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From World War II Radar to Microwave Popcorn, the Cavity Magnetron Was There

From World War II Radar to Microwave Popcorn, the Cavity Magnetron Was There

By the summer time of 1940, World War II had been raging in Western Europe for almost a 12 months. During the Battle of Britain, German plane bombed London and industrial facilities and blockaded seaports. The United States, in the meantime, was nonetheless actively making an attempt to keep out of the conflict.

Against this backdrop, the physicist Edward “Taffy” Bowen traveled with a gaggle of different British scientists and navy officers to Washington, D.C. Bowen had been entrusted with a black steel field that contained technical secrets and techniques associated to England’s wartime R&D. The objective of the journey, formally known as the British Technical and Scientific Mission, was to share these secrets and techniques with the United States and Canada, in the hope that they’d produce workable weapons and different tools for the conflict.

Among the field’s contents was a curious-looking system: a disc with grooves round its rim and skinny pipes and wires extending from its ends [above]. This palm-size gadget, known as a cavity magnetron, produced high-power microwaves, and it might show to be by far the most necessary merchandise in the field.

That mentioned, the field had a hapless begin to its journey. En route to the practice station in London, it was strapped to the roof of a London taxi. At Euston station, a porter whisked it away earlier than Bowen may object. Boarding a ship in Liverpool, Bowen was accompanied in all places by a silent gentleman with a navy bearing. The field, in the meantime, was secured in order that its secrets and techniques would sink with the ship if the boat got here below assault.

How the cavity magnetron got here to be in the field in the first place is a narrative that started a lot earlier. The phrase “magnetron”—from “magnetic” and “electron”—entered the English language in 1921, when Albert W. Hull revealed his analysis on “The Effect of a Uniform Magnetic Field on the Motion of Electrons Between Coaxial Cylinders.” Hull, a physicist and electrical engineer at the General Electric Research Laboratory in Schenectady, N.Y., was making an attempt to circumvent a patent on the triode held by Lee de Forest. [PDF] Hull’s split-anode magnetron was finally used as an amplifier in radio receivers and in oscillators. Numerous researchers started investigating and experimenting with the system.

According to John H. Bryant’s historical past of microwave know-how [PDF, subscription required], over 200 papers on split-anode magnetrons have been revealed worldwide prior to 1940. Some described how to use Hull’s invention; others instructed methods to optimize what was primarily an inferior design.

In September 1939, John Randall and Henry Boot, physicists at the University of Birmingham, in England, started exploring a brand new course in magnetron design, below the steerage of Mark Oliphant. They relied on the work of the Dutch engineer Klaas Posthumus, who had clarified the theoretical operation of the magnetron. And they’d a really particular downside to clear up: With German plane terrorizing England, any enchancment in the nation’s radar capabilities may assist.

In addition to being a physics professor at Birmingham, Oliphant was a member of the labeled British radar program. At the begin of the conflict, the nation had a series of radar stations that operated on a wavelength of 10 to 13 meters and was testing airborne radar with a wavelength of 1.5 meters. Oliphant argued for radar working in the microwave vary, with a wavelength of 10 cm or much less and with a peak energy of 1 kilowatt. Such a system would enhance the decision of radar photos, allow smaller, lighter tools that may very well be put in in plane, and be much less inclined to interference from floor echoes. A greater magnetron was the key.

Within two months, Randall and Boot had a primary design for his or her cavity magnetron. It consisted of a cylindrical piece of steel that had a cathode operating by a central gap. The surrounding anode had a sequence of symmetrical holes, or cavities, organized in a circle round the central gap. The cross part appeared like the chamber of a Colt revolver, which occurs to have served as a template for a few of Randall and Boot’s early prototypes.

When energy was provided to the cathode and a magnetic subject surrounded the system, the oscillation of the electrical cost round the cavities led to the radiation of electromagnetic waves. Each cavity created its personal resonant frequency.

By February 1940, they’d a prototype exhibiting a wavelength of 9.Eight cm at 400 watts. In April, they contracted with the General Electric Co. in Wembley, close to London, to produce hardier specimens that would face up to extra thorough testing. Most of the prototypes had 6 cavities, however the 12th prototype had 8. It was this final one, E1189, Serial No. 12, created by E.C.S. Megaw at General Electric, that Bowen took with him to North America.

The British Technical and Scientific Mission was led by Sir Henry Tizard, who was chairman of Britain’s Aeronautical Research Committee and noticed the apparent benefits that will come from a superior radar system. He knew the researchers at Birmingham had made vital developments, however he additionally understood that Britain would have challenges with industrial manufacturing whereas combating the conflict.

The authorities in London, in the meantime, was uncertain that the United States may hold its secrets and techniques. Tizard first had to persuade Winston Churchill to reveal the know-how to the Americans, after which he had to persuade the U.S. Congress to cooperate with the British. Several weeks earlier than Bowen and the remainder of the delegation set out, Tizard headed to Washington to lay the groundwork.

