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 Early units utilised spark gap technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode (valve) based oscillators.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.