Editing Microwave oven (section)

=== Early developments ===
{{multiple image
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| image1   = Cooking with radio waves - Chicago Worlds Fair 1933.jpg
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| image2   = Cooking with radio waves - Short Wave Craft Nov 1933 cover.jpg
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| footer    = Demonstration by Westinghouse of cooking sandwiches with a 60 MHz shortwave radio transmitter at the [[1933 Chicago World's Fair]]
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The exploitation of high-frequency [[radio wave]]s for heating substances was made possible by the development of [[vacuum tube]] [[radio transmitter]]s around 1920. By 1930 the application of [[short wave]]s to heat human tissue had developed into the medical therapy of [[diathermy]].  At the [[1933 Chicago World's Fair]], [[Westinghouse Electric Corporation|Westinghouse]] demonstrated the cooking of foods between two metal plates attached to a 10&nbsp;kW, 60&nbsp;MHz [[shortwave]] [[transmitter]].<ref>{{cite journal | title = Cooking with Short Waves | journal = Short Wave Craft | volume = 4| issue = 7 | page = 394 | date = November 1933 | url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf | access-date = March 23, 2015}}</ref>  The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes.<ref>{{Cite journal |last1=Lovelock |first1=J. E. |last2=Smith |first2=Audrey U. |name-list-style=vanc |year=1956 |title=Studies on Golden Hamsters during Cooling to and Rewarming from Body Temperatures below 0 degrees C. III. Biophysical Aspects and General Discussion |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=145 |issue=920 |pages=427–442 |bibcode=1956RSPSB.145..427L |doi=10.1098/rspb.1956.0054 |issn=0080-4649 |jstor=83008 |pmid=13359396 |s2cid=6474737}}</ref>

The 1937 United States patent application by Bell Laboratories states:<ref>{{US patent|2147689}} Chaffee, Joseph G., ''Method and apparatus for heating dielectric materials'', filed August 11, 1937; granted February 21, 1939</ref>
{{Blockquote|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 and far field|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 30 to 15 meters, respectively).<ref name=  "patent">{{citation | url = http://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | publisher = United States Patent and Trademark Office | title = 2,147,689: Method and Apparatus for Heating Dielectric Materials | first = Joseph G. | last = Chaffee | date = February 21, 1939 | access-date = February 5, 2015 | archive-date = March 19, 2022 | archive-url = https://web.archive.org/web/20220319201814/https://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | url-status = dead }}</ref> 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.