Hacking When It Counts: The Magnetron Goes To War

In 1940, England was in a dangerous predicament. The Nazi war machine had been sweeping across Europe for almost two years, claiming countries in a crescent from Norway to France and cutting off the island from the Continent. The Battle of Britain was raging in the skies above the English Channel and southern coast of the country, while the Blitz ravaged London with a nightly rain of bombs and terror. The entire country was mobilized, prepared for Hitler’s inevitable invasion force to sweep across the Channel and claim another victim.

We’ve seen before that no idea that could possibly help turn the tide was considered too risky or too wild to take a chance on. Indeed, many of the ideas that sprang from the fertile and desperate minds of British inventors went on to influence the course of the war in ways they could never have been predicted. But there was one invention that not only influenced the war but has a solid claim on being its key invention, one without which the outcome of the war almost certainly would have been far worse, and one that would become a critical technology of the post-war era that would lead directly to innovations in communications, material science, and beyond. And the risks taken to develop this idea, the cavity magnetron, and field usable systems based on it are breathtaking in their scope and audacity. Here’s how the magnetron went to war.

Short But Powerful

For most of the early days of radio, most of the innovation was geared toward providing communication at a distance. Whether it be radiotelegraphy, radiotelephony, or even radio broadcasting, distance was the driver. The further a single transmitter could reach, the fewer links would be required to make a communicate between any two points, and the larger the audience that could be reached with a broadcast. Due to the physics of radio frequency propagation, this put all the action in the longer wavelengths, the space on the spectrum with wavelengths between about 10 meters and 100 meters or so.

Some researchers were looking further up the spectrum, where wavelengths better measured in centimeters are found. These frequencies wouldn’t be of much use for long-haul communications or broadcasting, but they needed to be explored and exploited, and so the tools and technologies needed to produce them would have to be developed.

One of the devices used to generate short-wavelength radio frequency waves was almost an accidental discovery, and like many great ideas, it started as a way to get around patent protections on somebody else’s great idea. Such was the case in 1917 when Albert Hull, a researcher at General Electric in the USA, was working on ways to control the flow of electrons in a vacuum tube without running afoul of Lee DeForest’s triode patents. DeForest’s tube used electric fields to control electron flows, so Hull experimented with magnetic fields to do the same. Hull named his invention a “magnetron.” His employer’s purchase of the triode patent put further development of the magnetron on the back burner, but not before Hull discovered that by carefully controlling the magnetic field at a critical value, the tube could emit radio frequency waves with a much shorter wavelength — less than a meter — than anything else at the time.

At the same time, physicists around the world were working on similar technologies to produce shorter wavelengths. German and Czech scientists used modified triodes to produce the first real microwave (1 GHz and above) oscillator, and Japanese scientist Hidetsugu Yagi, later famed for the Yagi-Uda antenna, developed his own magnetron design that produced wavelengths as short as 5.6 cm. Dutch, French, and English researchers got into the act as well, pushed down the wavelengths while increasing the radiated power by changing the design of the magnetrons.

Boot and Randall

Cavity magnetron by Mrjohncummings CC-BY-SA 2.0

With war on the horizon, few of them missed the fact that powerful, short-wavelength radio waves would be immensely useful for remote detection of aircraft and myriad other uses. Competition between countries for microwave supremacy was fierce — worldwide, no fewer than 2,000 patent applications were filed between 1920 and the end of the war in 1945. The simultaneous invention process was a sort of artificial selection process, where the good ideas were rapidly identified and used to improve designs. This process led to the cavity magnetron, a design with a cylindrical central cavity surrounded with similar holes arranged around it and connected to it by narrow slots. In use, a central cathode emits electrons which circle around the central cavity under the influence of a strong magnetic field, sweeping past the slots and inducing strong microwave fields inside the resonating cavities.

Correctly engineered, cavity magnetrons can produce powerful microwaves. For the British, the task of turning the cavity magnetron into a working device fell to John Randall and Harry Boot, physicists at the University of Birmingham. They were given a simple but lofty set of goals: build a microwave source of 1,000 watts with a 10-cm wavelength, specifically for airborne radars that could be used against the Germans and their near nightly bombing raids.

