A solenoid consists of an insulated wire twisted around a metallic rod (typically iron). It yields a magnetic field as long as a current runs through the windings. The strength of the magnetic field is proportional to the number of turns of wire. Its advantage over a permanent magnet is that the intensity of the magnetic field can be varied and turned on and off. In the graphic, as electricity is supplied by a battery, the nail and the clip are attracted to the solenoid.