<P> An electromagnet is a type of magnet in which the magnetic field is produced by an electric current . The magnetic field disappears when the current is turned off . Electromagnets usually consist of wire wound into a coil . A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil . The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet . </P> <P> The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding . However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field . </P> <P> Electromagnets are widely used as components of other electrical devices, such as motors, generators, electromechanical solenoids, relays, loudspeakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment . Electromagnets are also employed in industry for picking up and moving heavy iron objects such as scrap iron and steel . </P>

What is one advantage of using electromagnets instead of permanent magnets
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