At one time, magnetic amplifiers were considered as an alternative to vacuum tubes because of their rugged construction, high reliability, and lower working voltages. Consequently, vacuum tubes had a definite lifespan due to heater filaments that eventually burned out over time, or a loss of vacuum resulting in potentially-destructive arc-overs within the tube.
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Introduction
Any discussion of magnetic amplifiers must begin with the early experiments of Heinrich Hertz and Nikola Tesla. It was Hertz who first recognized the concept of an electromagnetic current through his discovery of radio waves, utilizing induction coils and spark gaps. James Clerk Maxwell later validated Hertz's claims of an electromagnetic force and further suggested that this "force" was an oscillating, or alternating, current.
These earlier works paved the way for Nikola Tesla's experiments in high-frequency currents and magnetism. Indeed, the results of Tesla's work culminated in his invention of the high-frequency Tesla Coil, the alternating-current (AC) dynamo, and an entire system of AC production and transmission (the Tesla "polyphase" system), which included the AC transformer.
Tesla's experiments predated the work of Ernst F. W. Alexanderson by as much as twenty years and proved that high-frequency, alternating currents could be produced without the need of any mechanical apparatus. Although he didn't know it at the time, Tesla had proven the concept of the magnetic amplifier.
In later years, improved vacuum tubes, followed by the transistor, would come into play for the production and regulation of high-power AC and radio waves. Recently, however, a resurgence in the interest and applications of the magnetic amplifier is resulting in new uses for this venerable technology.
History
Early development
In the early 20th Century, the General Electric Company, under the direction of engineer E. F. W. Alexanderson, developed a system of transoceanic radio communications, utilizing continuous wave transmission over great distances. Alexanderson drew upon the work of Nikola Tesla and Reginald Fessenden as the inspiration for his system.
The result of this work was the 2-kilowatt Alexanderson Alternator, which produced radio frequencies from 50,000 to 100,000 cycles per second, and which critics had previously denounced as impractical. Later, Guglielmo Marconi took a vested interest in the project, and in 1915, witnessed a demonstration of a new, 50-kilowatt, 50,000-cycle alternator.
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 that snaked across the bottom of the Atlantic Ocean. The 50-kilowatt alternator was commandeered by the US Navy and put into service in January 1918 and was used until 1920, when a 200-kilowatt generator-alternator set was built and installed.
Usage in radio
Magnetic amplifiers were used early on to control large, high-power alternators by turning them on and off for telegraphy or to vary the signal for voice modulation. However, 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 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.
For trans-oceanic communication, frequencies of between 12 and 17 kHz were easily produced using a magnetic amplifier at high power levels. This capability would later prove useful for communications with submerged submarines and surface ships thousands of miles away from land.
References
Alexanderson, E. F. W., "Transoceanic Radio Communication," General Electric Review, October 1920, pp. 794-797.
Cheney, Margaret, "Tesla: Man Out of Time," 1981, New York: Simon & Schuster, Inc.
Chute, George M., "Magnetic Amplifiers," Electronics in Industry, 1970, New York: McGraw-Hill, Inc., pp. 344-351.
Trinkaus, George, "The Magnetic Amplifier: A Lost Technology of the 1950s," Nuts & Volts, February 2006, pp. 68-71.
Trinkaus, George, editor, "Magnetic Amplifiers: Another Lost Technology," 1951: Electronics Design and Development Division, Bureau of Ships, United States Navy.
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