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Microwave

Fall 2010 | Volume 25 |  Issue 3
Era:
1940s

In early 1945 Laurence Marshall contemplated the imminent financial ruin of his company. Raytheon had enjoyed a lucrative business supplying the U.S. military with magnetrons, electron tubes that generated microwaves, a key component in the nascent technology of radar and the detection of enemy airplanes. But World War II seemed likely to end soon, and with it Raytheon’s lucrative military contracts. Raytheon needed to come up with something it could sell to civilians. He gathered a half-dozen of his top employees at his house in Cambridge, Massachusetts, and asked, “What shall we do after the war?”

His managers, who had been working 70-hour weeks developing techniques for mass producing magnetrons as well as inventing radar devices for ship and airborne use, were dead tired. But one of them, Percy Spencer, had an idea. What if Raytheon built on its radar expertise and created a microwave oven for consumer use?

Marshall had depended on Spencer since 1925 to turn his abstract ideas into operable devices. Most often this called for a solution in the form of vacuum tubes. Spencer had only a grade-school education, but he had learned the rudiments of vacuum tubes while serving in the Navy during World War I. Inspired by the heroism of the Titanic’s wireless operators, he “got hold of textbooks and taught myself while I was standing watch at night.”

At Raytheon Spencer continued his self-education, assigning one of his engineers to gather all the new vacuum tube patents every week. The engineer then wrote a paragraph-long critique of each patent, focusing on how it might be applied to Raytheon products. Spencer’s careful study of these reports (along with his colleague’s engineering expertise) led to many of Raytheon’s cardinal successes in its first two decades.

During the war it was common in winter for Raytheon engineers walking past banks of magnetrons to warm their hands on the heat they emitted. An oft-told, possibly apocryphal story relates how Spencer walked by a magnetron with a chocolate candy bar in his shirt pocket. The heat melted the candy and gave him the idea that microwaves could be used to heat food. In the early 1940s he and other Raytheon engineers began experimenting with using radar tubes to heat food.

Using radiation to cook food, of course, is nothing new, dating back to meat cooked over glowing coals in prehistoric times. But the visible and infrared radiation that has always been used for cooking cannot penetrate the surface of most foods; the heat only diffuses into the interior. In contrast, microwaves—a type of radio wave—pass through the outer layer of food and heat the interior directly by agitating molecules of water, fats, sugars, and other food components.

Spencer and the Raytheon staff laid a garbage bin on its side and stuck a radar tube into it through a hole. They popped corn in the bin and also found that eggs would explode when heated rapidly, because the yolks absorbed heat faster than the whites. The speed with which microwaves cooked food impressed Marshall, who authorized further development of an oven that would work in restaurants, which were seen as its main market.

Spencer turned to Marvin Bock, a young Raytheon engineer, to design and build the new oven. The main difficulty facing Bock proved to be the frequency of the radiation. The magnetron he started with produced radiation with a wavelength of about 5 inches, an ideal size because Spencer had determined that radiation heating worked best when the wavelength matched the size of the food being cooked. However, Bock’s chosen wavelength was also comparable in size to the oven cavity, which in turn meant the microwaves could easily set up a standing wave inside the device and result in uneven heating. As one engineer described it, “If you put a hot dog in that, it would be cooked in the middle and raw on the ends.” A rotating turntable reduced the problem but did not eliminate it.

Bock could not increase the size of the oven because it needed to fit within a projected restaurant kitchen space no bigger than about 12 inches on a side. Neither could he adjust the wavelength. Federal regulations forced him to use a frequency of either 915 or 2450 megahertz—a wavelength of 12.9 or 4.8 inches, respectively—so as not to compete with other emergent microwave devices such as FM radio, long-distance telephone relays, aircraft guidance systems, and facsimile transmissions. (The Federal Communications Commission had designated use of the 915 and 2450 megahertz frequencies for “industrial, scientific, and medical” purposes.) Bock chose the latter frequency because its shorter wavelength would create smaller standing waves and thus smaller (though more numerous) hot and cold zones.

Bock then turned to building a magnetron that was stable and long-lived. He subjected Raytheon tubes to a series of stringent tests; he varied the voltage, measured their power output, and recorded how long it took each one to burn out. He determined that water cooling was the best means of carrying off the tube’s copious output of excess heat.

Next he turned to the problem of uneven heating, which he could only overcome by continually modulating the standing waves within the oven. The most promising solution was to vary the size of the oven cavity while the food was being heated. He achieved some success by moving one wall of the oven back and forth with a small motor, but this strategy proved impractical. He then started exploring the idea of changing the oven capacity virtually.

He hung rotating rods from the oven’s upper wall. A motor turned them at two to three revolutions per second. As the radiation bounced from top to bottom of the oven, this “mode stirrer” alternately blocked and let pass certain parts of the waves, creating a constantly shifting set of standing waves. While the solution was not perfect—even today, no microwave oven warms food perfectly—it distributed the microwave energy through the cavity well enough.

The first production microwave oven weighed 670 pounds, stood 62 inches tall, and measured nearly two feet deep and wide. To install it an electrician had to put in a 220-volt line, and a plumber had to incorporate a water pipe to cool the oven’s radar tube. This first oven sold shortly after the war ended for more than $2,000, the equivalent of about $20,000 today.

In 1970, after decades of new improvements in microelectronics, Raytheon and other U.S. manufacturers sold 40,000 ovens at $300 to $400 apiece. By 1971 the Japanese had begun exporting low-cost models priced $100 to $200 less. Sales increased rapidly over the next 15 years, rising to 1 million by 1975 and 10 million by 1985, nearly all of them Japanese. In the decades since 1970 the microwave oven has followed the familiar path from high-priced wonder to cheap, ubiquitous necessity.

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