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!
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), floppy disks, 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.