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Where's North?

Science Snack
Where's North?
Hanging magnets reveal the earth’s magnetic field.
Where's North?
Hanging magnets reveal the earth’s magnetic field.

By observing the effects of the earth’s magnetic field on magnets, we can replicate some of the earliest experiments conducted on magnets.

Tools and Materials
  • Two donut-shaped magnets about 0.8–1.0 inches (2–2.5 cm) in diameter
  • Two pieces of cotton string or thread, each about 24 inches (60 cm) long
  • Masking tape
  • Two pencils or straws that fit through the “donut holes” of the magnets
  • Partner
  • Two sticky dots
  • Magnetic compass
Assembly
  1. Tie a piece of string to each magnet as shown in the photo below.

  2. Use the masking tape to hang the magnets from the edge of a table so they’re at least 1 meter (39 inches) away from each other, dangling in space below the edge of the table. Be sure to keep the magnets away from any metal parts of the table, nearby chairs, or even pipes in the walls or beneath the floor.
To Do and Notice

Watch the hanging magnets for a moment. What do you notice?

Gently slip a pencil (or straw) through each magnet’s “donut hole.” Center the pencil so that it’s balanced within the magnet’s hole. Wait until the magnets stop moving, and then notice the orientation of the pencils.

Stand with your partner so that one person is near each magnet. Then hold both arms straight out, indicating the direction in which the pencils are pointing. What do you notice about the directions you and your partner are pointing? What are the chances of this alignment happening by accident? Can you think of an experiment to help convince you that something about the magnets is special?

While you’re standing with your arms outstretched, note that your own two arms are pointing in opposite directions. With your partner, choose one of these two directions to continue pointing before continuing with this Snack. Drop the arm that isn’t pointing in the chosen direction so that you and your partner are now pointing in the same direction with the other arm. Then, attach a sticky dot onto the side of the magnet that’s facing your chosen direction. Call the faces of the magnets “dot” and “no-dot.”

Bring the two magnets close to each other and notice how they interact. Is the dot side attracted to or repelled by the no-dot side?

Use the magnetic compass to find the north side of the room. Is the dot side of your magnet facing the north side or the south side of the room?

What's Going On?

Each face of your donut-shaped magnet is a magnetic pole. Magnets dangling in space will naturally line up within the earth’s magnetic field. One pole of your magnet will point toward the north, and the other will point toward the south.

You’ll also have discovered that like poles repel one another, and unlike poles attract: North poles are attracted to south poles, and south poles are attracted to north poles. Two south poles, however, or two north poles, will repel each other.

The pole (or face) that points toward the north was originally called the “north-seeking pole.” (Why the north pole of a magnet “seeks” the north pole of the earth was discovered by William Gilbert in the 1600s.) Later, that name was shortened to “north pole.” The other end became the south pole.

The north pole of your donut magnet and the magnetic compass point to the earth’s north magnetic pole. And yet your experiments have proven that north-pole magnets are attracted to south-pole magnets. This fact points to a surprising conclusion: The earth’s magnetic north pole is actually a south-pole magnet. (Think about it: the original “north-seeking pole” can’t also be the north pole it was seeking!) The naming conventions surrounding magnets and poles are notoriously confusing.

Going Further

The first compasses were made from natural magnetic stones, made from the mineral magnetite, Fe3O4. Known as leading stones, or lodestones, it’s a matter of debate how these stones originally became magnetized. A leading theory is that the strong magnetic fields created by lighting are responsible—a theory supported by the fact that most lodestones are found only near the earth’s surface.

The earth’s magnetic field is generated by the motion of liquid iron in the planet’s core. This “geodynamo” occasionally reverses its polarity, with the magnetic north and south poles swapping places. The switch isn’t instantaneous, but takes place over a few thousand years, with the time between reversals varying from tens of thousands to tens of millions of years.

Different cultures throughout history have noticed how magnets align. Just as you did in this exploration, each chose a particular direction—north or south—to be significant. The Chinese chose the south-pointing end, while the Europeans chose the north-pointing end. In choosing to mark the face of your donut magnet, did you make the same choice as the Chinese or the Europeans?

The ancient Chinese belief in geomancy (interpreting mystical signs according to their physical orientation) gave extra value to the southerly direction, while European cultures, having adopted the North Star as a navigational tool, saw the magnetic needle of a compass as a way to locate north when the North Star wasn’t visible.

Many scientific definitions and benchmarks are determined by “standards committees,” groups that meet to define such things as the location of the north pole of a planet. By deciding to call attention to one of the two possible directions for the hanging magnets, you and your partner have become a standards committee.

Teaching Tips

Is the way the dangling magnets line up an accident? To remove any doubt, try hanging nonmagnetic stainless steel, zinc, or aluminum washers and compare their alignments to that of the magnets.

If you’re working with a group and want to find out more about the properties of magnets, try having participants design experiments to determine the rules about how the dot and no-dot sides of the donut magnets interact.