A microwave oven (often referred to colloquially simply as a "microwave") is a kitchen appliance that heats food by dielectric heating, using microwave radiation to excite polarized molecules within the food. Microwave ovens heat foods quickly and efficiently, because excitation is fairly uniform in the outer 1 inch (25 mm) to 1.5 inches (38 mm) of a dense (high water content) food item; food is more evenly heated throughout (except in thick, dense objects) than generally occurs in other cooking techniques.
Raytheon invented the first microwave oven after World War II from radar technology developed during the war. Named the 'Radarange', it was first sold in 1947. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, which had been acquired in 1965 by Raytheon.
Microwave ovens are popular for reheating previously-cooked foods and cooking vegetables. They are also useful for rapid heating of otherwise slowly-prepared cooking items, such as hot butter and fats, and melted chocolate. Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to do so. Exceptions occur in rare cases where the oven is used to heat frying-oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling water. The boiling-range temperatures produced in high-water content foods give microwave ovens a limited role in professional cooking, since it usually makes them unsuitable for achievement of culinary effects where the flavors produced by the higher temperatures of frying, browning, or baking are needed. However, additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to produce these other heating effects, and microwave heating may cut the overall time needed to prepare such dishes.
Raytheon invented the first microwave oven after World War II from radar technology developed during the war. Named the 'Radarange', it was first sold in 1947. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, which had been acquired in 1965 by Raytheon.
Microwave ovens are popular for reheating previously-cooked foods and cooking vegetables. They are also useful for rapid heating of otherwise slowly-prepared cooking items, such as hot butter and fats, and melted chocolate. Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to do so. Exceptions occur in rare cases where the oven is used to heat frying-oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling water. The boiling-range temperatures produced in high-water content foods give microwave ovens a limited role in professional cooking, since it usually makes them unsuitable for achievement of culinary effects where the flavors produced by the higher temperatures of frying, browning, or baking are needed. However, additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to produce these other heating effects, and microwave heating may cut the overall time needed to prepare such dishes.
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 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 is described in this patent, is (like induction heating) an electromagnetic heating effect which is 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 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 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.
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 by the Allies of World War II, using magnetron technology. Percy Spencer, an American self-taught engineer from Howland, Maine, was working at the time building magnetrons for radar sets, with the American 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 with microwaves. The first food to be 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 October 8, 1945 Raytheon filed a US patent 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. In 1947, the company built the "Radarange", the first commercial microwave oven. It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5000 each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was 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 1954 consumed 1.6 kilowatts and sold for US$2000 to US$3000. 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$1295, 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.
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, or an empty microwave oven where there is no object to absorb the microwaves, indefinitely. 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 US 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. By the late 1970s the technology had improved to the point where prices were falling rapidly.
Often called "electronic ovens" in the 1960s, the name "microwave ovens" later became 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 most kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make 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. Current estimates hold that over 90% of American households own a microwave oven.
However, lower frequency dielectric heating as is described in this patent, is (like induction heating) an electromagnetic heating effect which is 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 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 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.
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 by the Allies of World War II, using magnetron technology. Percy Spencer, an American self-taught engineer from Howland, Maine, was working at the time building magnetrons for radar sets, with the American 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 with microwaves. The first food to be 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 October 8, 1945 Raytheon filed a US patent 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. In 1947, the company built the "Radarange", the first commercial microwave oven. It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5000 each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was 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 1954 consumed 1.6 kilowatts and sold for US$2000 to US$3000. 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$1295, 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.
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, or an empty microwave oven where there is no object to absorb the microwaves, indefinitely. 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 US 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. By the late 1970s the technology had improved to the point where prices were falling rapidly.
Often called "electronic ovens" in the 1960s, the name "microwave ovens" later became 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 most kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make 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. Current estimates hold that over 90% of American households own a microwave oven.
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