This Day In History: One of Your Modern Convenience Was Invented, The Microwave

The Microwave was invented


A microwave oven (commonly referred to as a microwave) is a kitchen appliance 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 in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous, high water content food item; food is more evenly heated throughout (except in heterogeneous, dense objects) than generally occurs in other cooking techniques.


Percy Spencer is generally credited with inventing the modern microwave oven after World War II from radar technology developed during the war. Named the “Radarange”, it was first sold in 1946. 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, and their use has spread into commercial and residential kitchens around the world.


Microwave ovens are popular for reheating previously cooked foods and cooking a variety of foods. They are also useful for rapid heating of otherwise slowly prepared cooking items, such as hot butter, fats, and chocolate. Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to produce Maillard reactions. 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.


Microwave ovens have a limited role in professional cooking,[1] because the boiling-range temperatures produced in especially hydrous foods impede flavors produced by the higher temperatures of frying, browning, or baking. However, additional heat sources can be added to microwave ovens, or into combination microwave ovens, to produce these other heating effects, and microwave heating may cut the overall time needed to prepare such dishes. Some modern microwave ovens are part of over-the-range units with built-in extractor hoods.



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.[2] The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes.


“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 15 to 30 meters).[4] 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 magnetron was originally a crucial component in the development of short wavelength radar during World War II.[5] In 1937–1940, a multi-cavity magnetron was built by the British physicist Sir John Turton Randall, FRSE, together with a team of British coworkers, for the British and American military radar installations in World War II. A more high-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.[6]


Sir Henry Tizard travelled to the U.S. in late September 1940 to offer the magnetron in exchange for their financial and industrial help (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 magnetron was later described by American historian James Phinney Baxter III as “[t]he most valuable cargo ever brought to our shores”.[7] Contracts were awarded to Raytheon and other companies for mass production of the magnetron.



In 1945, the specific heating effect of a high-power microwave beam was accidentally discovered by Percy Spencer, an American self-taught engineer from Howland, Maine. Employed by Raytheon at the time, he noticed that microwaves from an active radar set he was working on started to melt a candy bar he had in his pocket. The first food deliberately cooked with Spencer’s microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.[8][9] 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 8 October 1945,[10] Raytheon filed a United States patent application 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.


Commercial availability

In 1947, Raytheon built the “Radarange”, the first commercially available microwave oven.[11] It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5,000 ($54,000 in 2016 dollars) each. It consumed 3 kilowatts, about three times as much as today’s microwave ovens, and was water-cooled. An early 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$2,000 to US$3,000 ($18,000 to $27,000 in 2016 dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952.[12] They tried to market a large 220 volt wall unit as a home microwave oven in 1955 for a price of US$1,295 ($12,000 in 2016 dollars), 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 ($4,000 in 2016 dollars).

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: an empty microwave oven where there is nothing to absorb the microwaves. 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 U.S. 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.

Residential use

Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in developed countries. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971;[13] the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.[13][14] In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily.[15] In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998.[16] In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.[17]


Adoption has been slower in less-developed countries, as households with disposable income concentrate on more important household appliances like refrigerators and ovens. In India, for example, only about 5% of households owned a microwave in 2013, well behind refrigerators at 31% ownership.[18] However, microwave ovens are gaining popularity. In Russia, for example, the number of households with a microwave grew from almost 24% in 2002 to almost 40% in 2008.[19] Almost twice as many households in South Africa owned microwaves in 2008 (38.7%) than in 2002 (19.8%).[19] Microwave ownership in Vietnam was at 16% of households in 2008—versus 30% ownership of refrigerators; this rate was up significantly from 6.7% microwave ownership in 2002, with 14% ownership for refrigerators that year.[19]



A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary radio waves but lower than infrared light. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they do not interfere with other vital radio services. Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimetres (12.9 in).[20] Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e. as both potential energy and kinetic energy of atoms), is heat. Sometimes, microwave heating is explained as a resonance of water molecules, but this is incorrect;[21] such resonances occur only at above 1 terahertz (THz).[22] Rather it is the lag in response of the polar water molecule to the impinging electromagnetic wave. This type of dieletric loss mechanism is referred to as dipole interaction.


Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted. Dielectric heating of liquid water is also temperature-dependent: At 0 °C, dielectric loss is greatest at a field frequency of about 10 GHz, and for higher water temperatures at higher field frequencies.[23]


Compared to liquid water, microwave heating is less efficient on fats and sugars (which have a smaller molecular dipole moment).[24] Sugars and triglycerides (fats and oils) absorb microwaves due to the dipole moments of their hydroxyl groups or ester groups. However, due to the lower specific heat capacity of fats and oils and their higher vaporization temperature, they often attain much higher temperatures inside microwave ovens.[23] This can induce temperatures in oil or very fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional broiling (UK: grilling), braising, or deep fat frying. Foods high in water content and with little oil rarely exceed the boiling temperature of water.


Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of synthetic lava. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.


A common misconception is that microwave ovens cook food “from the inside out”, meaning from the center of the entire mass of food outwards. This idea arises from heating behavior seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface. This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach a higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogenous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to broiling/grilling (infrared) or convection heating—methods which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating further.


Heating efficiency

A microwave oven converts only part of its electrical input into microwave energy. An average consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits, although the power consumed by the electronic control circuits of a modern microwave oven is negligible (< 1% of the input power) during cooking.


For cooking or reheating small amounts of food, the microwave oven may use less energy than a cook stove. Although microwave ovens are touted as the most efficient appliance,[25] the energy savings are largely due to the reduced heat mass of the food’s container.[26] The amount of energy used to heat food is generally small compared to total energy usage in typical residences in the United States.[27]



A microwave oven consists of:

a high-voltage power source, commonly a simple transformer or an electronic power converter, which passes energy to the magnetron
a high-voltage capacitor connected to the magnetron, transformer and via a diode to the chassis
a cavity magnetron, which converts high-voltage electric energy to microwave radiation
a magnetron control circuit (usually with a microcontroller)
a short waveguide (to couple microwave power from the magnetron into the cooking chamber)
a metal cooking chamber
a turntable or metal wave guide stirring fan.
a digital / manual control panel

Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED, liquid crystal or vacuum fluorescent display, in the 90s brands such as Panasonic and GE began offering models with a scrolling-text display showing cooking instructions, numeric buttons for entering the cook time, a power level selection feature and other possible functions such as a defrost setting and pre-programmed settings for different food types, such as meat, fish, poultry, vegetables, frozen vegetables, frozen dinners, and popcorn. In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. As such, the choice of power level does not affect the intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds, thus altering the large scale duty cycle. Newer models have inverter power supplies that use pulse-width modulation to provide effectively continuous heating at reduced power, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating.


The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the industrial, scientific, and medical (ISM) frequency bands set aside for non-communication purposes. For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in the ITU Region 2 while 2.45 GHz is available worldwide. Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8 GHz and 24.125 GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies. The third, centered on 433.92 MHz, is a narrow band that would require expensive equipment to generate sufficient power without creating interference outside the band, and is only available in some countries.


The cooking chamber is similar to a Faraday cage to prevent the waves from coming out of the oven. Even though there is no continuous metal-to-metal contact around the rim of the door, choke connections on the door edges act like metal-to-metal contact, at the frequency of the microwaves, to prevent leakage. The oven door usually has a window for easy viewing, with a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh is much less than the microwaves’ wavelength (12.2 cm for the usual 2.45 GHz), microwave radiation cannot pass through the door, while visible light (with its much shorter wavelength) can.


Variants and accessories

A variant of the conventional microwave is the convection microwave. A convection microwave oven is a combination of a standard microwave and a convection oven. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwaves are more expensive than conventional microwave ovens. Some convection microwaves—those with exposed heating elements—can produce smoke and burning odors as food spatter from earlier microwave-only use is burned off the heating elements.