This would prove to be no mean feat because at the time, Boot and Randall had not even heard about the cavity magnetron. But they did their homework, and within a few weeks had built a prototype that was able to produce 400 W at a wavelength of 9.9 cm. This success led rapidly to improvements; they made their performance goal of 1 kW a week later, and in subsequent weeks 10- and even 25-kW magnetrons were constructed. Thanks to Boot and Randall, the magnetron was almost ready for battle.

Across the Pond

Unfortunately, the British had a problem. Surrounded by the enemy and virtually cut off from the rest of the world while mobilizing against the coming invasion, England had no resources to turn Boot and Randall’s magnetron into usable weapons. They faced a difficult decision: keep the secret and risk dying with them, or share the secret so that it could be used against the enemy. It was time to look across the Atlantic for help.

The idea of a technology swap with the Americans was the brainchild of Henry Tizard, head of the Aeronautical Research Committee, under whose auspices the magnetron had been engineered. Tizard proposed what was essentially a gift to the Americans, a bundle of every major secret the British had. This passel of technological treasure included not only the Boot and Randall magnetron, but designs for proximity fuzes, plastic explosive, self-sealing fuel tanks, rockets, superchargers, gunsights, and even papers detailing the feasibility of a nuclear fission bomb. With Churchill’s approval, almost every one of Britain’s hard-won secrets would be delivered to the Yanks, no strings attached, with only the hope that the uncommon bond between the two nations and the isolation and industrial power of the then-neutral US would result in something to help the war effort.

Cavity magnetron built in the 1940’s being unboxed in 2015 by the US Office of Naval Research

Six men, mostly from the military, formed what would be known as “The Tizard Mission.” They accompanied the secret stash, which included a working prototype of the Boot-Randal magnetron, on its almost inconceivably risky journey across the U-boat infested waters of the North Atlantic. Somehow, the Tizard mission arrived safely in Halifax, Nova Scotia, and was swiftly shuttled to Washington DC for meetings.

Within weeks, the prototype magnetron was handed over to Bell Labs in New Jersey, where the first 30 copies of the eventual one million that would be produced in America were made. The magnetron would eventually become small and light enough to be used in airborne radar sets, with larger, more powerful versions turned into shipboard and land-based sets. The magnetron had been weaponized, and there’s little doubt that it impacted the prosecution of the war.

It was said by historian James Phinney Baxter that the cavity magnetron the Tizard mission brought to America was “the most valuable cargo ever brought to our shores.” While certainly not an understatement given the importance and prevalence of microwaves in everything from communications to cooking, it’s hard to fathom the impact that all the treasures that simple tin box contained would have on the second half of the 20th century. Both in war and in peace, the Tizard Mission laid much of the technological foundations for the post-war era, with the cavity magnetron leading the way.

36 thoughts on “Hacking When It Counts: The Magnetron Goes To War

  1. “One of the devices used to generate short-wavelength radio frequency waves was almost an accidental discovery, and like many great ideas, it started as a way to get around patent protections on somebody else’s great idea.”

    Or other restrictions. Stem cells comes to mind.

    ” With Churchill’s approval, almost every one of Britain’s hard-won secrets would be delivered to the Yanks, no strings attached, with only the hope that the uncommon bond between the two nations and the isolation and industrial power of the then-neutral US would result in something to help the war effort.”

    The constant U-boats sinking maritime shipping would have been incentive in the long run.

    1. The Bush administration’s restrictions on *public* funding of new embryonic stem cell research spurred R&D with adult stem cells, which has been far more fruitful.

      There wasn’t any restriction put on *privately funded* embryonic research, but medical research companies would much rather suck on the tax money teat.

    1. Too bad it leaves out almost the entire story.

      You forgot Chain Home, H2S, and virtually all the other British radar work. Go read “The Invention That Changed the World” [1] or perhaps the appropriate episodes of “The secret war” [2].

      This article makes it sound like the Brits were the very last to stumble into an obvious invention, then went crying to the USA to save their bacon.

