Microwave

or: wartime radar for lukewarm coffee

During World War II, engineers built a copper tube called a cavity magnetron to power radar that could spot enemy bombers¹. Then an engineer standing next to one noticed the candy bar in his pocket had melted, so naturally they built an oven out of it: the first one was nearly six feet tall, weighed more than 750 pounds, and cost $5,000². Now a smaller one sits in nearly every kitchen in America, flipping an electric field 2.45 billion times a second so the water in your food keeps twisting back and forth until it cooks itself³.

And we use it to reheat the same cup of coffee three times.

How it works, badly

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In the war, we built a tube that shoots microwaves for radar.
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Then a candy bar melted in an engineer’s pocket.
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The field flips 2.45 billion times a second, and water wiggles along.
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Then we reheat the same coffee three times.

Wait, is that real?

Yes, the kitchen microwave really is a radar part.

The radar tube↩

The cavity magnetron came out of radar research during World War II. It turns electricity into powerful microwaves, and it let British radar systems spot approaching German bombers. A smaller magnetron is still the heart of a kitchen microwave oven.

The candy bar↩

Percy Spencer, an engineer at Raytheon, noticed that a candy bar in his pocket had melted while he stood near an active radar set. Raytheon filed a patent for cooking with microwaves on October 8, 1945, and sold the first microwave oven, the Radarange, in 1946. It was nearly six feet tall, weighed more than 750 pounds, and cost $5,000, more than $50,000 in today’s money. A countertop model for homes went on sale in the 1950s for $495.

How it heats↩

A kitchen microwave runs at about 2.45 gigahertz, so its electric field flips direction about 2.45 billion times a second. Water molecules have a slightly positive end and a slightly negative end, so they keep turning to follow the field and bump into their neighbors, and that jostling is heat. According to the 2020 Residential Energy Consumption Survey, almost all U.S. households have at least one microwave.

The fine print↩

The microwaves are not tuned to a special frequency of water: 2.45 gigahertz is a band regulators set aside for this kind of use, water’s own resonances sit far higher, and a real oven’s output wanders between about 2.40 and 2.50 gigahertz. Fats, sugars and other molecules heat up too, not just water. Spencer’s candy bar story comes down to us from Raytheon and Spencer himself, and the details vary in the telling. Nobody has counted how many times the average coffee gets reheated; three is a guess.

Sources: APS News: first patent for the microwave oven; West Texas A&M: are microwave ovens tuned to water?; EIA: Residential Energy Consumption Survey 2020

Last fact-checked October 28, 2026.