In 2000,[28] some manufacturers began offering high power quartz halogen bulbs to their convection microwave models, marketing them under names such as “Speedcook”, “Advantium” , “Lightwave” and “Optimawave” to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food’s surface with infrared (IR) radiation, browning surfaces as in a conventional oven. The food browns while also being heated by the microwave radiation and heated through conduction through contact with heated air. The IR energy which is delivered to the outer surface of food by the lamps is sufficient to initiate browning caramelization in foods primarily made up of carbohydrates and Maillard reactions in foods primarily made up of protein. These reactions in food produce a texture and taste similar to that typically expected of conventional oven cooking rather than the bland boiled and steamed taste that microwave-only cooking tends to create.


In order to aid browning, sometimes an accessory browning tray is used, usually composed of glass or porcelain. It makes food crisp by oxidizing the top layer until it turns brown. Ordinary plastic cookware is unsuitable for this purpose because it could melt.


Frozen dinners, pies, and microwave popcorn bags often contain a susceptor made from thin aluminium film in the packaging or included on a small paper tray. The metal film absorbs microwave energy efficiently and consequently becomes extremely hot and radiates in the infrared, concentrating the heating of oil for popcorn or even browning surfaces of frozen foods. Heating packages or trays containing susceptors are designed for a single use and are then discarded as waste.


Microwave-safe plastics

Some current plastic containers and food wraps are specifically designed to resist radiation from microwaves. Products may use the term “microwave safe”, may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.[29]


Benefits and safety features

Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, unless it is an induction cooktop, leaves a potentially dangerous heating element or trivet that will stay hot for some time. Likewise, when taking a casserole out of a conventional oven, one’s arms are exposed to the very hot walls of the oven. A microwave oven does not pose this problem.


Food and cookware taken out of a microwave oven are rarely much hotter than 100 °C (212 °F). Cookware used in a microwave oven is often much cooler than the food because the cookware is transparent to microwaves; the microwaves heat the food directly and the cookware is indirectly heated by the food. Food and cookware from a conventional oven, on the other hand, are the same temperature as the rest of the oven; a typical cooking temperature is 180 °C (356 °F). That means that conventional stoves and ovens can cause more serious burns.


The lower temperature of cooking (the boiling point of water) is a significant safety benefit compared to baking in the oven or frying, because it eliminates the formation of tars and char, which are carcinogenic.[30] Microwave radiation also penetrates deeper than direct heat, so that the food is heated by its own internal water content. In contrast, direct heat can burn the surface while the inside is still cold. Pre-heating the food in a microwave oven before putting it into the grill or pan reduces the time needed to heat up the food and reduces the formation of carcinogenic char. Unlike frying and baking, microwaving does not produce acrylamide in potatoes,[31] however unlike deep-frying, it is of only limited effectiveness in reducing glycoalkaloid (i.e. solanine) levels.[32] Acrylamide has been found in other microwaved products like popcorn.


Heating characteristics

Microwave ovens are frequently used for reheating leftover food, and bacterial contamination may not be repressed if the safe temperature is not reached, resulting in foodborne illness, as with all inadequate reheating methods.


Uneven heating in microwaved food can be 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, or by a turntable or carousel that turns the food; turntables, however, may still leave spots, such as the center of the oven, which receive uneven energy distribution. The location of dead spots and hot spots in a microwave can be mapped out by placing a damp piece of thermal paper in the oven. When the water saturated paper is subjected to the microwave radiation it becomes hot enough to cause the dye to be released which will provide a visual representation of the microwaves. If multiple layers of paper are constructed in the oven with a sufficient distance between them a three-dimensional map can be created. Many store receipts are printed on thermal paper which allows this to be easily done at home.[33]


The second problem is due to food composition and geometry, and must be addressed by the cook, by arranging the food so that it absorbs energy evenly, and periodically testing and shielding any parts of the food that overheat. In some materials with low thermal conductivity, where dielectric constant increases with temperature, microwave heating can cause localized thermal runaway. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting.[34]


Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of frozen food while the inside of the food remains frozen. Another case of uneven heating can be observed in baked goods containing berries. In these items, the berries absorb more energy than the drier surrounding bread and cannot dissipate the heat due to the low thermal conductivity of the bread. Often this results in overheating the berries relative to the rest of the food. “Defrost” oven settings use low power levels designed to allow time for heat to be conducted within frozen foods from areas that absorb heat more readily to those which heat more slowly. In turntable-equipped ovens, more even heating will take place by placing food off-centre on the turntable tray instead of exactly in the centre, assuming the food item so placed covers less of the center “dead zone”.


