What is Compass ?

An important feature found on many compasses is automatic declination adjustment. Declination, also known as variance, is the difference between magnetic North (the direction to which the needle points) and true North.

To use a compass like this, you first figure out which direction is north.

This difference exists because Earth's magnetic field does not align exactly with its North and South poles. The amount of declination varies from place to place on Earth's surface. If the amount of declination is known for a particular area, automatic declination adjustment allows the compass user to read true direction directly from the compass rather than having to add or subtract the amount of declination every time the compass is used.

You let the needle settle then rotate the compass card so the needle lines up with the north-south axis and the end of the needle colored red, marked with an arrow, or printed 'N' points north.

By 500 b.c., it was known that lodestone, a naturally occurring form of iron oxide also known as magnetite, had the ability to attract iron. No one knows where or when it was first noticed that a freely moving piece of lodestone tended to align itself so that it was pointing North and South. Written records indicate that the Chinese used magnetic compasses by 1100 a.d., western Europeans and Arabs by 1200 a.d., and Scandinavians by 1300 a.d.

You can then instantly see which direction is south, east, or west and (with the help of a map) set off in the direction you need to go.

Early compasses consisted of a piece of lodestone on a piece of wood, a cork, or a reed floating in a bowl of water. Somewhat later, a needle of lodestone was pivoted on a pin fixed to the bottom of a bowl of water. By the thirteenth century, a card marked with directions was added to the compass.

Magnetism is one of the first bits of science we learn in school and just about the first thing we discover is that "like poles repel, unlike poles attract."

In other words, if you hold two bar magnets so their north poles are almost touching, they'll push away from one another; if you turn one of the magnets around so one magnet's north pole is near the other magnet's south pole, the magnets will pull toward one another.

By the middle of the sixteenth century, the bowl of water was suspended in gimbals, which allowed the compass to remain level while being used aboard a ship being tossed by the ocean.

That's all there is to a compass: the red pointer in a compass (or the magnetized needle on your home-made compass) is a magnet and it's being attracted by Earth's own magnetism (sometimes called the geomagnetic field—"geo" simply means Earth).

In 1745, the English inventor Gowin Knight developed a method for magnetizing steel for long periods of time. This allowed needles of magnetized steel to replace needles of lodestone. During the early nineteenth century, iron and steel began to be used extensively in shipbuilding.

As English scientist William Gilbert explained about 400 years ago, Earth behaves like a giant bar magnet with one pole up in the Arctic (near the north pole) and another pole down in Antarctica (near the south pole). Now if the needle in your compass is pointing north, that means it's being attracted (pulled toward) something near Earth's north pole.

This caused distortions in the operation of magnetic compasses. In 1837, the British Admiralty set up a special commission to study the problem. By 1840, a new compass design using four needles was so successful at overcoming this difficulty that it was soon adopted by navies around the world.

Since unlike poles attract, the thing your compass is being attracted to must be a magnetic south pole. In other words.

Until the middle of the nineteenth century, navigators used both dry-card compasses, in which the needle pivoted in air, and liquid compasses, in which the needle pivoted in water or another liquid. Dry-card compasses were easily disturbed by shocks and vibrations, while liquid compasses tended to leak and were difficult to repair.

the thing we call Earth's magnetic north pole is actually the south pole of the magnet inside Earth.

In 1862, improvements in the design of liquid compasses quickly made the dry-card compass obsolete for naval use. By World War 1, the British Army used liquid compasses on land, and liquid compasses are still the standard for the best hand held magnetic compasses.

That's quite a confusing idea, but it'll make sense if you always remember that unlike poles attract.

The needle of a magnetic compass must be made of a metallic substance, which can be magnetized for an extended period of time. The most common substance used for compass needles is steel. Steel is an alloy of iron and a small amount of carbon.

Earth's magnetic field is actually quite weak compared to the "macho" forces like gravity and friction that really dominate our lives.

The raw materials used to produce steel are iron ore and coke (a carbon-rich substance produced by heating coal to a high temperature in the absence of air). Other substances such as cobalt are often added to the steel to produce alloys, which can be magnetized for a very long time.

For a compass to be able to show up the relatively tiny effects of Earth's magnetism, we have to minimize the effects of these other forces.

The housing that holds the needle in place is often made of acrylic plastic. Acrylic plastics are produced from various derivatives of the chemical compound acrylic acid. The most important of these derivatives is methyl methacrylate. Thousands of molecules of methyl methacrylate are linked into a long chain to form polymethyl methacrylate, known by the trade names Lucite and Plexiglas. Polymethyl methacrylate has the advantages of being strong and transparent.

That's why compass needles are lightweight (so gravity has less effect on them) and mounted on frictionless bearings (so there's less frictional resistance for the magnetic force to overcome).