Students know how to build a simple compass and use it to detect magnetic effects, including Earth’s magnetic field.

Earth has a magnetic field because it has a metallic core, and because it is spinning round and round on its axis. While we normally think that Earth’s magnetic field is permanent, it has actually reversed orientation many times in our planet’s billions of years of existence. However, it is very reasonable to expect that Earth’s north magnetic pole will remain north in our and our students’ lifetimes. Scientists still have many unanswered questions about Earth’s magnetic field.

As noted in the California Science Framework (page 57), students can build a compass by magnetizing a needle and then placing it on a piece of cork or sponge floating in a small bowl of water. Many texts include this activity. However, they rarely show the control, which would be to float an unmagnetized needle. Doing this control first, and then magnetizing the needle and floating it, would make a much stronger connection between the magnetizing of the needle and its orienting in the north-south direction.

The Framework also recommends comparing the floating needles with a commercial compass. The Framework also notes (page 58) that “large amounts of steel (such as the supporting beams of modern buildings) and operating electronic devices (such as television sets and computers) may distort the effects of Earth’s magnetic field and cause an inaccurate compass reading.

People get used to calling one end of a bar magnet its north pole and the other end its south pole. This can cause confusion when they start talking about the magnet’s or the compass’ north pole pointing toward Earth’s north magnetic pole. What is the confusion? In a different part of the magnetism lessons, we discover and discuss that like poles (such as two north poles) repel each other. So, why should the north pole of a compass or magnet point toward the north pole of the planet? It should be repelled by it and point away from it.

The best way to avoid this confusion is to use a slightly different terminology for the poles of a magnet or a compass. These should be called the “north seeking pole” and the “south seeking pole.” Whenever we refer to the north pole of a compass, we need to remember that we are really talking about its north seeking pole.