device in which magnetism is produced by an electric current. Any electric current produces a magnetic field, but the field near an ordinary straight conductor is rarely strong enough to be of practical use. A strong field can be produced if an insulated wire is wrapped around a soft iron core and a current passed through the wire. The strength of the magnetic field produced by such an electromagnet depends on the number of coils of wire, the magnitude of the current, and the magnetic permeability of the core material; a strong field can be produced from a small current if a large number of turns of wire are used. Unlike the materials from which permanent magnets are made, the soft iron in the core of an electromagnet retains little of the magnetism induced in it by the current after the current has been turned off. This property makes it more useful than a permanent magnet in many applications. Electromagnets are used to lift large masses of magnetic materials, such as scrap iron. They are essential to the design of the electric generator and electric motor and are also employed in doorbells, circuit breakers, television receivers, loudspeakers, atomic particle accelerators, and electromagnetic brakes and clutches. Electromagnetic propulsion systems can provide motive power for spacecraft. Electromagnets are also essential to magnetic levitation systems. Such systems often use a special kind of electromagnet whose coil is made of a superconducting metal. Because the coils of a superconducting electromagnet offers no resistance to the flow of electricity, no energy is wasted by the development of heat, and the magnetic field produced by the magnet can be very strong. Superconducting magnets are used in magnetic-resonance imaging, and can also be used for energy storage. The first practical electromagnet was invented early in the 19th cent. by William Sturgeon.
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Device consisting of a core of magnetic material such as iron, surrounded by a coil through which an electric current is passed to magnetize the co
Any object that possesses a magnetic field (displays magnetism), either permanently or temporarily through induction, causing it to attract materials
Figure 51.1 A permanent magnet. Figure 51.2 A simple electromagnet. Figure 51.3 A superconducting system. Figure 51.4 A high field open syst