Magnetic amplifier, also called a saturable reactor, is a device used to control large amounts of electric power. It is used where currents are too large, or other conditions are too severe, for transistor or vacuum tube amplifiers. For example, magnetic amplifiers are often used to control the speed of large motors or the brightness of airport runway lights.
A magnetic amplifier consists of two coils of wire–a main coil and a control coil–wound around an iron core. An alternating current flows through the main coil and creates a changing magnetic field around the core. This changing field limits the amount of current that can flow. But if direct current is passed through the control coil, the core becomes saturated (completely magnetized). The saturation cancels the limiting effect of the changing field, and enables much more current to flow through the main coil.
