History

Early developments

The exploitation of high-frequency radio waves for heating substances was made possible by the development of vacuum tube radio transmitters around 1920. By 1930 the application of short waves to heat human tissue had developed into the medical therapy of diathermy. At the 1933 Chicago World's Fair, Westinghouse demonstrated the cooking of foods between two metal plates attached to a 10 kW, 60 MHz shortwave transmitter. The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes.
The 1937 United States patent application by Bell Laboratories states and also in Canada:

"This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field."

However, lower-frequency dielectric heating, as described in the aforementioned patent, is (like induction heating) an electromagnetic heating effect, the result of the so-called near-field effects that exist in an electromagnetic cavity that is small compared with the wavelength of the electromagnetic field. This patent proposed radio frequency heating, at 10 to 20 megahertz (wavelength 15 to 30 meters). Heating from microwaves that have a wavelength that is small relative to the cavity (as in a modern microwave oven) is due to "far-field" effects that are due to classical electromagnetic radiation that describes freely propagating light and microwaves suitably far from their source. Nevertheless, the primary heating effect of all types of electromagnetic fields at both radio and microwave frequencies occurs via the dielectric heating effect, as polarized molecules are affected by a rapidly alternating electric field.

Cavity magnetron

Microwave ovens, several from the 1980s

The cavity magnetron developed by John Randall and Harry Boot in 1940 at the University of Birmingham.

The invention of the cavity magnetron made possible the production of electromagnetic waves of a small enough wavelength (microwaves). The magnetron was originally a crucial component in the development of short wavelength radar during World War II. In 1937–1940, a multi-cavity magnetron was built by the British physicist Sir John Turton Randall, FRSE, together with a team of British coworkers, for the British and American military radar installations in World War II. A more high-powered microwave generator that worked at shorter wavelengths was needed, and in 1940, at the University of Birmingham, John Randall and Harry Boot produced a working prototype.
Sir Henry Tizard travelled to the U.S. in late September 1940 to offer the magnetron in exchange for their financial and industrial help (see Tizard Mission). An early 6 kW version, built in England by the General Electric Company Research Laboratories, Wembley, London, was given to the U.S. government in September 1940. Contracts were awarded to Raytheon and other companies for mass production of the magnetron.

Discovery

In 1945 the specific heating effect of a high-power microwave beam was accidentally discovered by Percy Spencer, an American self-taught engineer from Howland, Maine. Employed by Raytheon at the time he noticed that microwaves from an active radar set he was working on started to melt a candy bar he had in his pocket. The first food deliberately cooked with Spencer's microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters. To verify his finding, Spencer created a high density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly.

On 8 October 1945, Raytheon filed a United States patent application for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing. The first time the public was able to use a microwave oven was in January 1947, when the Speedy Weeny vending machine was placed in Grand Central Terminal to dispense "sizzling delicious" hot dogs. Among those on the development team was robotics pioneer George Devol, who had spent the last part of the war developing radar countermeasures.

Commercial availability

Raytheon RadaRange aboard the NS Savannah nuclear-powered cargo ship, installed circa 1961

In 1947, Raytheon built the "Radarange", the first commercially available microwave oven. It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5,000 ($52,988 in today's dollars) each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2,000 to US$3,000 ($18,000 to $26,000 in today's dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952. They tried to market a large 220 volt wall unit as a home microwave oven in 1955 for a price of US$1,295 ($11,439 in today's dollars), but it did not sell well. In 1965, Raytheon acquired Amana. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$495 ($3,513 in today's dollars).

In the 1960s, Litton bought Studebaker's Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton then developed a new configuration of the microwave: the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a trade show in Chicago, and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was faster in Japan, due to a re-engineered magnetron allowing for less expensive units. Several other companies joined in the market, and for a time most systems were built by defense contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.

Residential use

1971 Radar Range [RR-4]

By the late 1970s, technological advances led to rapidly falling prices. Often called "electronic ovens" in the 1960s, the name "microwave oven" later gained currency, and they are now informally called "microwaves".
Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in developed countries. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971; the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.^[12][13] In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily. In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998. In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.
Adoption has been slower in less-developed countries, as households with disposable income concentrate on more important household appliances like refrigerators and ovens. In India in 2013, for example, only about 5% of households owned a microwave, well behind refrigerators at 31% ownership. Microwave ovens are gaining popularity, however. In Russia, the number of households with a microwave grew from almost 24% in 2002 to almost 40% in 2008. Almost twice as many households in South Africa owned microwaves in 2008 (38.7%) than in 2002 (19.8%). Microwave ownership in Vietnam was at 16% of households in 2008—versus 30% ownership of refrigerators—but this rate was up significantly from 6.7% microwave ownership in 2002—and 14% for refrigerators.