The mission turned out to be a simple promote to American scientists, who have been astounded by the cavity magnetron. It took a bit extra negotiating with the U.S. and Canadian governments to set the phrases for analysis, manufacturing, and supply. The U.S. National Defense Research Committee contracted with Bell Telephone Laboratories to replicate 30 copies of the system.

The committee additionally funded the creation of the Radiation Laboratory, or Rad Lab, at the Massachusetts Institute of Technology, to provide the allied forces with microwave radar. The lab finally produced 150 distinct radar techniques, which ranged from light-weight, compact models for plane to the enormous microwave early-warning system transported in 5 vehicles.

The Germans discovered that the British have been utilizing the cavity magnetron for radar in February 1943, after they examined a downed bomber close to Rotterdam. An explosive cost meant to destroy the radar set earlier than it fell into enemy arms failed to go off.

After the conflict, all secrets and techniques have been put aside, and the cavity magnetron discovered many peaceable business makes use of. Not solely did it develop into normal for radar in civil aviation, it additionally turned the coronary heart of each microwave oven. Thanks to these wartime efforts, we will all take pleasure in microwave popcorn.

One of the challenges of writing about the historical past of a know-how developed for navy functions is that the preliminary analysis was typically executed in secrecy. Such information circulates poorly, particularly in wartime. It’s subsequently frequent for the individuals concerned in the discovery course of to imagine they’ve invented one thing new when in actual fact the identical factor had already been invented elsewhere. This downside is compounded when the victors write the first draft of the story, and nationwide delight units a tone for his or her heroic story of invention.

While wanting into the historical past of the cavity magnetron, I got here throughout an often-quoted passage about the Tizard mission’s significance, written by James Phinney Baxter III, the official historian of the Office of Scientific Research and Development throughout World War II. In his Pulitzer Prize–successful e book, Scientists Against Time (1946), Baxter wrote that when the mission introduced the cavity magnetron to America in 1940, “they carried the most respected cargo ever introduced to our shores.” Bowen later strengthened the narrative of the British invention and U.S. growth of the cavity magnetron in his 1987 e book, Radar Days.

There is little doubt that Randall and Boot invented the cavity magnetron, however I additionally found competing narratives and precedence claims that make the historical past extra advanced, nuanced, and attention-grabbing. Articles by Paul Redhead [PDF] and by Yves Blanchard, Gaspare Galati, and Piet van Genderen [PDF] present that in the 1920s and ’30s, many individuals round the globe have been experimenting with completely different designs for the magnetron, not merely optimizing Hull’s split-anode model. In 2010, there was a whole convention devoted to the topic of the cavity magnetron’s origins, that includes papers about the contributions of Czech, Dutch, French, German, Russian, and Ukrainian engineers and scientists.

And so though Randall and Boot thought their system was the first, a number of others independently invented the cavity magnetron. If there was a primary, it’s in all probability Arthur L. Samuel of Bell Telephone Laboratories. He filed a U.S. patent software for a four-cavity magnetron in 1934. Unfortunately, his design was not very sensible.

N.F. Alekseev and D.D. Malairov developed a profitable multicavity magnetron in 1937, however this work wasn’t recognized exterior of the Soviet Union till 1940 [PDF]. In Japan, a joint analysis program of the Japanese Navy and the Japan Radio Company led to an eight-cavity magnetron by 1939. But lack of supplies hampered its manufacture.

Each of those gadgets predates the invention by Boot and Randall, however every one additionally got here with a caveat that saved it from gaining acceptance. One of the basic classes of the Tizard mission is that when scientific information is shared, growth can transfer ahead shortly. This 2015 video reveals the unboxing of a cavity magnetron made by Sylvania, considered one of a number of firms that manufactured the gadgets throughout World World II:

The cavity magnetron pictured at high is the very one which Bowen introduced to Washington. When the Tizard delegation returned to Britain, they left the E1189 with officers of the National Research Council of Canada to function a template for future analysis. It remained there till 1969, when it was given to the Canada Science and Technology Museum, in Ottawa.

Some museumgoers could view the magnetron for example of how know-how can get wrapped up in diplomacy. Those taken with the science of magnetrons could discover the simplicity of the system instructive. To me, it demonstrates how a single object can’t actually embody the total advanced historical past of even its personal invention. History is at all times a lot richer and extra deeply textured than it seems at first look.

An abridged model of this text seems in the November 2018 print situation as “The Mighty Magnetron.”

Part of a unbroken sequence taking a look at images of historic artifacts that embrace the boundless potential of know-how.

About the Author

Allison Marsh is an affiliate professor of historical past at the University of South Carolina and codirector of the college’s Ann Johnson Institute for Science, Technology & Society.

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