What Is a Magnetic Compass?

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Almost everyone has used some sort of device that allows for determination of traditional directions – north, south, east, west and combinations thereof. The days of youngsters romping though the woods with hand-held models fitted with an actual compass needle, however, have largely fallen into the dustbin of navigational history.

Today, virtually all smartphones come equipped with Global Positioning System (GPS) receivers that allow users to figure out where they are on Earth's directional "grid" within a few meters. This technology relies on a network of satellites in a continuous orbit high above Earth's atmosphere. But before modern rocketry, navigators relied on a now-outdated but extraordinarily clever way of determining direction.

A magnetic compass is a tool that fundamentally allows for the determination of a reference point or region on Earth corresponding to magnetic north. This is slightly different from true north, but with the varying correction factors required at different points around the globe now known, a good magnetic compass remains good enough to get a practiced user from place to place quite nicely.

Magnets and Magnetic Field Basics

Magnetism is a term describing a mathematically predictable set of effects on particles and systems in the branch of physics known as electromagnetics. As with its inseparable partner, electricity, magnetism is not something that can be "seen," but many of its effects in the real world are well known and have been incorporated into countless critical aspects of modern technology.

Magnetic "fields," which may be thought of as lines of influence on particles subject to the physical effects of magnetism, are drawn as originating from a north magnetic pole and flowing outward through space and back toward a south magnetic pole. In the case of a bar magnet (a rectangular magnet), this means a series of roughly C-shaped lines "flowing" from magnetic north to magnetic south.

  • Unlike the case with electric charges, there is no such thing as a "magnetic monopole." In other words, there can be no point source of a magnetic field in the way an electric field can be created and defined by a single point charge.

Magnetic fields are created by moving electric charges. This can be explicit and a function of purposeful engineering, as when a coil of current-carrying wire is wrapped many times around a piece of metal, creating an electromagnet. These are used in the generation of electrical power and in other critical industrial applications worldwide. The key trait of an electromagnet is that it ceases to be a magnet of any consequence once the current source is removed.

Alternatively, the source of moving charges underlying magnetic fields can "hide," being produced at the level of individual atoms in certain elements (e.g., iron, copper and nickel). Thanks in part to the "spin" characteristics of these elements' electrons, magnetic moments are created in the atoms in question, and in these ferromagnetic elements, local magnetic moments are additive rather than canceling in pairs (to simplify, the norm in most elements). The result is a piece of metal you know as a magnet.

Earth's Magnetic Field

The Earth is divided into Northern Hemisphere and a Southern Hemisphere, or "top" and "bottom" halves. The farthest points on the globe from a line drawn around the widest part of Earth in the direction of its rotation, called the equator, are known as poles. The Earth's rotation axis passes through and defines the North Pole and the South Pole. The former sits on ice, while the latter is located on a large continental land mass (Antarctica).

You've already learned that magnetic field lines are drawn from magnetic north to magnetic south. Yet when you see a diagram if the Earth's magnetic field, you see lines, most of them far above the surface, originating at the South Pole and ending at the North Pole. This is because the North Pole, by mere chance, constitutes a south magnetic pole, and correspondingly for the South Pole. No confusion was meant by this; the geography just happened to not line up with the physics because of the happenstance placement of a large deposit of iron ore in Canada (more on this soon).

Thus the reason a compass needle points in the direction humans have labeled "magnetic north" is that the needle is compelled to orient itself in the same direction as Earth's magnetic field, owing to a shift in the electrons in the atoms of the needle's material in response to the field. Think of arrow at the tip of a compass needle as being analogous to the arrow at the tip of the magnetic field lines: They point in the same direction.

Magnetic North Versus True North

The needle on your magnetic compass points not at the true North Pole, but at a point that is presently about 500 kilometers (about 310 miles) from the North Pole, on Ellesmere Island in northern Canada. This is owed to the presence of a large deposit of iron ore, which serves as a sort of "magnetic sink," and "sucks" one end of the needle toward the deposit of ore.

Note that it would be equally fair to say that the other end of the needle "points" south, while the other end is simply spun about as a consequence; it is really a matter of sailors centuries ago originally having chosen north as a fundamental navigational starting point, due to their location in the Northern Hemisphere.

Because navigation across large distances has been so critical for so long, correction factors for true versus magnetic north have been available for various points on Earth since well before computerization rendered this a more mundane task.

History of the Magnetic Compass

The Chinese are believed to have understood the properties of the lodestone as long as 2,000 years ago. This rare mineral is called a natural magnet today. When it happens to come in a long, oblong shape like an oversized needle, it will orient itself in Earth's magnetic field when suspended from above. The Chinese noticed this, but were stymied as to why it occurred.

By the 11th or 12th century A.D., the Chinese were using magnetic compasses for navigation. They were followed in short order (on a historical scale) by explorers from Europe and elsewhere. Initially, these pioneers failed to understand two important things: The reference point they called "north" thanks to their compasses was not in fact fixed during long journeys, and it differed by different amounts in different places.

This realization led to the development of a de facto database of correction factors for the entire world. Until the age of satellites, even the most elite military units relied on what now seems outlandishly archaic land navigation using the highest-tech magnetic compasses anywhere.

How to Make a Magnetic Compass

All you need to make your own magnetic compass is a bowl of water, a piece of cork, an ordinary sewing needle, a refrigerator magnet and an existing compass.

First, rub the sewing needle rapidly 50 times along an ordinary refrigerator magnet. Important: Do this in one direction only; in other words, not back and forth.

Then, place the cork in the bowl of water, and place the needle gently on top of the cork. Put the compass next to this assembly, so that you can see where north is. Soon, if you have managed to magnetize the needle, the needle will orient itself in the same direction as the compass needle.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.