There are microwave ovens on the market that allow full-power defrosting. They do this by exploiting the properties of the electromagnetic radiation LSM modes. LSM full-power defrosting may actually achieve more even results than slow defrosting.[35]


Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain ceramic or aluminium flakes, which are designed to absorb microwaves and heat up, which aids in baking or crust preparation by depositing more energy shallowly in these areas. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in colour, usually making them easily identifiable; the cardboard sleeves included with Hot Pockets, which have a silver surface on the inside, are a good example of such packaging. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way. The technical term for such a microwave-absorbing patch is a susceptor.[36]


Effects on food and nutrients

Comparative cooking method studies generally find that, if properly used, microwave cooking does not affect the nutrient content of foods to a larger extent than conventional heating, and that there is a tendency towards greater retention of many micronutrients with microwaving, probably due to the reduced preparation time.[37] Microwaving human milk at high temperatures is contraindicated, due to a marked decrease in activity of anti-infective factors.[38]


Any form of cooking will destroy some nutrients in food, but the key variables are how much water is used in the cooking, how long the food is cooked, and at what temperature.[39] Nutrients are primarily lost by leaching into cooking water, which tends to make microwave cooking healthier, given the shorter cooking times it requires.[40] Like other heating methods, microwaving converts vitamin B12 from an active to inactive form; the amount of inactivation depends on the temperature reached, as well as the cooking time. Boiled food reaches a maximum of 100 °C (212 °F) (the boiling point of water), whereas microwaved food can get locally hotter than this, leading to faster breakdown of vitamin B12. The higher rate of loss is partially offset by the shorter cooking times required.[41] A single study indicated that microwaving broccoli loses 74% or more of phenolic compounds (97% of flavonoids), while boiling loses 66% of flavonoids, and high-pressure boiling loses 47%,[42] though the study has been contradicted by other studies.[43] To minimize phenolic losses in potatoes, microwaving should be done at 500 W.[44]


Spinach retains nearly all its folate when cooked in a microwave; in comparison, it loses about 77% when boiled, leaching out nutrients. Bacon cooked by microwave has significantly lower levels of carcinogenic nitrosamines than conventionally cooked bacon.[39] Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.[39] Microwave blanching is 3–4 times more effective than boiled water blanching in the retaining of the water-soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% is lost (vs. 16% with boiled water blanching).[45]


Use in cleaning kitchen sponges

Studies have investigated the use of the microwave to clean non-metallic domestic sponges which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for two minutes (at 1000 watt power) removed 99% of coliforms, E. coli and MS2 phages. Bacillus cereus spores were killed at 4 minutes of microwaving.[46]



High temperatures

Water and other homogeneous liquids can superheat[47][48] when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients such as powdered creamer or sugar. This can result in spontaneous boiling (nucleation) which may be violent enough to eject the boiling liquid from the container and cause severe scalding.[49]

Closed containers, such as eggs, can explode when heated in a microwave oven due to the increased pressure from steam. Intact fresh egg yolks outside the shell will also explode, as a result of superheating. Insulating plastic foams of all types generally contain closed air pockets, and are generally not recommended for use in a microwave, as the air pockets explode and the foam (which can be toxic if consumed) may melt. Not all plastics are microwave-safe, and some plastics absorb microwaves to the point that they may become dangerously hot.

Products that are heated for too long can catch fire. Though this is inherent to any form of cooking, the rapid cooking and unattended nature of the use of microwave ovens results in additional hazard.


