Most commonly used in microwave ovens, a magnetron coverts electrical and magnetic currents into a very powerful form of heat. The main component of a magnetron tube is a filament that radiates heat after receiving an electrical charge, producing electrons as the heat increases. As the electrons move away from the filament in an attempt to latch onto positive electrodes, they are influenced by a negative magnetic field.
The negative field repels the electrons, confining them to a spot where they begin spinning in circular motion, generating even more heat, eventually producing energy powerful and hot enough to cook food.
Early in the 20th century, General Electric employee Albert Hull invented the magnetron. He was unable to find any uses for his creation and the device sat idle for about a decade.
In the pre-World War II era, engineers Harry Boot and John Randall pursued their own exploration of the magnetron, making one crucial change. Rather than encasing it in glass, they used copper, allowing an output of roughly 400 watts in an area less than a half-foot. During World War II, the tubes were used to increase the capability of sonar equipment, helping submarine crews spot hostile vessels more quickly.
After the war ended, it was not long before the magnetron was on its way to the kitchen. Percy Spenser of the Raytheon Corp. began experimenting with magnetron tubes in his laboratory. On one occasion, he noticed a candy bar in his pocket had dissolved into a gooey mess while he was tinkering. Now curious, he tossed some popcorn by the magnetron to see what would happen. The kernels popped. In a matter of months, he built the first microwave oven, which weighed more than a quarter ton, was over five feet tall, and cost about $5,000.
Besides ovens and radar, magnetrons, which range in output power from 600 watts to one million watts, are used in plasma screens and various scientific apparatus in which power must be produced in a microwave frequency.