Magnetic amplifier

Also called mag amp, the magnetic amplifier is a device that is used for the purpose of modulating or changing the voltage across a load in an alternating-current circuit. The magnetic amplifier consists of an iron-core transformer with an extra winding to which control signal can be applied.
Two windings appear in series with the alternating-current power supply and the load. These windings are connected on opposing phase, and together they are called the output winding.

A third coil is wound around the center column of the transformer core. This coil is called the input coil. When a direct current is applied to the input coil, the impedance of the utput coil changes. This occurs because of the saturable-reactor principle (read below).

A small change in the current through the input coil rsults in a large fluctuation in the output-coil impedance. Therefore, amplification occurs. A magnetic amplifier can if properly designed, produce considerable power gain.

The saturable reactor

A saturable reactor, is an inductor with a ferromagnetic core having special properties. The core of the saturable reactor achieves magnetic saturation at a fairly low level of coil current. This makes it possible to change the effective permeability by passing dc through the windings.
The saturable reactor exhibits its maximum inductance and therefore the maximum inductive reactance, when there is no direct current passing through the coil. As the current increases, the inductive reactance decreases, reaching a minimum when saturation occurs in the core material.