      [1] https://www.amazon.ca/Invention-That-Changed-World-Technological/dp/0684835290
      [2] http://www.dailymotion.com/video/x123k8x

      1. Just an ironic factoid regarding the Chain Home stations. The CH RF transmitters were 8 foot cubes of brass angle and sheet containing the high power valves with tuning controlled via a goniometer knob which turned a set of long shafts through to the gubbins.
        These shafts and insulators had to be non-conductive non-magnetic and when they were being built in 1936, the project director specified they were to be made of a marble-like substance called ‘Calit’. The only source of Calit was from Germany.

        This is from page the amazing book ‘RADAR How It All Began’ by Jim Brown. It is a book like no other on WWII Radar. It does not cover the politics, military strategies or history like other books. However if you want to know what size washers, bolts, dimensions of metal plates, the tetrode valve types and materials used in the CH RF transmitters then the author recalls all this from his memory, as he built them.

        It also has some basic RF circuit diagrams (again from memory, as the CH blueprints were routinely destroyed by order) and a detailed description of building the CH system then this is the book for you.

  2. Let us not forget the next big one: Ground Controlled Approach. And guess who worked on it, eh? A chap named Clarke. Yes the writer. His only book that wasn’t Sci-Fi gave an interesting explanation for it. But all-in-all a good write up. By the way, the Info-Age place in NJ has an amazing collection there, including early RADAR examples.

          1. There wasn’t a 2001 book except for the one written along with the movie. Still, that late 1960s movie did show a very iPad like device. But so did Star Trek, which was about the same time period.

    1. There are two powerful words in the English language. The first children learn at an early age. “No”, with the other being “yes”. As in “yes, I’ll take that million dollars off your hands”.

  3. For anyone curious about the historian quoted at the end of the article, Baxter wrote the pulitzer-prize winning “Scientists Against Time”. Looks like a great read.

    On another note, Amazon shows a currently available title “Scientists Against Time: The Role of Scientists in World War II” by H.A. Feiveson. Seems unfortunate to use the same main title as Baxter’s book without any obvious credit, at least on the Amazon page. Does anyone know anything about this one?

    1. One cool thing about the header art: (in my opinion) The greatest flag ever flown. Nowhere else will you find a better looking flag. Shame it represents so much restriction of freedom

  4. Many names missing from a much larger story , Dr Taffy Bowen was one developers and traveled to the US. To demonstrate and transfer the technology
    While visiting the US a number of years after the war he was able to recover His original Magnetron from a major museum ( much to their displeasure).
    This piece of priceless history was handed around at my local radio club here in Sydney AU during a lecture he gave to the club before his passing in 1991 .

  5. My head of department when I was at university was involved with this. He described one peculiar difficulty they experienced. The American machinists could not at first produce a working magnetron and the British sent over one of theirs. Using exactly the same blueprints he made a working magnetron. The American machinists then started churning out working magnetrons without changing a single step in the process.

  6. No doubt the cavity magnetron was an impressive step and helped the brits massively at the end of the war, but the implication that one day there was not radar and the next day the Brits invented it is frustrating. Germans already had the Kammhuber Line before “The Tizard Mission” even started. Once the war started, captured German tech steered much of the allies research. Hell, Germans had 200 mile range phased array ground radar and even radar in planes in 1942.

    1. As a child I read a children’s book about a British plan of crossing the Channel and briefly seizing a German radar site and removing some of its parts before scampering off. IIRC, it was from the point of view of a French boy who assisted them in their effort.

  7. In “Empire of the Air”, they mention MIT’s (I might be wrong about MIT) airborne radar research. IIRC, they were having trouble understanding waveguides. As they were testing an experimental radar in an aircraft, they kept requesting the pilot to find a bigger ship target to see if they could acquire it. After, a number of such requests, the pilot responded, the previous ship was the Queen Mary.

  8. The story does not mention one of the most remarkable hacks of ww2. The internal shape of the magnetron was complex and very difficult and expensive to produce, until someone thought of making it from thin laminates which could be cheaply stamped and then stacking them together.

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