The candy bar that melted by a radar tube
A magnetron is a tube that produces powerful centimeter-wavelength radio waves. It was perfected in Britain in 1940 so radar could fit inside an aircraft, and thousands of these tubes were then made. Many people knew it got warm near switched-on equipment. Raytheon engineer Percy Spencer was the one who didn't walk past: according to the company's own account, he stopped by an operating magnetron and found a melted candy bar in his pocket.
He then put corn kernels beside it and got popcorn. Then an egg - reportedly, it burst. A patent application was filed in fall 1945, and in 1947 the first Radarange oven appeared: about 340 kilograms, with water cooling. It wasn't installed in kitchens, but in restaurants, on ships, and on trains.
- Radar
- Magnetron
- Candy bar
- Oven
Home models had been attempted earlier, but the oven became an ordinary kitchen appliance after 1967, when the tube and cooling system were shrunk to countertop size. More than 20 years passed between the melted candy bar and the kitchen - not because people didn't understand the idea, but because it had to be made smaller.
Why food heats up while the plate stays cool
A water molecule is a dipole: it has a positive and a negative end. The field in the chamber oscillates 2.45 billion times per second, the molecules turn with it and bump into their neighbors. That bumping is the heating. Glass, porcelain, and most plastics can't turn this way, so they warm only from the food itself.
The wave enters food only shallowly, by a couple of centimeters. Beyond that, heat travels by ordinary conduction - from the outside in, not the other way around. That's why a large piece can have a cold center, and why there's a rotating tray: without it, there are spots in the chamber the wave barely reaches.
The 2.45-gigahertz frequency was chosen because it is water's resonance: water heats best at that frequency.
It's more the opposite. Liquid water's absorption peak is noticeably higher in frequency, and there the wave would be absorbed in the first millimeters of the food, heating only the crust. 2.45 gigahertz is a frequency permitted for household appliances, away from the peak, where the wave can reach into the food.
The magnetron in a modern microwave is a direct relative of the radar tube. It has become smaller and cheaper, but the principle hasn't changed: the same cathode, the same resonators, the same centimeter wave. A military device simply moved into the kitchen.
And stopped standing out. In the 2020 US housing survey, a microwave was found in nearly every home - it is more common than a dishwasher.
US homes with a microwave, 2020
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Sources
- Percy Spencer's patent, "Method of Treating Foodstuffs" (filed 1945, granted 1950)
- Raytheon's history of Radarange ovens
- reviews of dielectric heating at 2.45 GHz
- 2020 US Residential Energy Consumption Survey (RECS)