What does it take to notice that your chocolate bar has melted?

He fetched popcorn. Spencer put kernels near the magnetron and they popped, which confirmed that microwaves could penetrate food and heat it quickly. A follow-up test with an egg ended in a burst shell and a clear conclusion. Raytheon filed a patent for a microwave cooking process later that year. Others may have noticed the same thing before and never capitalized on it.

Spencer was orphaned at eighteen months, raised by an aunt and uncle in rural Maine, and left school in the fifth grade to help support his family. He taught himself electrical engineering from books while working at a paper mill, later joined the Navy and learned wireless communications, and rose to prominence at Raytheon.

His wartime work was unglamorous and decisive. Magnetrons, the vacuum tubes powering Allied radar, were notoriously hard to manufacture, and only about twenty a day could be made. Spencer went after a simpler and more efficient way to produce them, and output rose to two thousand six hundred a day.

In nineteen forty-five, while testing an active magnetron, he noticed a candy bar in his pocket had melted. Where others might have dismissed it as a quirk of the equipment, he put popcorn kernels near the magnetron and watched them pop, then tried an egg, which burst. Raytheon filed a patent for a microwave cooking process later that year.

Getting from there to a kitchen took decades. The nineteen forty-seven Radarange was nearly six feet tall and cost about five thousand dollars; the Tappan Model RL-1 of nineteen fifty-five, the first intended for homes, still cost one thousand two hundred and ninety-five. Radiation seals around the oven door were an early obstacle, costly and only marginally effective at first. Progress otherwise came from semiconductor advances, magnetron miniaturization and, by the early nineteen seventies, Japanese mass production of cheap magnetrons. By nineteen seventy-five microwaves were outselling gas ranges.