The Magnetron is the base of the microwave oven. The magnetron vacuum tube was invented in England during World War II, and was the central part of a highly secret military technology: radar. Eventually the benefits of this technology in the civilian sector were recognized and the secret military technology ended up in appliance stores everywhere.
A magnetron has an extremely hot central filament surrounded by a ring of resonant electromagnetic cavities. These cavities act like electromagnetic "tuning forks". Electromagnetic waves oscillate back and forth inside each cavity at the cavity's resonant frequency. The charge moves at a frequency determined by the shape and size of the cavity and these are carefully controlled so that the cavity's natural resonance frequency is 2.45 gigahertz. As the filament heats up electrons start breaking of its surface. A high voltage attracts them toward the walls of the cavities. The resonant cavities have small amounts of electric charge oscillating back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities' positively charged walls than to their negatively charged walls. However, a magnetic field present in the magnetron deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge the newly arrived electrons add to it. As electrons flow to the resonant cavities, the charge starts to grow in the resonant cavities and these cavities accumulate huge amounts of energy. A small wire loop and a microwave antenna then draw some of this energy. This antenna radiates the energy from the cavities into a metal channel that leads away from the magnetron and into the cooking chamber. This energy from the resonant cavities is delivered to the food in the oven and results in the heating of the food.
How a Microwave Oven Cooks
It's like waving a magic wand and your food is ready. The only difference is that your wand is shaped like a touch pad and beeps when you touch it. The magic words appear of the LCD display and, presto, the food is done. How does a microwave oven cook food?
Let us first look at a conventional oven. A conventional oven cooks food on the principles of conduction. First the outer layers of the food are heated and then this heat has to migrate (by conduction) from the outside of the food towards the middle. Since the outer layers get more heat and they are also surrounded by heated air, the water in them evaporates faster thus forming the crust that oven cooked foods normally have. If the heat is turned up to high you might get a crust that's burnt and an inside that isn't even warm.
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