Microwave oven

A microwave oven is an electric appliance that cooks food by bombarding it with electromagnetic radiation in the microwave frequency range. This radiation causes polar molecules within the food to spin, generating heat through a process called dielectric heating. The result is fast and efficient cooking, with fairly uniform heating in the outer inch or so of a uniform, high-moisture food item.

The cavity magnetron, invented in the United Kingdom, made it possible to produce microwaves short enough to heat water molecules efficiently. American engineer Percy Spencer is widely credited with creating and patenting the first commercial microwave oven, the Raytheon "Radarange," which hit the market in 1947. His design drew on British radar technology developed before and during World War II.

Raytheon later licensed its patents for a home-use model, first sold by Tappan in 1956, but it remained too bulky and costly for most households. Sharp Corporation introduced the first microwave with a turntable between 1964 and 1966, which provided more even heating than the earlier stirrer design. The countertop microwave oven debuted in 1967 from Amana Corporation, then a Raytheon subsidiary. Once prices dropped in the late 1970s, microwaves became affordable for home use, and their adoption spread globally, with costs falling sharply through the 1980s. Beyond cooking, microwaves are also used for heating in various industrial processes.

Today, microwaves are common in homes and kitchens, popular for reheating leftovers and cooking a wide range of foods. They quickly heat items that can easily burn or become lumpy in conventional pans, like butter, fats, chocolate, or porridge. However, microwaves rarely reach temperatures high enough for browning or caramelizing food through Maillard reactions. An exception occurs when heating oily items like bacon, which can get much hotter than boiling water.

Microwaves play a limited role in professional cooking because their boiling-range temperatures cannot produce the flavorful chemical reactions from frying, browning, or baking at higher heat. Some hybrid appliances, such as convection microwave ovens, combine microwaves with infrared radiation and hot air.

**History**

**Early developments**

The use of high-frequency radio waves for heating became possible with vacuum tube radio transmitters around 1920. By 1930, short waves were being used medically for diathermy, heating human tissue. At the 1933 Chicago World's Fair, Westinghouse demonstrated cooking foods between two metal plates connected to a 10 kW, 60 MHz shortwave transmitter. The team, led by I. F. Mouromtseff, found that steaks and potatoes could be cooked in minutes.

A 1937 Bell Laboratories patent application described a system for heating dielectric materials uniformly throughout their mass by subjecting them to a high-voltage, high-frequency field. This lower-frequency heating, like induction heating, relies on near-field effects in a cavity smaller than the electromagnetic wavelength. The patent proposed radio frequencies of 10 to 20 megahertz. In contrast, modern microwave ovens use far-field effects from freely propagating radiation, since the wavelength is small relative to the cavity. Despite this difference, the primary heating mechanism for both radio and microwave frequencies remains dielectric heating, where polarized molecules are agitated by a rapidly alternating electric field.

**Cavity magnetron**

The cavity magnetron enabled the production of microwaves with short enough wavelengths. It was a critical component for short-wavelength radar during World War II. Between 1937 and 1940, British physicist Sir John Turton Randall and coworkers built a multi-cavity magnetron for military radar. A higher-power, shorter-wavelength generator was needed, and in 1940, Randall and Harry Boot at the University of Birmingham produced a working prototype. They invented a valve that could generate pulses of microwave energy at a 10 cm wavelength—an unprecedented achievement.

In late September 1940, Sir Henry Tizard traveled to the US to offer Britain's most valuable technical secrets, including the cavity magnetron, in exchange for American financial and industrial support (the Tizard Mission). An early 6 kW version, built in England by the General Electric Company Research Laboratories in Wembley, was given to the U.S. government in September 1940. American historian James Phinney Baxter III later described the cavity magnetron as "[t]he most valuable cargo ever brought to our shores."