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From: Food Quality & Safety magazine, October/November 2013

Percy Spencer’s Microwave Revolutionizes Fast Cooking

by Lori Valigra

Innovators In food quality & Safety

Like many food safety innovations, the microwave had its roots in a war-time effort, but in this case, it was radar devices to detect Nazi planes at night. British scientists had developed radar, which worked successfully to thwart raids, but they couldn’t perfect a critical part known as a magnetron, which converts electrons into microwaves. They sought help from Raytheon Co., a U.S. military contractor, and more specifically, Percy LeBaron Spencer (1894-1970), a self-taught engineer who, with only a grade school education and creative thinking, promptly came up with a simpler manufacturing technique that also improved radar’s performance.

Spencer first became interested in electricity at an early age. Born in the remote town of Howland, Maine, he worked in mills from the age of 12 and became intrigued by a local paper mill that was going to start using electricity, about which little was known in his hometown. He joined the U.S. Navy at age 18 and taught himself to become an expert in radio technology by reading textbooks during night watch duty. By 1939, he was one of the world’s experts in radar tube design and was employed by Raytheon.

But Spencer’s idea to apply microwave technology to cooking was more of a fortuitous coincidence. It occurred in the lull of business after World War II ended, when product development was shifting from military to civilian applications. While standing near a magnetron in a Raytheon laboratory in 1946, he noticed a tingling sensation. He also discovered that the candy bar in his pocket had melted. Others had noticed the same effects earlier, but Spencer’s curiosity drove him to experiment, so he brought raw popcorn near the magnetron–it started popping. He built a simple metal box with a magnetron in it, which he tested to cook an egg and then used to reheat his lunches. Though rudimentary, it was the first microwave oven.

Spencer built the first true microwave oven by attaching a high-density electromagnetic field generator to an enclosed metal box. The magnetron device used for radar caused the stream of electrons interacting with a magnet to resonate in a high-powered vacuum tube, resulting in microwave radiation. In the case of the microwave oven, the magnetron emitted microwaves into a box, which blocked their escape and allowed for controlled experimentation. Spencer tested it further by placing various types of food in the box, observing the effects on it by the microwaves and monitoring temperatures.

Spenser wrote a report on his findings and Raytheon patented a high-frequency dielectric heating apparatus in 1946, after which it began selling microwave ovens for industrial use. The early models, available in 1954, were a far cry from today’s compact and tastefully designed kitchen models: Each weighed in at more than 750 pounds, stood 5 feet 6 inches tall and cost upwards of $3,000. Initially the monstrous cooking machines were used in restaurants, railroad cars, and ocean liners, which needed to cook large quantities of food quickly.

But chefs soon noticed the oven’s shortcomings, including turning some vegetables limp and colorless and not browning meats. After being refined for a couple decades, the microwave entered the homes of the average consumer, where it has become ubiquitous in the kitchen. The idea of rapid and safe cooking caught on and Raytheon’s subsidiary, Amana, started selling the consumer Radarange countertop model in 1967 for about $495.

Microwaves have three characteristics that allow them to cook food: They are absorbed by foods, they are reflected by metal, and they pass through glass, paper, plastic, and similar materials. Microwaves reflected within the metal interior of the oven are absorbed by food, causing water molecules within the food to vibrate and produce heat, which in turn cooks the food. However, the microwave energy does not contaminate or make the food radioactive because the microwave energy is changed to heat as the food absorbs it. Contrary to popular thinking, microwave ovens don’t cook food from the inside out. Instead, the outer layers of thick food are cooked mostly by the microwaves, but the inside is cooked mainly by the conduction of heat from the hot outer layers.

The microwave oven has revolutionized consumer dining culture. In fact, more than five million microwave ovens are sold yearly in the U.S., according to market estimates, outselling range ovens since 1975. But Spencer’s reward for his invention was small. He received a one-time $2 “gratuity” from Raytheon—at the time, the company paid all of its inventors on payroll for company patents.


Valigra is a writer based in Harrison, Maine. Reach her at lvaligra@gmail.com.

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