Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).
Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.
The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.
Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.
The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the
Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.
The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.
Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.
In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.
Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.
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— Rashid Riaz