The microwave celebrated its 50th anniversary in 2017. The early versions didn’t catch fire with the American public right away not only because they were sold at a high price tag but also because of safety concerns regarding leaked dangerous radiation. Japanese and Korean manufacturers entered the scene in the mid-1980s with their smaller and more affordable microwaves.
This led to about a quarter of American homes owning a microwave by 1986. It then increased to 96 percent in 2009 as microwaves enabled individuals to multitask and families began eating together less frequently.
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History
The microwave, one of the indispensable kitchen appliances, was an accidental invention.
As the Second World War came to an end, the market for the magnetron tubes that were used to create microwaves for close range military radar was no longer in use. Magnetron manufacturers such as Raytheon were eager to find new applications for this technology.
At that time, the idea of heating food substances using radio waves was not a new phenomenon since Bell Labs, RCA, and General Electric had already introduced the idea and were working on this technology.
Dielectric heating
It was already a well-known fact that radio waves heat dielectric materials and its use in medical and industrial concepts was quite common.
The first exploitation of high-frequency radio waves for heating materials was initiated by the development of vacuum radio tube transmitters in the 1920s. By the 1930s, the application of short waves to human heat tissues was in use as medical therapy.
During the World’s fair in 1933 held in Chicago, Westinghouse presented a 10-kilowatt shortwave radio transmitter that cooked potatoes and steak between two metal plates. However, not much came from these culinary escapades.
In 1937, Bell Laboratories’ US patent stated that the invention related to heating systems for dielectric substances and the object of the development is to heat such substances evenly and significantly simultaneously throughout their volume. It was therefore proposed to heat such materials simultaneously throughout the volume using the dielectric loss developed in them when they are subjected to a high voltage, high-frequency field.
The patent describes low-frequency dielectric heating, which is like induction heating – the outcome of electromagnetic heating. It results from near-field effects that are experienced in an electromagnetic cavity. This is smaller compared with the wavelength of a typical electromagnetic field. The patent brought up the idea of radio frequency heating at a frequency of 10 to 20 megahertz.
The heat that comes from microwaves with a wavelength smaller than its cavity results from far-field effects. This is the phenomenon that is used in modern microwave technology. All the same, the primary heating effect of all kinds of electromagnetic fields at both microwave and radio frequencies happens because of the dielectric heating effect since polarized molecules are affected by an electric field that alternates rapidly.
Cavity magnetron
The discovery of the cavity magnetron made the production of small enough electromagnetic waves possible. The magnetron was initially an important part of the making of short wavelength radar, especially during the Second World War. Between 1937 and 1940, a British physicist named Sir John Turton Randall, alongside a team of other scientists, developed a multi-cavity magnetron. The invention was to be used in the American and British military radar installations. The machine was a high-powered microwave generator that produced the much needed short waves.
In 1940, Harry Boot and Randall invented a working prototype at the University of Birmingham. They made a valve that would spit out bits of microwave radio energy on a wavelength of about 10 cm — something that nobody expected would ever come to pass.
Sir Henry Tizard would later travel to the United States during late September in 1940 to give a magnetron exchange for their industrial and financial assistance. A 6 kW version that had been built earlier on in London by the General Electric companies was presented to the U.S. government in September 1940. This magnetron would later be described by an American historian as “The most valuable cargo ever brought to our stores.” Raytheon and other companies were awarded contracts to facilitate the mass production of the magnetron.
The accidental discovery
The breakthrough for the microwave came in 1945. The development was purely coincidental, as the man behind it was not after creating a microwave gadget but was rather going on with his duties inside the laboratory. The American self-taught engineer, Dr. Percy Spencer, was the one who discovered the specific heating effect of a high power microwave beam.
Spencer was an employee of Raytheon. As he was working inside the lab on a research plan about radars using a microwave beam, he noticed that the chocolate bar in his pocket was heated up. He was curious to find out if it was the heat from the magnetron tube that caused the heating. Therefore, the next day he bought popcorn kernels and put them close to the tube. On close observation, he noted that the popcorn had started cracking after a few minutes. Within no time, they were popping all over the laboratory.
The next day, Spencer and a colleague of his decided to try putting an egg at the end of the magnetron tube. Again, the egg started moving in no time. However, it exploded after a few minutes. The reason for this was that the yolk heat up faster than the outer part, building up the internal pressure and eventually exploding.
After these three experiments, Spencer concluded that the low-density microwave energy could be utilized to cook food. Even though they had an egg in their face eventually, their invention felt much better. The final inference of this invention was where Dr. Spencer took a metal box, fed it with microwave power and used it to create an electromagnetic field of high density. They placed the food inside this metal box. The result was just as they had observed previously – the food was heated in just a short time.
