I’m very pleased to see two recently released movies that document the lives of some of the world’s foremost scientists. There’s “The Theory of Everything”, a biopic centered around Stephen Hawking‘s early success in physics, his shattering motor neuron disease diagnosis, but also his love life. Then there’s “The Imitation Game”, a film about mathematician Alan Turin, the legendary pioneer of computer science and artificial intelligence. In the latter movie, the public learns how Turing was an indispensable figure for the British World War II effort, having devised a code breaking machine that cracked the Nazi Enigma. Yet, not all science projects spurred by the urgency of war have been turned into a movie, despite some of them have a interesting history. Such a story worth telling is how radar technology unexpectedly and inadvertently led to the invention of the microwave oven – one of the most widely used home appliance in the world.

Radar and microwaves

Before and throughout the war, British ground radar technology was rather well matched by German scientific advances. Here, mid-war ground radar station FuMO 214 Würzburg-Riese [US National Archives]

Before and throughout the war, British ground radar technology was rather well matched by German scientific advances.
Here, mid-war ground radar station FuMO 214 Würzburg-Riese
[US National Archives]

In 1920, a young physicist called  Albert Hull, then working at the General Electric Research Laboratory in Schenectady, New York, invented the magnetron tube – a coaxial cylindrical anode and cathode with an axial magnetic field produced by an external coil. Basically, it was a magnet that controlled electrical current inside an evacuated electron tube, or vacuum tube. Hull believed the magnetron would be successful as an electrical converter, but 20 years later it would prove most useful in telecommunications. During WWII, the British were looking to devise a  higher-frequency radar technology for the war effort. A radar locates distant objects by bouncing radio waves off of them and then analyzing the reflections. Locating the enemy from afar was crucial to the war effort.

Engineers planed to built a  new radar system based on electromagnetic waves in the microwave region of the radio spectrum. Such a system would require smaller antennas and detect smaller objects than lower-frequency, longer-wavelength radars.  Microwaves correspond to a region in the EM spectrum defined by having wavelengths between approximately 1 meter and 1 millimeter, corresponding to frequencies between 300 MHz (Mega = 106 Hz = 106 sec-1) and 300 GHz (Giga = 109). Today, microwaves are often used to transmit data from satellites in space to satellite dishes on Earth, but back then building a  high-power source of microwave radiation proved to be a challenge.

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In 1940, John Turton Randall and Harry Boot, two young physicists working in England at the University of Birmingham, found a way to modify Hull’s original magnetron tube to make it produce microwaves with high enough power. The improved designed was called  a cavity magnetron tube, and shortly after its first test runs it became the heart of the Allies’ advanced radar systems that were so essential, perhaps decisive, to the overall Allied victory in World War II.

During the war, one of the leading suppliers of cavity magnetron tubes was an US company called  Massachusetts-based Raytheon Manufacturing Company. Working there was a self-taught engineer by the name of Percy Spencer. One day in 1946 while testing a new magnetron unit, Spencer felt a strange tingling sensation and suddenly noticed that the candy bar in his pocket had melted. He then placed popcorn, eggs and other foods in front of the device and they all cooked – actually the egg exploded all over his friend’s face!  Shortly after the accidental discovery, engineers at Raytheon went to work on Spencer’s new idea, developing and refining it to be of practical use. A year later, the first commercial product hit the market. After a few decades of turmoil, myths and legends regarding microwave use, public demand began to swell with acceptance until the sales of microwave ovens eventually surpassed those of gas ranges in 1975. Furthermore, in 1976 the microwave became a more common household appliance than the dishwasher as it found its home in nearly fifty-two million U.S. households, or 60% of U.S. homes.

Introduced in 1967, the Amana Radarange microwave oven would forever change the way American families prepare meals. Image: SMECC

Introduced in 1967, the Amana Radarange microwave oven would forever change the
way American families prepare meals. Image: SMECC

Although microwave ovens today have advanced since the very first designs, at their core they still use the same cavity magnetron tube  harnessed so effectively for WWII radar.

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How a microwave oven works

Inside the Magnetron: Large magnets impose a field that causes the outward-flowing cloud to revolve (left). As it does, it forms spokes that pass each cavity between the plates (right). A passing spoke provides negative charge to the cavity, which then falls off until the next spoke arrives. The rise and fall creates an electromagnetic field in the cavities that oscillates at 2.45 gigahertz. Image: GEORGE RETSECK

Inside the Magnetron: Large magnets impose a field that causes the outward-flowing cloud to revolve (left). As it does, it forms spokes that pass each cavity between the plates (right). A passing spoke provides negative charge to the cavity, which then falls off until the next spoke arrives. The rise and fall creates an electromagnetic field in the cavities that oscillates at 2.45 gigahertz. Image: GEORGE RETSECK

The microwave oven is quite a feat of physics and engineering. At its core, the oven exploits the polarity of water molecules which tend to rotate themselves into alignment with their positive ends in the direction of an electric field. With each rotation the water molecule’s electrostatic potential energy is transferred into thermal energy. An analogy would be a very crowded room, when everyone is told to turn and face the stage. In doing so, people brush up against one another as they turn and friction causes the conversion of some of their energy into thermal energy. The magnetron reverses its electric field very fast, so water molecules flip back and forth at a rate of billions of times per second.

Magnetron High voltage is sent to the cathode filament. After it heats up, it emits electrons that the positively charged anode plates attract. The attached antenna resonates at 2.45 gigahertz and emits microwaves from its tip--just like a radio-transmission antenna. Image: GEORGE RETSECK

Magnetron High voltage is sent to the cathode filament. After it heats up, it emits electrons that the positively charged anode plates attract. The attached antenna resonates at 2.45 gigahertz and emits microwaves from its tip–just like a radio-transmission antenna. Image: GEORGE RETSECK

This heat is what actually cooks food in the oven. Because all particles in the food are vibrating and generating heat at the same time, food cooked in the microwave cooks much more swiftly than food cooked in a conventional oven where heat must slowly travel from the outside surface of the food inward. The same radio waves that cook your food pass harmlessly through plastics, glass, and ceramics. It is this characteristic that keeps plastic plates from melting and glasses from exploding. It is also this feature of microwaves that makes them so energy efficient; they heat only the food and nothing more.

Oven Transformer, diode and capacitor raise household electricity from 120 to 3,000 volts or more and deliver it through a wire to a magnetron. The magnetron generates microwaves, sent by an antenna through a waveguide into the cooking chamber, where the waves reflect off metal walls. A platter rotates food through the waves for uniform heating. Models without a platter have a small spinning blade (not shown) at the end of the guide to thoroughly distribute the microwaves. Image: GEORGE RETSECK

Oven Transformer, diode and capacitor raise household electricity from 120 to 3,000 volts or more and deliver it through a wire to a magnetron. The magnetron generates microwaves, sent by an antenna through a waveguide into the cooking chamber, where the waves reflect off metal walls. A platter rotates food through the waves for uniform heating. Models without a platter have a small spinning blade (not shown) at the end of the guide to thoroughly distribute the microwaves. Image: GEORGE RETSECK

As you might have learned from experience (ouch!), metals reflect microwaves which is why they line the walls of the microwave such that no waves escape and cook anyone in the kitchen!

Source: Scientific American

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