Permanent and Temporary Magnets
There are two basic kinds of magnets—permanent and temporary. A permanent magnet retains its magnetic properties for a long time. A temporary magnet acts as a magnet only as long as it is in the magnetic field produced by a permanent magnet or an electric current. Magnetic materials from which permanent magnets are made are called hard magnetic materials and those from which temporary magnets are made are called soft magnetic materials.
A lodestone is a naturally occurring permanent magnet composed of magnetite, an iron-bearing mineral. Such magnets have been known since ancient times. Virtually all magnets used commercially today are made from synthetic magnetic materials. The most common such materials are alnicos—iron alloys containing aluminum, nickel, and cobalt. Magnetic materials containing such rare-earth elements as samarium or neodymium form very strong permanent magnets. Ferrites, which consist of ferric oxide (an oxide of iron) combined with the oxides of one or more other metals, are widely used in electronic devices. Flexible magnets are made by combining magnetic materials with plastics.
Permanent magnets are typically made into U-shaped horseshoe magnets, with the poles side by side; and bar magnets, with the poles at opposite ends.
Every object that is lifted or moved by a magnet acts as a temporary magnet. Such an object ordinarily loses its magnetism when the permanent magnet is removed, although in certain cases it will retain weak magnetic properties.
An electromagnet is a temporary magnet that is magnetized by the magnetic field produced by an electric current in a wire. Electromagnets have magnetic properties only while the current is flowing.