Magnets produce magnetic fields, or areas in which potentially magnetic materials, such as iron, are polarized and attracted to the magnet. There are two types of magnets: permanent magnets and electromagnets. Permanent magnets have a constant magnetic field, while electromagnets only produce a field when an electric current runs through the coil that comprises part of their structure. Ring magnets are permanent magnets that are distinguished by their shape: they are round with a hole in the middle, and because of this shape they are sometimes referred to as donut magnets.
A variety of materials are used to create permanent magnets. Often, they are made of one of a group of materials called rare earth, which are mixtures of elements — usually neodymium, iron and boron or samarium and cobalt. Alnico magnets are mixtures of aluminum, nickel and cobalt. There are also weaker magnets that are made of magnetic materials, like iron oxide, mixed with nonmagnetic materials, such as plastic or ceramics. The magnetic elements produce the field, while the nonmagnetic elements give the magnet shape.
These materials are not naturally magnetic, however — they simply have magnetic potential. In the factories where they are made, workers first form the material into the desired shape, and they may coat the magnet to make it more colorful. Then, they pass the object through a strong electromagnet, which induces magnetic properties in the object that remain even after the electromagnet is turned off. If the electromagnet is strong enough, this process creates a permanent magnet.
The process of magnetization creates poles on the magnet that are labeled north and south, and each repels similar poles and attracts opposite poles. The location of the north and south parts of ring magnets depends on the way they are polarized; one half is always north and one half is south. Sometimes, the magnet is divided so that one side of the ring is north and the other is south, but different kinds of polarization can create magnets that are split into quarters or eighths. North and south segments always alternate around the ring.
Ring magnets are most commonly used in science experiments, although they also have medical applications. Some people have implantable cardioverter-defibrillators, or ICDs, that automatically apply shocks to their hearts if the rhythm develops irregularities. If the devices malfunction, they can shock the patients unnecessarily, leading to irregular rhythms and possibly death. Medical personnel sometimes place these magnets on patients’ chests over the ICDs to disable the devices.