How does a compass work?

A compass is a device for finding geographic directions. It usually consists of a magnetized needle that is mounted or suspended horizontally and is free to move around a pivot point until it is aligned with Earth's magnetic field.

The position of the needle or magnet indicates the direction of magnetic north.

To understand this, imagine a bar magnet vertically inside Earth: the north and south magnetic poles are where the field lines emerge from the north and south ends of the magnet. Magnetic north on Earth is actually like the south end of a magnet, since the north end of a compass needle points to it.

What's the difference between true north and magnetic north?

Earth's spin axis defines the North and South Poles, so true north always stays in the same location. But Earth's magnetic poles shift around.

Right now, the magnetic North Pole is located in the far northern Canadian Arctic Ocean, fairly close to the geographic North Pole — just 600 miles away. So the direction indicated by the compass needle is close to true north.

The angle between geographic north and magnetic north is about 11 degrees. To find true north using a compass, you must make a small correction for the difference between magnetic north and geographic north. This correction depends on your location on Earth.

Take the road to the magnetic North Pole.
This site tells the story of how magnetic north changes over time, and has an image showing both magnetic north and geographic north. Scroll down the page to see a map of the movement of magnetic north between 1600 and 2000.

Credit: NOAA

Building a simple compass

The classic way to build a simple compass is to lay a magnetized sewing needle on a thin slice of cork (or a small piece of Styrofoam) and let the cork float in a glass of water. The needle will slowly turn and orient itself to point north. The purpose of the cork floating on the water is to ensure that the needle can move freely.

To magnetize a needle, stroke it along a magnet about 50 times in the same direction — from the center of the magnet to the tip. (Don’t run it back and forth over the magnet!) The needle is magnetized when it picks up iron filings.

Using a compass

The dial on a commercial compass is marked with the cardinal points: north, east, south, and west. There are 360 degrees around the dial. North is at 0, east at 90 degrees, south at 180 degrees , and west at 270 degrees. The north-pointing end of a compass is usually marked in red.

How can you tell which end of your compass needle points north?

It depends where on Earth you are!

If you live in the northern hemisphere, take the compass outside around noon. The Sun is to the south. The end of the needle that is pointing away from the Sun is the north-pointing end.
If you live in the southern hemisphere, take the compass outside around noon. The Sun is to the north. The part of the needle that points toward the Sun is the north-pointing end.

A compass may be used in the laboratory to investigate magnetic fields that are stronger than Earth's. For example, a compass shows the direction of the magnetic field around a wire carrying an electric current.

When is a compass of no use at all?

If you’re using a compass at the north magnetic pole, the needle tries to point straight down into the earth and just points whichever way the compass is tilted. A compass is useless at the North Pole!
A compass is also useless in locations where deposits of iron ore or other magnetic substances distort the local magnetic field. This happens, for example, in the Ramapo Mountains of northeastern New Jersey, where there are iron ore deposits, and near Grants, New Mexico, where there are magnetic lava fields.