1920 and 3000 Hz as these were the easiest frequencies to produce with the equipment available. As with many technology-based fields it was the advent of World War II which led to huge developments in the utilization of induction heating in the production of vehicle parts and munitions. Over time, the technology advanced and units in the 3 to 10 kHz frequency range with powers outputs to 600 kW became common place in induction forging and large induction hardening applications. The motor generator would remain the mainstay of MF power generation until the advent of high voltage semiconductors in the late 1960s and early 1970s. Early in the evolutionary process it became obvious to engineers that the ability to produce a higher radio frequency range of equipment would result in greater flexibility and open up a whole range of alternative applications. Methods were sought to produce these higher RF power supplies to operate in the 200 to 400 kHz range. Development in this particular frequency range has always mirrored that of the radio transmitter and television broadcasting industry and indeed has often used component parts developed for this purpose. Early units utilised spark gap technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode (valve) based oscillators. Indeed, many of the pioneers in the industry were also very involved in the radio and telecommunications industry and companies such as Phillips, English Electric and Redifon were all involved in manufacturing induction heating equipment in the 1950s and 1960s. The use of this technology survived until the early 1990s at which point the technology was all but replaced by power MOSFET and IGBT solid state equipment. However, there are still many valve oscillators still in existence, and at extreme frequencies of 5 MHz and above they are often the only viable approach and are still produced. Mains frequency induction heaters are still widely used throughout manufacturing industry due to their relatively low cost and thermal efficiency compared to radiant heating where piece parts or steel containers need to be heated as part of a batch process line. Valve oscillator based power supply Due to its flexibility and potential frequency range, the valve oscillator based induction heater was until recent years widely used throughout industry. Readily available in powers from 1 kW to 1 MW and in a frequency range from 100 kHz to many MHz,