Induction heaters use an electrical source to drive alternating current (AC) through a coil to heat metal objects. The passage of current through the electrically conductive coil generates an intense and rapidly changing magnetic field that causes eddy currents to flow through the workpiece. The high frequency used in induction heating also forces AC to flow in a thin layer towards the surface of the workpiece, increasing the effective resistance of the metal object to the passage of the large current. For ferrous materials, the alternating magnetic field inside the coil repeatedly magnetizes and de-magnetizes the iron crystals, causing considerable friction and heating. Heating due to hysteresis loss is greatest for workpiece materials that have a large area. Induction heaters usually incorporate the coil with a tank circuit so that either the current or the voltage becomes sinusoidal. This minimizes inverter losses and radio frequency (RF) interference.