In NMR Six Miles Deep Bob Kleinberg explains how the nuclei of some atoms can be made to line up with a magnetic field. When that field disappears the nuclei "relax," returning to their previous state of alignment. Here's an experiment you can do to get a feel for how this works.

Some objects, such as refrigerator magnets, are magnetic all the time. These are called permanent magnets. Other objects become magnetic only when in a magnetic field caused by a permanent magnet or an electromagnet. These objects (e.g. nails, paper clips, staples, etc.) are called temporary magnets. When a temporary magnet is removed from the magnetic field it was placed in, it begins to relax, or loose its magnetism. Relaxation does not necessarily happen immediately or completely.

A magnetic compass is a device that tells us what direction is North. It does this by aligning a small magnet, the compass needle, with the magnetic field of the earth. The compass can be used to detect more than just the magnetic field of the Earth. In fact, it points in the direction of the strongest magnetic field in the area. If you bring a permanent magnet near a compass and move it around, the compass will react. In this experiment, we will use a compass to study the rate at which a temporary magnet relaxes.

This Science Lab activity is in two parts. First you will build and test an electromagnet. Then you will use the electromagnet to explore the phenomenon of magnetic relaxation.