How a Compass Works

Humans have learned a lot of tricks for finding their way around this planet—the location of the sun as it sets on the horizon, the pattern of stars in the night sky, even changes in wind direction throughout the seasons.  About a thousand years ago Chinese mariners began employing a new trick that used an invisible force of nature to help chart their course.

That force, magnetism, had been observed in naturally occurring magnetic minerals for hundreds of years. Then one day, some long forgotten person discovered that a small piece of this magnetic material, if suspended from a string or floated on a liquid surface, always lined up in the same North/South direction.

Try this experiment yourself.  Rub the pointy end of a needle on a magnet to “magnetize” it, then place it gently in a cup of water.  The surface tension of the water should keep the needle from sinking, but if it doesn’t, float the needle on a small piece of cork.  The needle will slowly swing to point north.

The compass needle, like the magnetized sewing needle in our experiment, is also a magnet, balanced so that it can swing freely on a little pedestal. Magnetism is an invisible field associated with certain objects—magnetite and the planets in our solar system, for example. Why magnetism occurs is a question that has received a great deal of scientific attention and there are several good articles available on the web.

More central to our discussion, however, is the question of HOW magnetism works.  We have all had the experience of playing with two magnets and observing how they only align with each other in a certain direction. This phenomenon is called polarity.  Magnets have north poles that attract south poles, and vice-versa.  North poles repel each other, as do south poles.

When magnets repel and attract in this manner, it is the invisible lines of force that are interacting.  These lines of force, called flux, can be observed in another simple experiment with a magnet, sheet of paper, and some iron filings.  The image below shows the result of the experiment, and what important to note is that the metal filings too far away to be drawn tight to the magnet line up along the curved flux lines between the north and south poles.

Similarly, our tiny floating needle’s magnetic field lines up with the giant magnetic field of planet Earth.  The north-pointing end of a compass needle (usually painted red) is of the opposite polarity as the Earth’s magnetic north pole, so it is attracted rather than repelled, and it aligns parallel to Earth’s strong magnetic lines of flux.

The Earth’s magnetic field is oriented more or less north-south, but there are bumps and wrinkles in its lines of flux associated with metallic mineral deposits.  This knowledge highlights one small inconsistency in my statement earlier that our floating needle points north.  A more accurate way to describe the behavior of our needle would be to say that it lines up with the Earth’s magnetic field (which is lined up approximately north).

A more important distinction related to the term “north” is that Earth’s molten core sloshes around a bit, and causes to Earth’s magnetic poles to slowly wander around.  Currently, the north magnetic pole is several hundred miles away from the “True” north pole that maps line up with.  A magnetic compass points toward magnetic north, so a correction needs to be made if you are using a compass with a map.  See Understanding Magnetic Declination for details.

There are a few more details in the articles to follow that will help you learn how to use a compass, but these concepts are the same for all magnetic compasses:  the north pointing end of a compass needle aligns with the Earth’s magnetic field, pointing toward the magnetic north pole.