Editing Magnetic amplifier (section)

== Applications ==
Magnetic amplifiers were important as modulation and control amplifiers in the early development of voice transmission by radio.<ref name=Storm55>{{cite book |first=H.F. |last=Storm |title=Magnetic Amplifiers |series=Howard W. Sams photofact publication; FMA-1 |publisher=Wiley |location= |date=1955 |oclc=895109162 |pages=383 |hdl=2027/pst.000030030824}}</ref> A magnetic amplifier was used as voice modulator for a 2 kilowatt [[Alexanderson alternator]], and magnetic amplifiers were used in the keying circuits of large high-frequency [[alternator]]s used for radio communications. Magnetic amplifiers were also used to regulate the speed of Alexanderson alternators to maintain the accuracy of the transmitted radio frequency.<ref name=Storm55/> 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.  The alternator's frequency limits were rather low to where a frequency multiplier had to be utilized to generate higher radio frequencies than the alternator was capable of producing.  Even so, early magnetic amplifiers incorporating powdered-iron cores were incapable of producing radio frequencies above approximately 200&nbsp;kHz.  Other core materials, such as ferrite cores and oil-filled transformers, would have to be developed to allow the amplifier to produce higher frequencies.

The ability to control large currents with small control power made magnetic amplifiers useful for control of lighting circuits, for [[stage lighting]] and for advertising signs. Saturable reactor amplifiers were used for control of power to industrial furnaces.<ref name=Storm55/> Magnetic amplifiers as variable AC voltage controllers have been mostly replaced by [[silicon controlled rectifier]]s or [[TRIAC]]s.  Magnetic amplifiers are still used in some arc welders.

Small magnetic amplifiers were used for radio tuning indicators, control of small motor and cooling fan speed, control of battery chargers.

Magnetic amplifiers were used extensively as the switching element in early switched-mode ([[Switched-mode power supply|SMPS]]) power supplies,<ref>{{cite book |first=Abraham I. |last=Pressman |title=Switching Power Supply Design |publisher=McGraw-Hill |date=1997 |isbn=0-07-052236-7 }}</ref> as well as in lighting control. [[Semiconductor]]-based solid-state switches have largely superseded them, though recently there has been some regained interest in using mag amps in compact and reliable switching power supplies. PC [[ATX]] power supplies often use mag amps for secondary side voltage regulation. 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|Hall effect]] current transducers. The cables from two [[traction motors]] passed through the core of the device.  During normal operation the resultant flux was zero as both currents were the same and in opposite directions. The currents would differ during wheel slip, producing a resultant flux that acted as the Control winding, developing a voltage across a resistor in series with the AC winding which was sent to the wheel slip correction circuits.

Magnetic amplifiers can be used for measuring high DC-voltages without direct connection to the high voltage and are therefore still used in the [[HVDC]]-technique. The current to be measured is passed through the two cores, possibly by a solid bus bar. There is almost no voltage drop in this bus bar. The output signal, proportional to the ampere turns in the control current bus bar, is derived from the alternating excitation voltage of the magnetic amplifier, there is no voltage created or induced on the bus bar. The output signal has only a magnetic connection with the bus bar so the bus may be, quite safely, at any ([[High voltage|EHT]]) voltage with respect to the instrumentation.

Instrumentation magnetic amplifiers are commonly found on space craft where a clean electromagnetic environment is highly desirable. {{citation needed|date=April 2014}}

The German [[Kriegsmarine]] made extensive use of the magnetic amplifiers. They were used for the master stable element systems, for slow moving transmission for controlling guns, directors and rangefinders and train and elevation controls. Magnetic amplifiers were used in aircraft systems ([[avionics]]) before the advent of high reliability semiconductors. They were important in implementing early [[autoland]] systems and [[Concorde]] made use of the technology for the control of its [[Concorde#Powerplant|engine air intakes]] before development of a system using digital electronics.  Magnetic amplifiers were used in stabilizer controls of [[V-2 rocket|V2 rockets]].

===Usage in computing===
{{Main|Magnetic logic}}
Magnetic amplifiers were widely studied during the 1950s as a potential switching element for [[Mainframe computer|mainframe]] computers. 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. Another advantage is that a single mag amp could be used to sum several inputs in a single core, which was useful in the [[arithmetic logic unit]] (ALU) as it could greatly reduce the component count. Custom tubes could do the same, but transistors could not, so the mag amp was able to combine the advantages of tubes and transistors in an era when the latter were expensive and unreliable.

The principles of magnetic amplifiers were applied non linearly to create [[magnetic logic|magnetic digital logic gates]]. 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. Only one large-scale mag amp machine was put into production, the [[UNIVAC Solid State]], but a number of contemporary late-1950s/early-1960s computers used the technology, like the [[Ferranti Sirius]], [[Ferranti Orion]] and the [[English Electric KDF9]], or the one-off [[MAGSTEC]].