Microwave oven 

History

Microwave oven.
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The idea for using microwaves to cook food was discovered by Percy Spencer who was working for Raytheon and was building magnetrons for radar sets. One day he was working on an active radar set when he had noticed a sudden and strange sensation, and saw that a chocolate bar he had in his pocket had melted. The holder of 120 patents, Percy was no stranger to discovery and experiment, and realized what was happening. The first food to be deliberately cooked with microwaves was popcorn, and the second was an egg (which exploded in the face of one of the experimenters).

In 1946 Raytheon patented the microwave cooking process and in 1947, they built the first commercial microwave oven, the Radarange. It was almost 6 feet (1.8 m) tall and weighed 750 pounds (340 kg). It was water-cooled and produced 3000 watts, about three times the amount of radiation produced by microwave ovens today. This first step was so successful that Raytheon eventually purchased Amana to round-out a complete home applicance product suite.

A number of other companies joined in the market, and for a time most systems were built by defense contractors, who were the most familiar with the magnetron. Litton[?] was particularily well known in the restaurant business. By the early 1970s the technology had improved to the point where prices were falling rapidly. Formerly found only in large industrial applications, microwaves were increasingly becoming a standard fixture of most (western) kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make the ovens easier to use. By the 1980s they were almost universal.

Description

A microwave oven consists of:

• a magnetron control circuit (usually with a microcontroller),
• a magnetron,
• a waveguide, and
• a cooking chamber

A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz, through the food. Water molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Each water molecule is an electric dipole, meaning that it has a positive charge at one end and a negative charge at the other, and is therefore twisted to and fro as it tries to align itself with the alternating electric field induced by the microwave beam. This molecular movement creates heat. Microwave heating is sometimes incorrectly explained as resonance of water molecules, but this occurs only at much higher frequencies, in the tens of gigahertz.

The cooking chamber itself is a Faraday cage enclosure to prevent the microwaves escaping into the surroundings. The oven door is usually a glass panel, but has a layer of conductive mesh to maintain the shielding. Since the mesh width is much less than the wavelength of 12 cm, the microwave radiation can not pass the door, while light (with a much shorter wavelength) can.

Microwaving food is fast and popular, but there are hazards because food is heated for so short a time and often cooked unevenly. Microwave ovens are frequently used for reheating previously cooked food, and bacterial contamination may not be killed by the reheating, resulting in food poisoning. The uneven heating is partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, and by a turntable that turns the food. The second problem must be addressed by the cook, who should arrange the food so that it absorbs energy evenly, and periodically test and shield any parts of the food that overheat.

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