The magnetic amplifier was most prominent in power control and low-frequency signal applications from 1947 to about 1957, when the transistor began to supplant it.

For controlled saturation characteristics, the magnetic amplifier employs core materials that have been designed to have a specific B-H curve shape that is highly rectangular, in contrast to the slowly tapering B-H curve of softly saturating core materials that are often used in normal transformers.

The AC windings may be connected either in series or in parallel, the configurations resulting in different types of mag amps.

Unlike the silicon controlled rectifiers or TRIACs which replaced them, the magnitude of these harmonics decreases rapidly with frequency so interference with nearby electronic devices such as radio receivers is uncommon.

Magnetic amplifiers were also used to regulate the speed of Alexanderson alternators to maintain the accuracy of the transmitted radio frequency.

[2] Magnetic amplifiers were used to control large high-power alternators by turning them on and off for telegraphy or to vary the signal for voice modulation.

Even so, early magnetic amplifiers incorporating powdered-iron cores were incapable of producing radio frequencies above approximately 200 kHz.

Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS) power supplies,[5] as well as in lighting control.

Cores designed specifically for switch mode power supplies are currently manufactured by several large electromagnetics companies, including Metglas and Mag-Inc.

Magnetic amplifiers were used by locomotives to detect wheel slip, until replaced by Hall effect current transducers.

Instrumentation magnetic amplifiers are commonly found on space craft where a clean electromagnetic environment is highly desirable.

Like transistors, mag amps were somewhat smaller than the typical vacuum tube, and had the significant advantage that they were not subject to "burning out" and thus had dramatically lower maintenance requirements.

That era was short, lasting from the mid-1950s to about 1960, when new fabrication techniques produced great improvements in transistors and dramatically lowered their cost.

Thus, in principle, a saturable reactor is already an amplifier, although before 20th century they were used for simple tasks, such as controlling lighting and electrical machinery as early as 1885.

By 1916 Alexanderson added a magnetic amplifier to control the transmission of these rotary alternators for transoceanic radio communication.

[11][12] The experimental telegraphy and telephony demonstrations made during 1917 attracted the attention of the US Government, especially in light of partial failures in the transoceanic cable across the Atlantic Ocean.

Many are recorded in service through the mid-1990s and some are still in use at older generating stations, notably in hydroelectric plants operating in northern California.