output range typically incorporates 2.5 kW to 40 kW. Induction heaters in this range are used for smaller components and applications such as induction hardening an engine valve. MF induction generators work from 1 kHz to 10 kHz. The output range typically incorporates 50 kW to 500 kW. Induction heaters within these ranges are used on medium to larger components and applications such as the induction forging of a shaft. Mains (or supply) frequency induction coils are driven directly from the standard AC supply. Most mains-frequency induction coils are designed for single phase operation, and are low-current devices intended for localised heating, or low-temperature surface area heating, such as in a drum heater. History The basic principle involved in induction heating was discovered by Michael Faraday as early as 1831. Faraday's work involved the use of a switched DC supply provided by a battery and two windings of copper wire wrapped around an iron core. It was noted that when the switch was closed a momentary current flowed in the secondary winding, which could be measured by means of a galvanometer. If the circuit remained energized then the current ceased to flow. On opening the switch a current again flowed in the secondary winding, but in the opposite direction. Faraday concluded that since no physical link existed between the two windings, the current in the secondary coil must be caused by a voltage that was induced from the first coil, and that the current produced was directly proportional to the rate of change of the magnetic flux. Initially the principles were put to use in the design of transformers, motors and generators where undesirable heating effects were controlled by the use of a laminated core. Early in the 20th century engineers started to look for ways to harness the heat-generating properties of induction for the purpose of melting steel. This early work used motor generators to create the medium frequency (MF) current, but the lack of suitable alternators and capacitors of the correct size held back early attempts. However, by 1927 the first MF induction melting system had been installed by EFCO in Sheffield, England. At around the same time engineers at Midvale Steel and The Ohio Crankshaft Company in America were attempting to use the surface-heating effect of the MF current to produce localized surface case hardening in crankshafts. Much of this work took place at the frequencies of