The magnets that are used most commonly, such as the ones on compasses, those used for fridge decorations, and in many other everyday tools, are called permanent magnets. This type of magnet produces an external magnetic field that attracts or repels iron, and it may lose its strength when mistreated. Inside a magnet are groups of atoms called domains. The magnetizing process, which exposes a material to increasingly strong magnetic fields, aligns these domains in a single direction, where they become locked in a crystalline structure. High heat, radiation, strong electrical currents, or other nearby magnets, though, can damage that structure, nudging the domains out of alignment and diminishing the attractive force. Electromagnets, or non-permanent magnets, a less familiar type, have magnetic fields that rely on an electric current. They, thus, do not lose their strength; instead, the strength of the field can be varied as needed. This makes them appropriate for various applications, such as telephone receivers.