Editing Compass (section)

== Construction ==
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. For example, a magnetised rod can be created by repeatedly rubbing an iron rod with a magnetic [[lodestone]]. 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.

If a needle is rubbed on a [[lodestone]] or other magnet, the needle becomes magnetized. When it is inserted in a cork or piece of wood, and placed in a bowl of water it becomes a compass. Such devices were universally used as compasses until the invention of the box-like compass with a "dry" pivoting needle, sometime around 1300.

{{Main|Points of the compass}}
[[File:Boussole fantassin russe.jpg|thumb|Wrist compass of the Soviet Army with counterclockwise double graduation: 60° (like a watch) and 360°]]
Originally, many compasses were marked only as to the direction of magnetic north, or to the four cardinal points (north, south, east, west). Later, these were divided, in China into 24, and in Europe into 32 equally spaced points around the compass card. For a table of the thirty-two points, see [[points of the compass|compass points]].

In the modern era, the 360-degree system took hold. This system is still in use today for civilian navigators. The degree system spaces 360 equidistant points located clockwise around the compass dial. In the 19th century some European nations adopted the "[[grad (angle)|grad]]" (also called grade or gon) system instead, where a right angle is 100 grads to give a circle of 400 grads. Dividing grads into tenths to give a circle of 4000 [[decigrade]]s has also been used in armies.

Most military forces have adopted the French "[[millieme (angle)|millieme]]" system. This is an approximation of a milli-radian (6283 per circle), in which the compass dial is spaced into 6400 units or "mils" for additional precision when measuring angles, laying artillery, etc. The value to the military is that one [[angular mil]] subtends approximately one metre at a distance of one kilometer. Imperial Russia used a system derived by dividing the circumference of a circle into chords of the same length as the radius. Each of these was divided into 100 spaces, giving a circle of 600. The [[Soviet Union]] divided these into tenths to give a circle of 6000 units, usually translated as "mils". This system was adopted by the former [[Warsaw Pact]] countries, ''e.g.'', the Soviet Union, [[East Germany]], etc., often counterclockwise (see picture of wrist compass). This is still in use in Russia.

Because the Earth's magnetic field's inclination and intensity vary at different latitudes, compasses are often balanced during manufacture so that the dial or needle will be level, eliminating needle drag. 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 individual zone balancing prevents excessive dipping of one end of the needle, which can cause the compass card to stick and give false readings.<ref name=globalcompass>{{Cite web|title=Global Compasses|url=http://www.mapworld.co.nz/global.html|access-date=2023-03-16|website=www.mapworld.co.nz}}</ref>

Some compasses feature a special needle balancing system that will accurately indicate magnetic north regardless of the particular magnetic zone. Other magnetic compasses have a small sliding counterweight installed on the needle. This sliding counterweight, called a "rider", can be used for counterbalancing the needle against the dip caused by inclination if the compass is taken to a zone with a higher or lower dip.<ref name=globalcompass />

{{Main|Magnetic deviation}}
[[File:MuseeMarine-compas-p1000468.jpg|thumb|A [[binnacle]] containing a ship's standard compass, with the two iron balls which correct the effects of [[ferromagnetism|ferromagnetic]] materials. This unit is on display in a museum.]]
Like any magnetic device, compasses are affected by nearby ferrous materials, as well as by strong local electromagnetic forces. Compasses used for wilderness land navigation should not be used in proximity to ferrous metal objects or electromagnetic fields (car electrical systems, automobile engines, steel [[piton]]s, etc.) as that can affect their accuracy.<ref name=des122>[[#Johnson|Johnson]], p. 122</ref> Compasses are particularly difficult to use accurately in or near trucks, cars or other mechanized vehicles even when corrected for deviation by the use of built-in magnets or other devices. Large amounts of ferrous metal combined with the on-and-off electrical fields caused by the vehicle's ignition and charging systems generally result in significant compass errors.

At sea, a ship's compass must also be corrected for errors, called [[Magnetic deviation|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 [[ferromagnetism|ferromagnetic]] materials in the compass's environment can be corrected by two iron balls mounted on either side of the compass binnacle in concert with permanent magnets and a [[Flinders bar]].<ref>{{Cite web|url=https://msix.nga.mil/StaticFiles/HoMCA.pdf|title=Handbook of Magnetic Compass Adjustment|last=GEOSPATIAL-INTELLIGENCE AGENCY|first=National|date=2004|access-date=2019-05-09|archive-date=2019-05-09|archive-url=https://web.archive.org/web/20190509224408/https://msix.nga.mil/StaticFiles/HoMCA.pdf|url-status=dead}}</ref> The coefficient <math>a_0</math> represents 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.<ref>{{cite journal | last =Lushnikov | first =E. | title =Magnetic Compass in Modern Maritime Navigation | journal =TransNav, the International Journal on Marine Navigation and Safety of Sea Transportation | volume =9 | issue =4 | pages =539–543 | date =December 2015 | url =http://www.transnav.eu/Article_Magnetic_Compass_in_Modern_Lushnikov,36,613.html | doi =10.12716/1001.09.04.10 | access-date =11 February 2016 | doi-access =free }}</ref>

A similar process is used to calibrate the compass in light general aviation aircraft, with the compass deviation card often mounted permanently just above or below the magnetic compass on the instrument panel. Fluxgate electronic compasses can be calibrated automatically, and can also be programmed with the correct local compass variation so as to indicate the true heading.