A magnet (from Greek μαγνήτις λίθος magnḗtis líthos, "Magnesian stone") is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity.
An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as steel, which greatly enhances the magnetic field produced by the coil.
The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.
Read more about Magnet: Discovery and Development, Common Uses of Magnets, Medical Issues and Safety, Magnetizing Ferromagnets, Demagnetizing Ferromagnets, Electromagnets, Units and Calculations
Famous quotes containing the word magnet:
“It has lately been drawn to your correspondent’s attention that, at social gatherings, she is not the human magnet she would be. Indeed, it turns out that as a source of entertainment, conviviality, and good fun, she ranks somewhere between a sprig of parsley and a single ice- skate. It would appear, from the actions of the assembled guests, that she is about as hot company as a night nurse.”
—Dorothy Parker (1893–1967)
“A healthy soul stands united with the Just and the True, as the magnet arranges itself with the pole, so that he stands to all beholders like a transparent object betwixt them and the sun, and whoso journeys towards the sun, journeys towards that person. He is thus the medium of the highest influence to all who are not on the same level.”
—Ralph Waldo Emerson (1803–1882)
“Yes, but I do not travel to find comfortable, rich, and hospitable people, or clear sky, or ingots that cost too much. But if there were any magnet that would point to the countries and houses where are the persons who are intrinsically rich and powerful, I would sell all, and buy it, and put myself on the road to-day.”
—Ralph Waldo Emerson (1803–1882)