on. Consequently, the illumination of the display will fade.

Mariners' compasses can have two or more magnets permanently attached to a compass card, which moves 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.
[[File: CompassWick.jpg|thumb|A Greek maritime liquid compass with an additional wick compartment for illumination.]]
[[File:מצפן.jpg|alt=A close up photo of a geological compass|thumb|A close up photo of a geological compass]]
The magnetic compass is very reliable at moderate latitudes, but in geographic regions near the Earth's magnetic poles it becomes unusable. As the compass is moved closer to one of the magnetic poles, the magnetic declination, the difference between the direction to geographical north and magnetic north, becomes greater and greater. At some point close to the magnetic pole the compass will not indicate any particular direction but will begin to drift. Also, the needle starts to point up or down when getting closer to the poles, because of the so-called [[magnetic inclination]]. Cheap compasses with bad [[bearing (mechanical)|bearings]] may get stuck because of this and therefore indicate a wrong direction.

Magnetic compasses are influenced by any fields other than Earth's. Local environments may contain magnetic mineral deposits and artificial sources such as [[MRI]]s, large iron or steel bodies, electrical engines or strong permanent magnets. Any electrically conductive body produces its own magnetic field when it is carrying an electric current. Magnetic compasses are prone to errors in the neighborhood of such bodies. Some compasses include magnets which can be adjusted to compensate for external magnetic fields, making the compass more reliable and accurate.

{{main|Magnetic dip#Acceleration error}}

A compass is also subject to errors when the compass is accelerated or decelerated in an airplane or automobile. Depending on which of the Earth's hemispheres the compass is located and if the force is acceleration or deceleration the compass will increase or decrease the indicated heading. Compasses that include compensating magnets are especially prone to these errors, since accelerations tilt the needle, bringing it closer or further from the magnets.

{{main|Magnetic dip#Turning error}}

[[File:Northerly and southerly turning errors.png|thumb|The dipping effect causes compass card to lead in a northerly turning error (fig. A) and lag in a southerly turning error (fig. B).<ref>{{Cite PHAK|year=2016|chapter=8|page=26}}</ref>]]

Another error of the mechanical compass is the turning error. When one turns from a heading of east or west the compass will lag behind the turn or lead ahead of the turn. Magnetometers, and substitutes such as gyrocompasses, are more stable in such situations.