A
solenoid is a form of
electromagnet. In its simplest construction it consists of a number of turns of conductive wire through which a
current is passed. This creates a
magnetic field which is concentrated at the centre of the windings. Any
ferromagnetic material that is brought into proximity is attracted by the magnetic field. Generally the construction is arranged so that the ferromagnetic material is presented as a plunger within the coil which is free to move in and out and is held out by a spring when the current is switched off.
Much more efficient forms of solenoid are made by winding the coil around a ferrous C-shaped core and using a T-shaped ferrous core. When a current flows, this structure creates a tight magnetic loop, the T fitting into the C and touching at the lip and bottom.