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Microwave oven

The use of high-frequency electric fields for heating dielectric materials had been proposed in 1934,for example US patent 2,147,689 (application by Bell Telephone Laboratories, dated 1937) states"This invention relates to heating systems for dielectric materials and the object of the invention is toheat 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 thedielectric loss produced in them when they are subjected to a high voltage, high frequency field."

However, lower frequency dielectric heating as is described in this patent, is (like induction heating) anelectromagnetic heating effect which is the result of the so-called near field effects that exist in anelectromagnetic cavity that is small compared with the wavelength of the electromagnetic field. Thispatent proposed radiofrequency heating, at 10 to 20 megahertz (wavelength 15 to 30 meters).[2]Heating from microwaves that have a wavelength that is small in relation to the cavity (as in a modernmicrowave oven) is due to "far field" effects that are due to classical electromagnetic radiation thatdescribes freely-propagating light and microwaves suitably far from their source. Nevertheless, theprimary 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.

The specific heating effect of a beam of high-power microwaves was discovered accidentally in 1945,shortly after high-powered microwave radar transmitters were developed and widely disseminated bythe Allies of World War II, using magnetron technology. Percy Spencer, an American self-taughtengineer from Howland, Maine, was working at the time building magnetrons for radar sets, with theAmerican company Raytheon. He was working on an active radar set when he noticed that a Mr.Goodbar he had in his pocket started to melt. The radar had melted his chocolate bar withmicrowaves. The first food to be deliberately cooked with Spencer's microwave was popcorn, and thesecond was an egg, which exploded in the face of one of the experimenters.[3][4] To verify his finding,Spencer created a high density electromagnetic field by feeding microwave power from a magnetroninto 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 October 8, 1945[5] Raytheon filed a US patent for Spencer's microwave cooking process, and anoven that heated food using microwave energy from a magnetron was soon placed in a Bostonrestaurant for testing. In 1947, the company built the "Radarange", the first commercial microwaveoven.[6] It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost aboutUS$5000 each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, andwas water-cooled. The first Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah. An early commercial model introduced in 1954consumed 1.6 kilowatts and sold for US$2000 to US$3000. Raytheon licensed its technology to theTappan Stove company of Mansfield, Ohio in 1952.[7] They tried to market a large, 220 volt, wall unitas a home microwave oven in 1955 for a price of US$1295, but it did not sell well. In 1965 Raytheon

acquired Amana. In 1967 they introduced the first popular home model, the countertop Radarange, ata price of US$495.

In the 1960s, Litton bought Studebaker's Franklin Manufacturing assets, which had beenmanufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Littonthen developed a new configuration of the microwave, the short, wide shape that is now common. Themagnetron feed was also unique. This resulted in an oven that could survive a no-load condition, or anempty microwave oven where there is no object to absorb the microwaves, indefinitely. The new ovenwas shown at a trade show in Chicago, and helped begin a rapid growth of the market for home

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microwave ovens. Sales volume of 40,000 units for the US industry in 1970 grew to one million by1975. Market penetration was faster in Japan, due to a re-engineered magnetron allowing for lessexpensive units. Several other companies joined in the market, and for a time most systems were builtby defense contractors, who were most familiar with the magnetron. Litton was particularly well knownin the restaurant business. By the late 1970s the technology had improved to the point where priceswere falling rapidly. Often called "electronic ovens" in the 1960s, the name "microwave ovens" laterbecame standardized, often now referred to informally as simply "microwaves." Formerly found only in

large industrial applications, microwave ovens were increasingly becoming a standard fixture of mostkitchens. The rapidly falling price of microprocessors also helped by adding electronic controls tomake the ovens easier to use.[citation needed] By 1986, roughly 25% of households in the U.S.owned a microwave oven, up from only about 1% in 1971.[8] Current estimates hold that over 90% ofAmerican households own a microwave oven.