Electromagnet, a device that has magnetic properties only when an electric current is applied to it. The electromagnet is an essential part of electric generators and motors. It is also used in many other devices, including electric bells, tape recorders, circuit breakers, and scientific instruments called particle accelerators. Large electromagnets, attached to cranes, are used to lift huge loads of metal.
The simplest electromagnet is a coil of insulated wire. The magnetism it produces is weak, however, and for practical purposes the wire is wound around an iron core, greatly increasing the magnetic strength.
In 1820, Hans Christian Oersted, a Danish physicist, discovered that an electric current has magnetic properties. Using this principle, William Sturgeon, an English physicist, made the first electromagnet in 1823. Joseph Henry, a United States physicist, greatly improved Sturgeon's design in the late 1820's.A Simple Electromagnet can be made by wrapping doorbell wire around a nail.
A Relay is an electromagnet used as a switch to permit one electric circuit to control the operation of one or more additional circuits. The switch contacts are moved by the electromagnet rather than by hand. Relays are used in many control devices, including furnace controls and automobile electrical circuits.
A Solenoid can be used as a relay or to control mechanical equipment. It consists of a coil wound on a nonmagnetic cylinder, inside of which is an iron plunger free to move. When current is applied, the plunger is attracted into the coil; with this action the plunger either closes an electric switch or pushes against a lever.