The first model
On 8th October 1945, Raytheon Corporation applied for a US patent for Spencer’s innovation. They developed the first microwave appliance that was used in heating food. A specimen of the appliance was tested in Boston restaurant. They named the model Radarange. It was much larger than the modern version we have now. It was roughly 6 feet tall and weighed 750 pounds. Its price was $5,000. Since the magnetron tube used in the oven required water cooling, some plumbing installations were also included in the first model. The power needed to run this machine was 3 kilowatts.
Evolution
The first ever commercial model was created in 1954 but it didn’t perform well with regards to sales. In 1965, the Amana Corporation was bought by Raytheon, and they made the first home-applicable model of the microwave oven in 1967. It retailed at $495 and was quite popular even though it did not do well regarding sales. This initiated the era of an entirely new concept of cooking.
In the 1960s, another company known as Litton developed a newer model of the oven which was wide and short. They have almost the same appearance as the present model of the microwave. The model was made using an exceptional magnetron feed that helped support itself in a no-load condition for an imprecise period. The first time the model was exhibited was during a trade fair in Chicago. After the trade fair, sales grew at an increasingly fast speed. During the 1970s, their sales grew at an even faster pace. By the end of 1975, the company had made more than one million sales in the United States market.
About the same time, Japan has created a cheaper assortment of microwave appliances using a re-engineered magnetron. The newer magnetron was way less expensive than the first. During the late 1970s, other companies also launched their own versions with better features. The result was a steady drop in the price of the microwave appliance. To date, the market is still evolving and so you can easily find a microwave even if your budget is fairly low.
The principles
A microwave oven operates by passing microwave energy through food to heat it. Microwaves are a form of non-ionizing electromagnetic emission that is at a higher frequency than typical radio waves. It is, however, lower than the rate of infrared light. The ovens use frequencies in one of the Industrial, Scientific, Medical ads, which are reserved for this application. Therefore, they do not tamper with other important radio services.
Consumer ovens typically use 2.45 GHz, which is a wavelength of 4.8 inches. Industrial or commercial appliances use 915 MHz, a wavelength of 12.9 inches. Fat, water and other substances present in the food being heated in a microwave absorb energy from the waves in a process called dielectric heating. Many molecules — like that of water — are electric dipoles, implying that they have a partly positive charge at one end and are partly negative at the other end.
They rotate as they attempt to align themselves with a rotating electric field of the waves. Rotating molecules hit each other and put them in motion, distributing energy. The energy that is disbursed in the form of molecular vibrations, rotations and translations in liquids and solids increase the food’s temperature. The process is like that of heat transfer from a hotter body.
Microwave operation is more effective in liquid than a frozen water. This is because the motion of the molecules in the frozen water is more restricted than that of the liquid water. The dielectric loss is largest at 0 degree in a field of 10GHz.
Water is more efficient in microwave heating than sugars and fats. The reason is that fats and sugars have less molecular dipole movement. They absorb microwaves as a result of the dipole movements of the hydroxyl or ester groups. However, because of the low specific heat capacity of oils and fats and their high vaporization temperature, they attain high temperatures inside the oven. They can introduce temperatures in fatty and oily foods such as bacon above the boiling point of water. They are also high enough to cause browning reactions, just like in boiling temperature of water.
Heating characteristics
Microwaves interact with the food to produce heat within the food. Some people think that the microwave cooks food from the inside — a phenomenon that is a big misconception. If you experience uneven heating in the microwave, it could be a result of uneven distribution of microwaves and partly because of the varying rates of energy absorption in different parts of the food. The unevenness of microwaves is reduced by a stirrer which is a kind of fan that distributes the energy throughout the oven as it rotates.
The second problem of different absorption rates is because of differing food geometry and composition and is to be addressed by the cook. The cook may arrange the food such that it absorbs energy evenly and should test and shied parts of the food that is prone to overheating. For materials that have low thermal conductivity where the dielectric constant rises with temperature, microwave heating might result in a localized thermal runaway. Under specific conditions, glass shows a level of thermal runaway to the extent of melting.
Safety features
All microwaves have a timer that alerts the users when the food is ready and switches off the device. Cookware and food taken out of the microwave are barely hotter than 212 degrees Fahrenheit.
The microwave oven heats without getting hot itself so they are safer than most of the available heating methods. For instance, if you heat food using a stove, there is a dangerous heating element that would stay on it for a while.
Cookware and food taken from an oven rarely exceed 100 degrees Celsius. Cookware from a microwave is much cooler than that from a typical oven because it is transparent to microwaves.
The lower cooking temperature is safer than frying or baking in the oven since it gets rid of formation of chars and tar which are carcinogenic.
Caution
Homogenous liquids such as water can overheat in a microwave oven. The liquid can boil explosively such as when the user holds the container to remove it from the oven so that they can add other ingredients like sugar or water. This can lead to abrupt boiling that could be violent enough to remove the boiling liquid from the container and therefore cause scalding.