Metal objects

Any metal or conductive object placed into the microwave will act as an antenna to some degree, resulting in an electric current. This causes the object to act as a heating element. This effect varies with the object’s shape and composition, and is sometimes utilized for cooking.


Any object containing pointed metal can create an electric arc (sparks) when microwaved. This includes cutlery, crumpled aluminium foil (though some foil used in microwaves are safe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape.[50] Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m). The air forms a conductive plasma, which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. When dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are unhealthy in large quantities.

It is possible for metal objects to be microwave-oven compatible, although experimentation by users is not encouraged. Microwaving an individual smooth metal object without pointed ends, for example, a spoon or shallow metal pan, usually does not produce sparking. Thick metal wire racks can be part of the interior design in microwave ovens. In a similar way, the interior wall plates with perforating holes which allow light and air into the oven, and allow interior-viewing through the oven door, are all made of conductive metal formed in a safe shape.

The effect of microwaving thin metal films can be seen clearly on a Compact Disc or DVD (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, porcelain with thin metal films can also be destroyed or damaged by microwaving. Aluminium foil is thick enough to be used in microwave ovens as a shield against heating parts of food items, if the foil is not badly warped. When wrinkled, aluminium foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The USDA recommends that aluminium foil used as a partial food shield in microwave cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.[5]


Another hazard is the resonance of the magnetron tube itself. If the microwave is run without an object to absorb the radiation, a standing wave will form. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. For the same reason, dehydrated food, or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard.


Certain foods such as grapes, if properly arranged, can produce an electric arc.[52] Prolonged arcing from food carries similar risks to arcing from other sources as noted above.


Some other objects that may conduct sparks are plastic/holographic print thermoses (such as Starbuck’s novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell will burst off the object or melt.


The high electrical fields generated inside a microwave often can be illustrated by placing a radiometer or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device.


Direct microwave exposure

Direct microwave exposure is not generally possible, as microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which the oven is constructed. Furthermore, ovens are equipped with redundant safety interlocks, which remove power from the magnetron if the door is opened. This safety mechanism is required by United States federal regulations.[53] Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.[54] According to the United States Food and Drug Administration’s Center for Devices and Radiological Health, a U.S. Federal Standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately 5 cm (2 in) from the surface of the oven.[55] This is far below the exposure level currently considered to be harmful to human health.[56]


The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer risks associated with ionizing radiation such as X-rays and high-energy particles. Long-term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from 2.45 GHz microwave radiation even with chronic exposure levels (i.e. large fraction of life span) far larger than humans are likely to encounter from any leaking ovens.[57][58] However, with the oven door open, the radiation may cause damage by heating. Every microwave oven sold has a protective interlock so that it cannot be run when the door is open or improperly latched.


Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. There is some evidence that nutritional content of some foods may be altered differently by cooking in a microwave oven, compared to conventional cooking, but there is no indication of detrimental health issues associated with microwaved food.[59]


There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.[60][61] The general effect of this exposure will be physical burns to the body, as human tissue, particularly the outer fat and muscle layers, has similar composition to some foods that are typically cooked in microwave ovens and so experiences similar dielectric heating effects when exposed to microwave electromagnetic radiation.


Chemical exposure

Some magnetrons have ceramic insulators with beryllium oxide (beryllia) added. The beryllium in such oxides is a serious chemical hazard if crushed then inhaled or ingested. In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be handled. This is a danger if the microwave oven becomes physically damaged, if the insulator cracks, or when the magnetron is opened and handled, yet not during normal usage.



