A wire wound into a spiral or helix shape. A magnetic coil is an electromagnet whose magnetic field can be switched on and off as required. When an electric current flows through the conductor, a magnetic field is created around it. If the wire is twisted into a loop, the magnetic field at its centre is amplified. In a long magnetic coil (solenoid), the magnetic field has parallel field lines. Magnetic coils are used, for example, in magnetic confinement fusion, where the combination of magnetic fields generated by them creates a magnetic cage for hot plasma. The most common material for magnetic coils is copper, but when stronger magnetic fields are required, niobium-titanium, niobium-tin or ReBCO superconductors are used.