Once a symbol of futuristic convenience, the microwave oven has faded into the background of modern existence, its familiar beep and gentle hum echoing from dorm rooms to office break rooms and family kitchens alike.

The now-ubiquitous appliance originated in the laboratories of wartime America and owes its existence to a serendipitous snack and a radar engineer named Percy Spencer.

Before discovering microwave cooking, Spencer was already a key figure in one of World War II’s most critical technological efforts. A self-taught engineer with no formal education beyond grammar school, Spencer rose to prominence at the American defense and aerospace company Raytheon. His early work focused on a vital wartime challenge: streamlining the production of magnetrons, the vacuum tubes that powered Allied radar systems.

Radar technology gave the Allied forces a decisive edge, but in the early 1940s, magnetrons were notoriously difficult to manufacture. “Spencer was curious about creating a simpler and more efficient way to create magnetrons so they could be mass-produced,” says Rini Paiva, executive vice president of selection and recognition for the National Inventors Hall of Fame. "At the time, only about 20 a day could be manufactured.”

Thanks to Spencer’s work, production soared to 2,600 magnetrons daily, according to Paiva, a breakthrough that helped advance radar capabilities during the war. Spencer’s deep familiarity with radar technology and habit of noticing what others overlooked, led to his most unexpected contribution to history.

In 1945, Spencer noticed that a candy bar in his pocket had mysteriously melted while testing an active magnetron. Where others might have dismissed the incident as a trivial quirk of the equipment, Spencer saw possibility. His curiosity led him to experiment further, placing popcorn kernels near the magnetron. When they began to pop, his suspicion was confirmed: microwaves could penetrate food and heat it quickly. A follow-up test with an egg resulted in a burst shell and a clear conclusion: Spencer had found a new way to cook.

“Others may have made similar observations before but never capitalized on them while apparently Percy did,” says Joseph A. Schwarcz, director of McGill University’s Office for Science and Society. “As Louis Pasteur said, ‘chance favors the prepared mind.’ I guess Percy's mind was prepared.”

Recognizing the significance of what Spencer uncovered, Raytheon quickly filed a patent for a "microwave cooking process" later that year. But turning a wartime breakthrough into a practical appliance was no small task.

In 1947, Raytheon introduced the first commercial microwave oven, appropriately named the Radarange. Standing nearly six feet tall and weighing 750 pounds, the early Radarange required water cooling and cost around $5,000 ($73,397 in today’s dollars). Despite its size and cost, the Radarange proved useful in environments where food needed to be cooked quickly, such as military installations and hospital kitchens. But it took decades of engineering improvements before the microwave found its way into the average American home.

“One major technical hurdle was to develop practical designs for radiation seals around the oven door,” says John F. Gerling, former president of the International Microwave Power Institute. “Early designs were costly and only marginally effective. Nowadays, these seals are designed to be not only low-cost and highly effective (i.e., very low emissions) but robust enough for the rigors of everyday use (and abuse) by consumers.”

The Tappan Model RL-1, developed in collaboration with Raytheon and introduced in 1955, was the first microwave intended for home use, but its $1,295 price tag and large size made it impractical for most kitchens. The real breakthrough occurred during the 1960s and 1970s, driven largely by advancements in semiconductor technology and magnetron miniaturization.

“Although overall manufacturing costs were much lower, the magnetron was still the single most costly part in their [microwave ovens] construction,” says Gerling. “By the early 1970s, the Japanese had figured out how to mass-produce magnetrons at a much lower cost.”

The turning point came in 1967, when Amana (a division of Raytheon) introduced the Radarange RR-4. Priced at $495, it was the first compact countertop model, easier to install and affordable for middle-class families. Despite this breakthrough, it took another decade or two for microwave ovens to become common in American homes. Improvements in manufacturing and safety standards, along with the rise of dual-income households and the demand for time-saving technologies, helped drive widespread adoption in the years that followed.

By 1975, microwave ovens were outselling traditional gas ranges, and by the mid-1980s, they were present in over half of all U.S. households. This era cemented the microwave's status as a symbol of modernity and convenience. Today, over 90 percent of U.S. households own a microwave oven.

Spencer’s personal story is as remarkable as the invention itself. Orphaned at 18 months and raised by an aunt and uncle in rural Maine, Spencer left school in the fifth grade to help support his family. He taught himself electrical engineering by reading books while working at a paper mill, and later joined the Navy, where he learned about wireless communications.

Spencer died in 1970, just as the microwave oven was poised to become a staple in American homes. He held over 100 patents in his lifetime, but, other than the $2 bonus Raytheon typically awarded employees for their patented inventions, he never profited from his most successful “ah-ha” moment.