Magnetrons

by Chris Woodford. Last updated: November 23, 2020.

Want to cook a dinner in five minutes or make an airplane safer to fly in bad weather? You'll be needing some microwaves, then. Those are the invisible, super-energetic, short-wavelength radio waves that travel at the speed of light, doing the important stuff in microwave ovens and radar-navigation equipment. Making microwaves is easy if you have the right equipment—a handy gadget called a magnetron. What is it and how does it work? Let's take a closer look!

Photo: The CV64 cavity magnetron, developed in Birmingham in 1942, was small enough to fit inside an airplane. Devices like this made it possible for planes to use radar defenses for the first time. An exhibit at Think Tank (the science museum in Birmingham, England). Sorry about the slightly poor quality of the image: the exhibit is inside a glass case and hard to photograph.

How does a magnetron work?

Artwork: Right: One of the drawings of the high-energy magnetron developed in the 1940s by Percy Spencer, who went on to perfect the microwave oven while working at Raytheon. (I've colored it in to match my own artwork below.) You can see a bigger version of this drawing and read the full technical details via Google Patents. Artwork courtesy of US Patent and Trademark Office.

Magnetrons are horribly complicated. No, really—they're horribly complicated! To understand how they work, I find it helps to compare them to two other things that work in similar ways: an old-style TV set and a flute.

A magnetron has quite a lot in common with a cathode-ray (electron) tube, the sealed glass bulb that makes the picture in an old-style television set. The tube is the heart of a TV: it makes the picture you can see by firing beams of electrons at a screen covered in chemicals called phosphors so they glow and give off dots of light. You can read all about that in our main article on television, but here (briefly) is what's happening. Inside the TV, there's a negatively charged electrical terminal called a cathode that's heated to a high temperature so electrons "boil" off it. They accelerate down the glass tube, attracted by a positively charged terminal or anode and reach such high speeds that they race past and crash into the phosphor screen at the tube's end. But a magnetron doesn't have the same purpose in life as a TV. Instead of making a picture, it's designed to generate microwaves—and it does that a little bit like a flute. A flute is an open pipe filled with air. Blow across the top in just the right way and you make it vibrate at a specific musical pitch (called its resonant frequency), generating a sound you can hear that corresponds directly to the length of the pipe.

A magnetron's job is to generate fairly short radio waves. If you could see them, you could easily measure them with a school ruler. They're usually no shorter than about 1mm (0.04 in; the shortest division on a metric ruler) and no longer than about 30cm (12in; the length of a typical school ruler). The magnetron does its stuff by resonating like a flute when you pump electrical energy into it. But, unlike a flute, it produces electromagnetic waves instead of sound waves so you can't hear the resonant energy its making. (You can't see that energy either, because your eyes aren't sensitive to short-wavelength, microwave radiation).

A brief history of magnetrons

• 1920s: American engineer Albert W. Hull invents the first magnetron while working for General Electric. [1]
• 1934: Arthur L. Samuel of Bell Telephone Laboratories invents the cavity magnetron. [2]
• 1936–7: Soviet scientists Nikolay Alekseyev and Dmitrii Malyarov build a four-segment cavity magnetron. Although details of their work filters through to Germany, it remains unknown in Britain and the United States. [3]
• 1939: Two physicists, John Randall and Harry Boot, working at the University of Birmingham, England independently develop a much more powerful magnetron that is compact enough to fit into ships, planes, and submarines. [4]
• 1940s: American engineer Percy Spencer accidentally discovers that microwaves produced by a magnetron have enough power to heat and cook food. He patents the microwave oven in the 1950s.
• 1943: The British cavity magnetron is deployed for the first time. [3]
• 1976: MIT researchers George Bekefi and Thaddeus Orzechowski develop the relativistic magnetron, which is roughly 10–100 times more powerful than the cavity magnetron. They achieve a power of 900MW, compared to the 10MW or so that cavity magnetrons were then capable of producing. [5]
• 2009: University of Michigan researchers sponsored by the US Air Force announce the development of a more compact, higher power magnetron that could improve the resolution of radar navigation.

Photo: There's a magnetron tucked inside your microwave oven, usually just behind the control and instrument panel on the right. If you open the door, you can sometimes get a glimpse of the magnetron and its cooling fins through the perforated metal cage that separates it from the main cooking compartment.