  1. ^ Hervé This, Révélations gastronomiques, Éditions Belin. ISBN 2-7011-1756-9
  2. ^ “Cooking with Short Waves” (PDF)Short Wave CraftNew York: Popular Book Corp. 4 (7): 394. November 1933.Retrieved 23 March 2015.
  3. ^ U.S. Patent 2,147,689 Chaffee, Joseph G., Method and apparatus for heating dielectric materialsfiled 11 August1937; granted 21 February 1939
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  24. ^ “Efficient” here meaning more energy is deposited, not necessarily that the temperature rises more, because thelatter also is a function of the specific heat capacitywhich is often less than water for most substances. For apractical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have lessheat capacity than the water they replace.
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  45. ^ M. A. OSINBOYEJO; L. T. Walker; S. Ogutu & M. Verghese. “Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnips, foods, and greens using HPLC”NationalCenter for Home Food Preservation, University of GeorgiaRetrieved 23 July 2011.
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World War One: Meuse-Argonne, Preparations

On 26 September 1918, the American First Army launched a massive attack between the Argonne Forest (Forêt d’Argonne) and the Meuse River northwest of the storied French town of Verdun. By the time that the Germans agreed to an armistice forty-seven days later, the Meuse-Argonne Campaign would gain the distinction of being the largest and most costly military operation in American history. Over a million American soldiers, sailors, airmen, and marines, as well as 135,000 French soldiers, participated in the offensive. Although the First Army had committed to this battle long before most of its doughboys had mastered the skills required to fight a mass industrialized war, the Americans persevered and gradually ground down the German units opposing them. Unfortunately, this approach came at a high price: 26,277 men killed and another 95,786 wounded as the American Expeditionary Forces (AEF) learned how to wage a modern war against a skilled opponent. The Meuse-Argonne Offensive was the most important American military contribution to the Allied effort during the war. The AEF’s hard-won victory materially contributed to the collapse of the German Army and achieved President Woodrow Wilson’s strategic goal of securing for the United States a major role in crafting the peace that followed the Armistice.

The German Army in the Meuse-Argonne Region
By the time the Americans began their attack in the Meuse-Argonne, their German foes had occupied the region for four years. The area had been the scene of fierce fighting in 1914 and 1915, and the Germans used the region as the staging area for their attack on Verdun in 1916. To that end, the Germans had constructed fortifications and artillery shelters throughout the sector, and built a light rail line and other logistics nodes to support their operations in the region. After suffering an unsustainable rate of loss during the fighting at Verdun and the Somme, in September 1916 Ludendorff ordered a review of German defensive tactics and the establishment of a new series of fortifications, collectively known as the Siegfried Stellung (or the Hindenburg Line to the Allies), along the Western Front. The new German doctrine used a defense in depth to husband Germany’s declining manpower resources and to counter the growing effectiveness of Allied artillery and offensive tactics. The Germans planned to use skillfully sited field fortifications and interlocking defensive firepower to exhaust Allied attacks and serve as a base for timely, powerful counterattacks that would prevent the Allies from gaining any major foothold.

Although the Germans had dedicated less effort to engineering the defenses in the Meuse-Argonne than they had on the western sectors of the Hindenburg Line, nature provided them with ample defensive advantages to compensate for this shortfall. The western boundary of the U.S. First Army’s sector encompassed most of the Argonne Forest. The forest itself sat on a plateau bounded by the Aisne and Aire rivers. The Argonne was crisscrossed by a range of hills and draws running in a generally east-west direction; these features, along with dense vegetation, presented grave challenges to the mobility, command and control, and artillery support of the American attackers. To the east of the Argonne was the valley of the Aire River—a natural movement corridor for the Americans, but one dominated on the west by the hills of the Argonne and on the east by the large buttes of Montfaucon and Vauquois and other heights. The center of the American sector was the Barrois Plateau, a series of hills and highlands that started in the south at Montfaucon, ran to the northeast to Romagne-sous-Montfaucon and Cunel, and ended at the Barricourt Heights. East of the Barrois Plateau was the Meuse River valley, another natural movement corridor flanked by high ground on both banks. As Lt. General Hunter Liggett, commander of the U.S. I Army Corps, mused, “The region was a natural fortress beside which the Virginia Wilderness in which Grant and Lee fought was a park.”

