A compass is a device used to determine direction on the surface of the earth. The most familiar type of compass is the magnetic compass, which relies on the fact that a magnetic object tends to align itself with Earth's magnetic field. Other types of compasses determine direction by using the position of the Sun or a star, or by relying on the fact that a rapidly spinning object (a gyroscope) tends to resist being turned away from the direction in which its axis is pointing.
The basic parts of a magnetic compass are the needle (a thin piece of magnetic metal), the dial (a circular card printed with directions), and the housing (which holds the other parts in place). Inexpensive compasses, generally used as toys, may have no other parts. Compasses intended for more serious purposes usually have other parts to make them more useful. These other parts may include lids, covers, or cases to protect the compass; sights making use of lenses, prisms, or mirrors to enable the user to determine the direction of an object in the distance; and a transparent baseplate marked with a scale of inches or millimeters so that the compass can be used directly on a map.
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. 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.
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.
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. 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.
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. 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.
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. 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.
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. 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.
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.
Making the needle
- 1 Iron ore, coke, and limestone are heatedI in a blast furnace by hot pressurized air. The coke releases heat, which melts the ore, and carbon monoxide, which reacts with iron oxides in the ore to release iron. The limestone reacts with impurities in the ore such as sulfur to form slag, which floats on the molten iron and is removed. The product of this process is pig iron, which contains about 90% iron, 3-5% carbon, and various impurities.
- 2 To remove the impurities and most of the carbon, oxygen is blasted into the molten pig iron under high pressure. The impurities are released as slag and the carbon is released as carbon monoxide. The remaining molten steel is poured into molds and allowed to cool into ingots weighing thousands of pounds each.
- 3 The ingots are heated to about 2,200° F (1,200° C) and rolled between grooved rollers to form slabs. The slab is cut with giant shears, reheated, and rolled again until it is the proper thickness for needles. The thin sheet of steel is then stamped with a sharp die in the shape of the needle. The process is repeated to produce many needles from a single sheet of steel.
- 4 The needles are shipped from the steel manufacturer to the compass manufacturer. At the compass factory, the needles are inserted by hand into holders on an automated turntable. As the turntable spins the "North" end of the needle is sprayed with red paint and the "South" end of the needle is sprayed with white paint. As the needle continues, it is exposed to a strong magnetic field produced by an electronic magnetizer.
- 5 The magnetized needles are removed from the turntable and the paint is allowed to dry. The needles may also be baked in an oven to dry the paint. They are then placed in storage until needed for assembly.
Making the housing
6 Polymethyl methacrylate is formed by subjecting a solution of methyl
methacrylate to light, heat, or various chemical catalysts. The
components of the compass housing are then formed by a process known as
injection molding. Polymethyl methacrylate is heated until it melts into
a liquid. The molten plastic is then injected
Assembling the compass
- 7 When the compass manufacturer receives an order from a wholesaler, the plant manager arranges for the necessary parts to be issued from storage to workers on an assembly line. As the compass progresses along the assembly line, plastic components are snapped together. Some plastic components move through printers, which stamp them with markings such as a company logo, or with scale markings for use with maps.
- 8 One of the most critical components of a compass is the vial, which holds the needle. The needle is balanced on a pivot to enable it to move freely. Inexpensive compasses may have a steel pivot, but the best compasses have jeweled pivots in order to resist wear. Jeweled pivots are made of very hard materials such as an osmium-iridium alloy and are capped with a material such as artificial sapphire.
- 9 The vials are dipped in a liquid that will serve as a dampener. A dampener is a substance that causes the needle to come to rest more quickly when it is disturbed. Various liquids are used for dampeners. These liquids must be transparent and must not react with any of the components of the compass. A typical liquid used for this purpose might be a mixture of ethyl alcohol and water.
10 The vials filled with liquid are sealed using sonic welding. This
avoids exposing the needle to heat, which could disturb its magnetism.
In this process, ultra-sonic waves are used to melt the plastic at the
place where the vial is to be sealed. The plastic is then allowed to
solidify, forming a tight seal. Assembly of the compass continues as the
sealed vial is snapped onto a baseplate.
- 11 The completed compasses are packed in ways that protect them from theft and damage. They may be packed in clam packing, in which a plastic container resembling a clam shell surrounds the compass. They may also be packed in blister packing, in which a plastic bubble attached to a flat piece of cardboard surrounds the compass. The packaged compasses are placed in card-board boxes and shipped to the wholesaler.
At each step of the manufacturing process, the various components which make up the compass are visually inspected and removed if they are defective. Common imperfections include printing errors and bubbles in the dampening liquid. The most important part of the compass, the magnetic needle, is very unlikely to be defective. The few cases in which the needle does not work properly are usually caused by the consumer exposing the needle to a strong magnetic or electric field. In such cases, the needle may be remagnetized so that it points backwards, with the "North" end pointing south.
The most important part of quality control for a magnetic compass is the user's responsibility for learning how to use the compass properly. Compasses are very reliable instruments, but they are useless if the user does not know how to use them correctly. Knowing how to allow for declination is a critical skill in using a magnetic compass. In some parts of the world, failure to allow for declination could lead to an error of several degrees, causing the user to wind up many miles from the intended destination. An excellent way to learn proper use of a compass is to participate in the sport of orienteering. This sport involves using a map and compass to compete with others in finding a path from a starting point to a selected destination.
During the 1970s, the U.S. Navy began an ambitious project known as the Global Positioning System (GPS). The GPS project was taken over by the U.S. Air Force in the 1980s and completed in June 1993. GPS consists of a system of 24 satellites containing atomic clocks that broadcast extremely accurate time signals to Earth. By analyzing the exact time these signals arrive at a receiver, it is possible to determine position with great accuracy. Devices not much larger than an ordinary compass can determine location within about 100 ft (30 m).
At first glance, it may seem that GPS threatens to make the magnetic compass obsolete. In fact, the exact opposite is true. Because GPS indicates position but not direction, manufacturers of GPS equipment recommend that it be used with a compass. Compasses also have the advantage of requiring no energy supply. Unlike GPS, compasses can be used when heavy tree cover or large buildings block the reception of electronic signals. Although GPS promises to revolutionize navigation, traditional compasses will remain a vital component in how we find our way around.
Where to Learn More
Hogan, Paula Z. The Compass. Walker and Company, 1982.
Randall, Glenn. The Outward Bound Map and Compass Handbook. Lyons and Burford, 1989.
"Consumer's Guide to Compasses." Mechanix Illustrated (April 1978): 9.
Kleppner, Daniel. "Where I Stand." Physics Today (January 1994): 9-11.
— Rose Secrest