A microwave oven, or simply microwave, is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce thermal energy (heat) in a process known as dielectric heating. Microwave ovens heat food quickly and efficiently; the heating effect is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous, high-water-content food item. The development of the cavity magnetron in the United Kingdom made possible the production of electromagnetic waves of a small enough wavelength (microwaves) to efficiently heat up water molecules. American electrical engineer Percy Spencer is generally credited with developing and patenting the world's first commercial microwave oven, the Raytheon "Radarange", which was first sold in 1947. He based it on British radar technology which had been developed before and during World War II. Raytheon later licensed its patents for a home-use microwave oven that was first sold by Tappan in 1956, but it was still too large and expensive for general home use. Sharp Corporation introduced the first microwave oven with a turntable between 1964 and 1966, this feature created more even heating than the earlier microwave stirrer feature. The countertop microwave oven was introduced in 1967 by the Amana Corporation, which was, at the time, owned by Raytheon. After microwave ovens became affordable for residential use in the late 1970s, their use spread into residential kitchens around the world, and prices fell rapidly during the 1980s. In addition to cooking food, microwave ovens are used for heating in many industrial processes. Microwave ovens are a common kitchen appliance in households and kitchens and are popular for reheating previously cooked foods and cooking a variety of different foods. They rapidly heat foods which can easily burn or turn lumpy if cooked rapidly in conventional pans, such as butter, fats, chocolate, or porridge. Microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperature to produce Maillard reactions. Exceptions occur in cases where the oven is used to heat frying-oil and other oily items (such as bacon), which attain far higher temperatures than that of boiling water. Microwave ovens have a limited role in professional cooking, because the boiling-range temperatures of a microwave oven do not produce the flavorful chemical reactions that frying, browning, or baking at a higher temperature produces. However, there are hybrid appliances that combine infrared radiation, hot air, and microwaves, such as convection microwave ovens.
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:
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 30 to 15 meters, respectively). 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
The invention of the cavity magnetron made possible the production of electromagnetic waves of a small enough wavelength (microwaves). The cavity magnetron was a crucial component in the development of short wavelength radar during World War II. In 1937–1940, a multi-cavity magnetron was built by British physicist Sir John Turton Randall, FRSE and coworkers, for the British and American military radar installations in World War II. A higher-powered microwave generator that worked at shorter wavelengths was needed, and in 1940, at the University of Birmingham in England, Randall and Harry Boot produced a working prototype. They invented a valve that could produce pulses of microwave radio energy at a wavelength of 10 cm, an unprecedented discovery. Sir Henry Tizard traveled to the US in late September 1940 to offer Britain's most valuable technical secrets including the cavity magnetron in exchange for US financial and industrial support (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. The cavity magnetron was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores". Contracts were awarded to Raytheon and other companies for the mass production of the cavity magnetron.
Discovery
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