A chocolate bar melted in Percy Spencer’s pocket. That is, more or less, the entire origin story of the microwave oven.
Spencer was a self-taught engineer at Raytheon, working in 1945 on active radar for the US military. He was standing next to a running magnetron, the vacuum tube that generates the microwave radiation radar uses, when he reached into his pocket and found that a chocolate bar had melted through its wrapper. He had noticed this kind of warming effect near magnetrons before. He had not investigated it. This time, he did.
Spencer immediately went to work on controlled experiments. He tried popcorn kernels near the magnetron. They popped. He tried an egg, sealed inside its shell, which built up steam pressure and exploded in the face of a colleague who leaned in to observe. Within months, Spencer had filed a patent and Raytheon had begun development of a commercial product. The Radarange, introduced in 1947, was the first commercially available microwave oven. It was 1.8 meters tall, weighed about 340 kilograms, required a water cooling line, and cost roughly $5,000 in 1947 dollars. It was marketed primarily to restaurants and commercial kitchens.
Consumer-scale microwave ovens did not reach American homes at scale until the 1970s, when prices dropped below $500 and reliability improved. By 1986, roughly 25 percent of US households owned one. By 1997, that figure was over 90 percent. The entire transition from a chocolate bar melting in a hallway to a kitchen appliance in nearly every American home took about 50 years, which is fast by the standards of transformative technology.
