The dull halogen light. The spinning glass plate. The humming that terminates in a "BEEP." Today the sights, sounds, and smells of the microwave oven are immediately familiar to most Americans. There's a microwave in 90 percent of American homes heating everything from popcorn topork rinds in a hurry.

The microwave is beloved for its speed and ease of use. But what you might not know about your indispensable kitchen appliance is that it was invented utterly by accident one fateful day 70 years ago, when a Raytheon engineer named Percy Spencer was testing a military-grademagnetron and suddenly realized his snack had melted.

"It was a gooey, sticky mess"

The Knack

Spencer was no timid lab rat. "Gramps was loud, wanted to make everything happen at all times"—so the inventor's grandson George "Rod" Spencer Jr. told Popular Mechanics. "There were no 'challenges,' simply everything was a goddamn problem that needed to be solved. Everyone trusted him to do just that."

A chef using a Raytheon Radarange III, an early commercial microwave oven, circa 1958. Getty Images

Growing up poor around the turn of the century in the wilderness ofHowland, Maine, Spencer had little formal schooling and, unlike the millions of modern Americans who now heat up their lunch in his invention, often had to hunt for his food. Modern conveniences like the automobile and electricity were unfamiliar to him at young age, but he got into engineering anyway, thanks in large part to a natural curiosity that drew Spencer to the mills that populated the region. At 12 he got a job atthe spool mill one town over. At 14 Spencer got hired toinstall electricity at the nearby paper mill. A few years later he was soinspired by the heroic actions of the Titanic's radio operators that he joined the Navy and learned the new technology. Spencer would later explain, "I just got hold of a lot of textbooks and taught myself while I was standing watch at night."

After World War I, Spencer landed a job at thenewly-established American Appliance Company, co-founded by engineer Vannevar Bush, who today is most known for organizing the Manhattan Project and predicting many of the innovations that led to the computer revolution and the internet. In 1925, the company changed its name to Raytheon Manufacturing Company. It's still around today making missiles, military training systems and electronic warfare products.

In the '20's, Spencer became one of Raytheon's most valued and well-known engineers. During World War II, while Raytheon was working on improving radar technology for Allied forces, Spencer was the company's go-to problem solver. For example, he helped to developproximity fuses, or detonators that allowed you to trigger artillery shells so they'd explode in mid-air prior to hitting their mark. In an email to Popular Mechanics, current Raytheon engineer and part-time company historian Chet Michalak says Spencer "had a knack for finding simple solutions to manufacturing problems."

"There were no 'challenges'—simply everything was a goddamn problem that needed to be solved"

Spencer earned several patents while working on more efficient and effective ways to mass-produce radar magnetrons. A radar magnetron is a sort of electric whistle that instead of creating vibrating sound creates vibrating electromagnetic waves. According to Michalak, at the time Spencer was trying to improve the power level of the magnetron tubes to be used in radar sets. On that fateful day in 1946, Spencer was testing one of his magnetrons when he stuck his hand in his pocket, preparing for the lunch break, when he made a shocking discovery: The peanut cluster bar had melted. Says Spencer, "It was a gooey, sticky mess."

The Snack

Getty Images

A story this good can't help but change as it's passed down over the years. Some tellings of the legend say it was a melted chocolate bar that led to Spencer's eureka. But if you ask Rod Spencer today, he'll tell you that's dead wrong.

"He loved nature (due to his childhood in Maine)... especially his little friends the squirrels and the chipmunks," the younger Spencer says of his grandfather, "so he would always carry a peanut cluster bar in his pocket to break up and feed them during lunch." This is an important distinction, and not just for the sake of accurate storytelling. Chocolate melts at a much lower temperature (about 80 degrees Fahrenheit) which means melting a peanut cluster bar with microwaves was much more remarkable.

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Understandably curious just what the heck had happened, Spencer ran another test with the magnetron. This time he put an egg underneath the tube. Moments later, it exploded, covering his face in egg. "I always thought that this was the origin of the expression 'egg in your face'," Rod Spencer laughs. The following day, Percy Spencer brought in corn kernels, popped them with his new invention, and shared some popcorn with the entire office. The microwave oven was born.

At this point you might be wondering: How did Spencer know cooking with microwaves was safe? According to his grandson, he didn't. Today, we know that the low doses of electromagnetic radiation emitted by microwavesare generally considered safe (though, theFDA admits that no studies have been done to assess the impact of low levels of microwaves on humans over time, and there are those who still firmly believe microwaves are killing us). But back in the 1940s, this information was not available. "He didn't care about that," Rod Spencer ays. "This was when people would wear nuclear stuff around their neck to get rid of cancer."

Wikimedia Commons / Daniel Christensen

In 1947, just a year after Spencer's snack food serendipity, the first commercial microwave oven hit the market. Calledthe "Radarange," it weighednearly 750 pounds and cost more than $2,000. Needless to say, it wasn't a big seller. The first domestic microwave was introduced in 1955, but it too failed to launch because it was expensive and because microwave technology was still an unknown. It wasn't until 1967, two decades after its invention, that the microwave oven finally caught on in American homes in the form of Amana's compact "Radarange." By 1975,a million microwaves were sold every year.

Today, Rod Spencer is a project manager and engineer himself. He's writinga book about his grandfather. "I love telling these stories. I grew up with so many of them, my head is full. Some of the stuff he did - he was crazy, he was smart and everyone loved him." And thankfully, he liked feeding the squirrels.

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