Adapted with permission by the National Oceanic and Atmospheric Administration from Discover Your World with NOAA: An Activity Book
In ancient times, sailors found their way by observing stars and other celestial bodies — when they were visible through the clouds, that is. Thus,
one of the most important improvements to ocean navigation was the invention of the compass.
There is some disagreement about who should get credit for this invention. It’s pretty clear that the Chinese knew about magnetism as early as 2637 BC, but the first written description of a compass for navigation didn’t appear in Europe until 1190. Why did it take so long? After you do this activity, you may have at least one good answer!
- Sewing needle about one to two inches long
- Small bar magnet or refrigerator magnet
- Small piece of cork (corks from wine bottles work well, but not the plastic stoppers)
- Small glass or cup of water to float the cork and needle
- Pair of pliers
- Pair of scissors or knife (to cut cork)
First, some warnings: Needles, scissors, and knives are sharp — be careful! Also, magnets can damage cards with a magnetic stripe (credit cards, library cards, school IDs, etc), computer monitors, and some electronic devices. Keep magnets away from these things.
1. Rub a magnet over the needle a few times, always in the same direction. This action magnetizes the needle.
2. Cut off a small circle from one end of the cork, about 1/4-inch thick. Lay the circle on a flat surface.
3. Using a pair of pliers, carefully poke the needle into one edge of the circle and force the needle through the cork so that the end comes out the
other side. Push the needle far enough through the cork so that about the same amount of needle is sticking out each side of the cork. Be
careful not to stick yourself!
4. Fill the cup about half full of water, and put the cork and needle assembly on the surface of the water.
5. Place your “compass” on a flat surface and watch what happens. The needle should point towards the nearest magnetic pole — north or
south, depending upon where you live.
6. Try placing a magnet near your compass and watch what happens. How close does the magnet have to be to cause any effects? Try this
again with a nail or other steel object. You can see why it’s important to keep metal objects away from compasses on ships!
7. Imagine you are on the deck of a ship tossing back and forth on the open ocean. How well do you think your compass would work? When the
cork floats on the water it creates a sort of low friction bearing. This kind of bearing is essential to allow the needle to rotate in response to Earth’s
magnetic field. But a cup of water probably wouldn’t last long on the deck of a rolling ship! The need for a sturdy low-friction bearing was
one of the reasons that it took a long time for mariners to use compasses at sea, even though the basic principles had been known for centuries.
Magnetic fields are areas that contain a force created by moving electrical charges. Earth produces a magnetic field. This field is very weak, but it is
sufficient to align magnetized objects — such as your needle — that are free to rotate. By floating the needle on the cork, you allow it to rotate freely so the needle becomes lined up with Earth’s magnetic field, and points toward the north or South Pole of the planet.