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Ride the (electromagnetic) Wave with Percy Spencer!

On January 24, 1950 Percy Spencer was granted US2495429, a Method of Treating Foodstuffs. This a significant day in the history of appliance technology, though it would be more than two decades before the microwave oven would be widely available and affordable enough to begin appearing in households across the globe. This appliance revolutionized the way people prepared food, but the process, like many other discoveries, was entirely accidental.

Enlargement of drawing from Percy Spencer’s patent application: Method of Treating Foodstuffs.

Percy Spencer was a self-taught engineer who had several jobs before joining the U.S. Navy where he learned about radar technology. After leaving the Navy he held several positions, ultimately ending up at Raytheon Manufacturing Company, a major producer of magnetrons and also where Spencer invented what he is most well known for: the microwave oven.

How this happy accident happened began when Spencer had a chocolate candy bar in his pocket that melted while he was observing radar sets in operation, though none of the sources I came across named the candy bar. These radar sets emitted electromagnetic waves, but the ones we’re interested in are those that range between about 2 and 40 gigahertz in frequency. According to the IEEE standard, the radar-frequency letters associated with this range are S-Ka, and this range equates to between 300mm and 7.5mm in length.

Spencer experimented by pointing microwaves at various food items surrounded by a metal box. He observed that the waves actually cooked the items and these observations led to his construction of the first microwave oven. While this design was submitted on October 8, 1945 the United States Patent Office granted the patent for Spencer’s invention on January 24, 1950 as US2495429, a Method of Treating Foodstuffs.

One of the most impressive examples he includes in his application is the following:

“With the system described, I have found that an egg may be rendered hardboiled with the expenditure of 2 kw.-sec. [kilowatt second] This compares with an expenditure of 36 kw.-sec. to conventionally cook the same. I have also found that with my system a potato requires the expenditure of about 240 kw.-sec., which compares with 72,000 kw.-sec. necessary to bake the same in an electric oven.”

Raytheon microwave oven on the NS Savannah, Baltimore, Maryland, USA. Photo by Acroterion, 19 May 2012 under Creative Commons Attribution-Share Alike 2.0 Unported license.

His invention also included a “conveyor system” that brought the food to the area exposed to the microwaves, which was a good idea back when a microwave oven was nearly 6 feet tall and weighed around 750 pounds! These monoliths were marketed mainly to restaurants beginning in 1947 and it wasn’t until decades later when the counter-top models began being commercially available that microwaves began to be found in the home.

Resources on Percy Spencer and the microwave:


  1. W Cuirle
    January 26, 2021 at 8:38 am

    The unit kw-sec is pronounced “kilowatt second,” not “kilowatt per second.” A kw-sec is a unit of energy equal to 1000 joules or about 1 BTU.

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