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Find out more

On this website

• Electromagnetic spectrum
• Microwave ovens
• Radar
• Radio
• Television

Books

• Microwaves: Introduction To Circuits, Devices And Antennas by M. L. Sisodia. New Age International, 2007. Introductory guide, but aimed at undergraduates.
• Electromagnetic Theory for Microwaves and Optoelectronics by Keqian Zhang and Dejie Li. Springer, 2007. Another undergraduate text.

Articles

Easy-to-read

• From World War II Radar to Microwave Popcorn, the Cavity Magnetron Was There by Allison Marsh. IEEE Spectrum, October 31, 2018. Exploring the development of the military magnetron.
• A Brief History of the Microwave Oven by Evan Ackerman. IEEE Spectrum, September 30, 2016. How Raytheon's Percy Spencer pioneered a new way of cooking—with waves.
• Microwave Ovens Posing as Astronomical Objects by Alexander Hellemans. IEEE Spectrum, May 5, 2015. How magnetrons in microwaves have caused problems for astronomers.

History and development of magnetrons

• Andrei Haeff and the Amazing Microwave Amplifier by Jack Copeland and Andre A. Haeff. IEEE Spectrum, August 25, 2015. Exploring the work of a forgotten figure from microwave history.
• [PDF] The invention of the cavity magnetron and its introduction into Canada and the USA by Paul A. Redhead. Physics in Canada, Nov/Dec 2001. This is an excellent, concise account of how magnetrons developed around the time of World War II in the United States, the UK, and Canada. [Archived via The Wayback Machine.]
• The Cavity Magnetron in World War II: Was the Secrecy Justified? by Bernard Lovell, Notes and Records of the Royal Society of London, Vol. 58, No. 3 (Sep., 2004), pp. 283–294.
• Personalities in Science: Albert W. Hull, Scientific American, Vol. 168, No. 5, May 1943, p. 195. A short biography of the magnetron pioneer—and why his work mattered so much in wartime.
• The Cavity Magnetron: Not Just a British Invention by Yves Blanchard et al, IEEE Antennas and Propagation Magazine, October 2013

More technical

• Review of the relativistic magnetron by Dmitrii Andreev, Artem Kuskov, and Edl Schamiloglu. Matter and Radiation at Extremes 4, 067201 (2019). Includes a great review of general magnetron history and many useful citations.
• Historical Notes on the Cavity Magnetron by H.A.H. Boot and J.T. Randall. Transactions of the Institute of Electrical and Electronics Engineers, Number 7, July 1976, p.724. How two British pioneers developed early military magnetrons.

Patents

Artwork: Illustrations of Arthur Samuel's original cavity magnetron from his US Patent #2,063,342: Electron discharge device, courtesy of US Patent and Trademark Office. As in the artworks above, the anode is colored red, the cathode yellow, and the coil surrounding the glass discharge tube is dark gray.

If you want to read detailed technical descriptions of how magnetrons are designed and how they work, patents are an excellent place to start. They're not always that easy to understand, but the descriptions are extremely detailed and there are generally very clear labeled diagrams. Here are a few to start you off; you'll find lots more if you search at the USPTO (or Google Patents) using the keyword "magnetron":

• US Patent #2,099,533: Magnetron by Dietrich Prinz, Telefunken Gesellschaft, 30 July 1935. An early German magnetron design.
• US Patent #2,063,342: Electron discharge device by Arthur L. Samuel, Bell Telephone Laboratories, 8 December 1936. The first cavity magnetron.
• US Patent #2,408,235: High-efficiency Magnetron by Percy L. Spencer, Raytheon Manufacturing Company, 24 September 1946. The full text of Percy Spencer's cavity magnetron patent, illustrated up above.
• US Patent #7,906,912: Magnetron by Takeshi Ishii et al Panasonic Corporation, 15 March 2011. A very detailed description of the kind of magnetron you'll find in a modern microwave oven.

References

1. ↑   Personalities in Science: Albert W. Hull.
2. ↑   US Patent #2,063,342: Electron discharge device by Arthur L. Samuel.
3. ↑   The Cavity Magnetron in World War II: Was the Secrecy Justified? by Bernard Lovell. Nikolay Alekseyev and Dmitry Malyarov—Lifelines of inventors of multiresonator magnetron by N. A. Borisova, 2011 21st International Crimean Conference "Microwave & Telecommunication Technology", Sevastopol, 2011, pp. 97–99.
4. ↑   Historical Notes on the Cavity Magnetron by H.A.H. Boot and J.T. Randall.
5. ↑   Review of the relativistic magnetron by Dmitrii Andreev, Artem Kuskov, and Edl Schamiloglu.