A solenoid is device that uses the principle of an electromagnet to convert electrical current into mechanical movement. Most typically it is a coil of insulted wire, which, when electric current flows through the wire, has an induced magnetic field inside the coil. This field is used to magnetize a metallic armature or plunger inside the coil--an electromagnet.
Hans Christian Oersted's (1775-1851) discovery in 1820 that an electric current in a wire produced a magnetic field set off a flurry of activity among scientists. In the forefront was André Ampère (1775-1836), whose experiments with electricity and magnetism showed that two parallel wires with electric currents flowing in the same direction attracted each other. When the current flowed in opposite directions, the wires repelled one other. He also experimented with magnetic fields that were created by current flowing in a circular wire and theorized that a helix of wires, or a wire coiled like a bedspring, would behave like a bar magnet, and consequently have a north and south pole. He called his helix a solenoid, but did not actually build the device.
The person who first did build a solenoid was William Sturgeon (1783-1850), in 1823. He took an iron bar, wrapped a wire around it eighteen times, and sent electric current through it. The result was remarkable; he had built an electromagnet that could lift nine pounds (four kilograms), or nine times its own weight. Joseph Henry then improved on Sturgeon's design. By painstakingly hand-wrapping his wire in silk for insulation, Henry was able to increase the number of turns of wire in his coil. His electromagnet could lift 750 pounds (340 kg); a later design doubled that amount. Henry also used his device to attract a small metal bar that used a spring to snap it back into place when the current was turned off-- a relay. He set up a "relay" of devices, so one relay could operate another, thus remotely controlling a series of electromagnets. (This was the forerunner of the telegraph.)
When an iron bar is placed within a tube that has been wrapped with a coil of wire, it will move within the coil. When the current is turned on, the bar is pulled inward; when the current is switched off, a spring attached to the bar pulls it back out. The strength of the magnetic field within the coil is fairly constant, but at the ends it drops off to about half of the strength at the center. This movement gives the solenoid the capability of controlling other circuits. It can operate relays, switches, shutters, valves, and numerous other spring-loaded devices. The starter of an automobile is actuated by a solenoid; turning the key energizes the coil, which pulls the solenoid in to complete the starting circuit, and the engine starts. The most common use of a solenoid in the home is found in doorbell chimes.
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