MICROWAVE MAGIC by Pam North Most households have them, and you are likely to use one at least once a week. Although common today, they once were rare. Only in 10% of homes in 1977, 85% of households had one by 1995. The microwave oven - chances are your home is more likely to have this appliance than a dishwasher. In 1940, British scientists invented the magnetron, a tube that produces microwaves, the tube probably would have remained a strictly military item had it not been for the experience in 1946 of Dr. Percy Spencer, a self-taught engineer at Raytheon Corporation, a United States defense contractor. While testing the new vacuum tube, Spencer happened to step near it with a chocolate bar in his pocket, and he noticed a few minutes later that the candy had melted. Curious that the chocolate was hot but he wasn't, Spencer decided to test the magnetron with a bag of popcorn kernels, which sputtered and then popped a few seconds later. He experimented with an egg, which first quaked and trembled, then exploded. Intrigued, he devised a metal box with an opening for the microwave power, and placed food inside; the temperature of the food rose with the microwave energy. Spencer revealed his discovery to Raytheon, with the suggestion that his employer manufacture magnetron-powered ovens for sale to the public. Raytheon, having the capacity to produce magnetron tubes at a weekly rate of 10,000, was interested; the end of World War II had cut back military purchases to almost nothing, so Spencer's proposal appeared to be an excellent idea to recover lost sales. Raytheon developed and refined Spencer's concept, and by late 1946, Raytheon filed a patent on the first "high-frequency dielectric heating apparatus." Raytheon's first product to hit the market was nearly six feet tall, weighed more than 750 pounds, and cost $5,000, which limited its customer base mainly to commercial use by high-end restaurants, ocean liners and railroads. Raytheon then offered another model in 1947, called the Radarange (the winning entry in a naming contest among employees); it was housed in a refrigerator-sized cabinet, and cost between $2,000 and $3,000 (that was the price tag on some small homes at that time). Tappan managed to maufacture a smaller version between 1952 and 1955, but the price, $1,295, was still prohibitive. It wasn't until 1964 that a miniaturized magnetron, perfected by the Japanese, made it possible for Raytheon, in 1965, to produce a microwave oven (100 volts) that could fit on a countertop and sell for $500, still somewhat expensive for the average family. Demand for the ovens grew, and by 1975 sales of microwaves exceeded sales of gas ranges; by 1976 microwave ovens had reached about 52 million (nearly 60%) of American homes. Technical improvements in the 1980s continued to bring improved quality and lower prices to the ovens, and they became even more practical and affordable. The food industry, recognizing the advantages the oven offered, began to shift its focus toward making food products microwaveable, and the microwave oven quickly became America's favorite appliance. Next week: How a microwave oven works.