A magnet made from a piece of steel is called a permanent magnet because, once magnetized, it keeps its magnetism. An electromagnet is a temporary magnet associated with a flowing electric current–turning off the current turns off the magnet.
A simple electromagnet consist of a length of iron, called the core, wrapped around and around with a length of insulated wire. When the ends of the wire are connected to a current supply such as a battery, the iron becomes magnetized and the whole arrangement acts just like a permanent magnet. It is not surprising that the branch of physics that deals with this interaction of electricity and magnetism is called electromagnetism.
The British scientist Michael Faraday championed this area of science in the 1830s, although the first electromagnets had been constructed a few years earlier by the American physicist Joseph Henry. There were three key stages in the scientific development of electromagnetism. The first was the 1820 observation by the Danish physicist Hans Christian Oersted that there is a magnetic field surrounding a wire carrying an electric current. He deduced this fact when he observed the deflection of a compass needle place near a current-carrying wire.
The second major step was taken about 10 years later when Faraday proved experimentally that a magnetic field that is changing induces a current in an associated circuit. The third and final stage came in the 1870s when Scottish theoretical physicist James Clerk Maxwell explained the interaction between electricity and magnetism in a set of mathematical equations. He showed that a changing electric field should produce a magnetic field and predicted the existence of electromagnetic waves that travel at the speed of light. Indeed light is such a wave, as are radio waves and all the other types of electromagnetic radiation discovered after Maxwell's time.
Simple electromagnets have limited uses, perhaps the best-known being to lift pieces of iron and steel in a scrap yard. This application demonstrates one of the great advantages of an electromagnet: it can be on to pick up scrap and then off again to dump it.
Other applications of electromagnets include dynamos, electric motors, electric bells, solenoids, and relays. A solenoid is a simple switching device, consisting of a sliding, spring-loaded electromagnet. Its movement is commonly used for opening and closing the contacts in a relay (a type of high-voltage switch).
Electromagnets are also key components in some kinds of microphones, loudspeakers and audio and video tape decks. MRI scanners and particle accelerators use some of the most powerful electromagnets in the world.
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