this type of unit found widespread use in thousands of applications including soldering and brazing, induction hardening, tube welding and induction shrink fitting. The unit consists of three basic elements: High voltage DC power supply The DC (direct current) power supply consists of a standard air or water cooled step-up transformer and a high voltage rectifier unit capable of generating voltages typically between 5 and 10 kV to power the oscillator. The unit needs to be rated at the correct kilovolt-ampere (kVA) to supply the necessary current to the oscillator. Early rectifier systems featured valve rectifiers such as GXU4 (high power high voltage half wave rectifier) but these were ultimately superseded by high voltage solid state rectifiers. Self exciting class 'C' oscillator The oscillator circuit is responsible for creating the elevated frequency electric current, which when applied to the work coil creates the magnetic field which heats the part. The basic elements of the circuit are an inductance (tank coil) and a capacitance (tank capacitor) and an oscillator valve. Basic electrical principles dictate that if a voltage is applied to a circuit containing a capacitor and inductor the circuit will oscillate in much the same way as a swing which has been pushed. Using our swing as an analogy if we do not push again at the right time the swing will gradually stop this is the same with the oscillator. The purpose of the valve is to act as a switch which will allow energy to pass into the oscillator at the correct time to maintain the oscillations. In order to time the switching, a small amount of energy is fed back to the grid of the triode effectively blocking or firing the device or allow it to conduct at the correct time. This so-called grid bias can be derived, either capacitively, conductively or inductively depending on whether the oscillator is a Colpitts, Hartley oscillator, Armstrong tickler or a Meissner. Means of power control Power control for the system can be achieved by a variety of methods. Many latter day units feature thyristor power control which works by means of a full wave AC (alternating current) drive varying the primary voltage to the input transformer. More traditional methods include three phase variacs (autotransformer) or motorised Brentford type voltage regulators to control the input voltage. Another very popular method was to use a two part tank coil with a primary