To this “natural fortress,” the Germans added their own skills at defense to present the Americans with a formidable set of obstacles to overcome. Within the sector, the Germans had constructed four major defensive belts arrayed over a depth of fifteen to twenty-four kilometers. Most of their engineering efforts had gone into strengthening the third position, composed of two lateral sections of the Hindenburg Line, the Brunhild Stellung and the Kriemhilde Stellung. In the area of the main American advance, the line ran from Grandpré on the west across the heights of Côte Dame Marie, Romagne, and Cunel, to Brieulles on the Meuse. It consisted of warrens of concrete-reinforced shelters and machine gun nests, earthen strong points, and support and communications trenches. These defensive positions made adroit use of terrain and barbed-wire belts to canalize attackers into a web of interlocking machine gun fields of fire and preplanned artillery targets. The other belts in the sector followed a similar design but made less use of hardened fortifications.

The German high command had split the defense of the Meuse-Argonne region between the Third Army under General Karl von Einem and the Fifth Army under General Georg von der Marwitz. Each reported to different army group commanders, which would hamper German unity of command and effort in the opening days of the American offensive. The German Third Army was responsible for the Argonne Forest and the area running west into the French Fourth Army’s sector, while the German Fifth Army’s area of operation extended from the Aire Valley to east of St. Mihiel. The Third Army’s Group Aisne placed the 76th Reserve Division on the extreme left of the American sector, while the Group Argonne had the 2d Landwehr Division and the 1st Guards Division in line. The corps was responsible for the defense of the Argonne Forest and the area around Varennes. Although the Landwehr units had been stripped of most of their youngest troops to fill the ranks of the assault divisions for Ludendorff’s Spring Offensives, they had been stationed in the Argonne since September 1914 and early 1915 and were well acquainted with the terrain that they would defend. The 1st Guards Division, meanwhile, was an elite unit, but it had been worn down by four years of fighting and by being actively engaged in operations since March 1918.

The Fifth Army’s Group Meuse West placed the 117th Division and the 7th Reserve Division in the line facing the Americans. The AEF rated both of these as second-class divisions, and the 117th was still recovering from the heavy losses it had suffered at the Battle of Amiens in early August. When it became clear to the German high command that a major Franco-American attack was looming in the Argonne region, it sent two more divisions, the 5th Guards and the 5th Bavarian Reserve, to reinforce the sector. The 5th Guards was an excellent but battered unit that had seen action against the American 2d Division north of Château-Thierry in June. The 5th Bavarian Reserve Division had been spared much of the fighting in 1918, but the AEF still considered it to be only a second-class unit.

Although all of the German divisions that the U.S. First Army would face in the opening days of the Meuse-Argonne were understrength, tattered, and tired, their core of experienced veterans and leaders intended to make the Americans pay heavily for the ground that the Kaiser’s army defended. They also had a massive array of firepower. When the German 123d Infantry Division entered the Meuse-Argonne sector at Cunel on 11 October 1918, it had been hammered by fighting the Americans at St. Mihiel and was down to only 89 officers and 1,705 men. However, the division could still field 198 heavy and light machine guns—one gun for every nine soldiers. The German defenders of the Meuse-Argonne were likewise well provided with artillery, leading one doughboy to bitterly note that it seemed as if “every goddamn German there who didn’t have a machine gun had a cannon.” Given the daunting terrain and determined enemy in the Meuse-Argonne, the doughboys would face a rough fight.

The First Army Plans the Campaign
Pershing’s insistence on conducting the St. Mihiel Offensive while preparing for the Meuse-Argonne Campaign gave the AEF and the First Army’s staff only twenty-three days to plan and organize the largest military operation in American history. The AEF was fortunate to draw on the talents of Brig. Gen. Fox Conner, Col. Walter S. Grant, and Lt. Col. George C. Marshall for this monumental task. The most important challenge was moving and staging the massive number of troops required for the operation and for relieving the French forces operating in the sector. The relief in place of the French Second Army by the U.S. First Army would be a complex ballet that would move 220,000 soldiers out of the front while simultaneously deploying over 600,000 soldiers and 3,823 artillery pieces to the sector. To further complicate matters, some units earmarked for the start of the Argonne drive were already committed to the St. Mihiel operation and had to start moving out of the salient before that battle had concluded.

The mass armies of the First World War required extensive logistical support to operate in the field, especially when conducting major offensives. The size of the AEF’s logistics command, the Services of Supply (SOS), dwarfed all previous American military logistics efforts. By the Armistice, the SOS contained 546,596 soldiers, more men than were in the combined armies of Ulysses S. Grant and Robert E. Lee in 1864. Despite the phenomenal growth of the SOS, the Supreme War Council’s decision to give priority to shipping infantry and machine gun units to France in the spring of 1918 meant that the SOS was still short of the men and special units it needed to properly execute its missions. The AEF’s logisticians persevered against great odds to establish nineteen railheads; thirty-four evacuation hospitals; and fifty-six ordnance, quartermaster, ammunition, petroleum, gas warfare, and engineer depots to supply the Meuse-Argonne drive. In the seventeen days before the offensive began, the SOS also pre-positioned 40,000 tons of shells to support the first five days of artillery fire.

The First Army’s operational plan was ambitious. Pershing envisioned that the American offensive would occur in four. In the first phase, three American corps would attack on a thirty-kilometer front stretching from the Argonne Forest to the Meuse River. On the first day of the battle, 26 September, these corps would drive sixteen kilometers through the first three German defensive belts. After breaking through the enemy’s main defenses, the Americans would reconnect with the other arm of the Franco-American offensive, the French Fourth Army, north of the Argonne Forest at Grandpré. Pershing’s decision to devote most of his veteran divisions to the St. Mihiel Offensive meant that five of the nine divisions slated for the Meuse-Argonne’s initial assault had little to no previous exposure to combat. Right from the outset, Pershing expected a great deal from his inexperienced soldiers.

The First Army anticipated that the second phase of the operation would begin on 27 September with another sixteen kilometer drive to push the Germans back beyond the line of Stenay to Le Chesne. In the third phase, the French XVII Corps, under American command, would attack east of the Meuse River to clear the Heights of the Meuse and protect the right flank of the First Army’s drive north. The last phase of the operation would carry the combined Franco-American attack to the rail heads of Sedan and Mézières.

During the first phase, General Liggett’s I Corps would attack down the Aire Valley on the army’s left flank. Working with units from the French Fourth Army, the I Corps would clear the Argonne Forest. In the center, Maj. Gen. George H. Cameron’s V Corps would seize Montfaucon and the other heights of the Barrois Plateau. On the right, Maj. Gen. Robert L. Bullard would push the III Corps through the valley of the Meuse and rout the Germans from their sector up to the town of Brieulles-sur-Meuse. In carrying out this phase, the V Corps faced the most difficult tactical challenges. The German defenses on Montfaucon loomed above the corps’ front and had withstood several French attacks in 1914 and 1915. To make matters worse, Cameron would have to rely on three green units, the 91st, 37th, and 79th Divisions, to storm the high ground in his sector. The First Army planners recognized the V Corps’ dilemma and gambled that the rapid advance of the I and III Corps on Cameron’s flanks would turn the Germans out of their defenses on the Barrois Plateau and thus aid the V Corps’ advance. To help its corps accomplish their missions, the First Army dedicated 419 tanks to support the 26 September attacks. Lt. Col. George S. Patton Jr.’s 1st U.S. Tank Brigade, with 127 American crewed Renault FT light tanks reinforced with twenty-eight French-crewed Schneider tanks, was directed to support the 35th Division in the I Corps’ sector. The First Army also assigned 250 French-crewed tanks to the 3d U.S. Tank Brigade to assist

SOURCE: U.S Army Campaigns of World War I (United States Army Center of Military History): CONTRIBUTOR: Cade Pommeraan

World War One: Meuse-Argonne; 27-31 Oct. 1918 Preparing for the Final Push

World War One: Meuse-Argonne; 1-11 Nov. 1918 Victory & Defeat