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Neon Blubs Show Microwave Distribution in Oven
Videos for Engineers

Here is a really cool demonstration showing the distribution of microwaves inside a kitchen microwave oven. Experimenter Zeke Kossover drilled an array of holes in an acrylic panel, into which he inserted neon bulbs. Per Zeke, "Microwaves are invisible, so you can't see them inside microwave oven, but their presence can be detected with neon lamps. The changing electromagnetic field from the microwaves will make charged particles move, and so the electrons in the metal legs will move creating current. This current makes the lamps glow." You can see how the field changes as the panel rotates, and also how the presence of a substance that absorbs the energy affects the pattern.

RF Cafe - Neon Bulbs (AC & DC), image from WikipediaA typical NE-2 type neon bulb, likely used in the experiment, turns on at about 65 VAC or 90 VDC. Once the bulb starts conducting, the resistance goes down and the voltage drop across the bulb lowers by about 10 volts. The nearly constant voltage made neon bulbs useful as voltage regulators before semiconductor devices like Zener diodes became available. Since the trigger voltage is what determines when the neon begins to conduct, that explains why 65 VAC, with a peak of 65 Vrms x √2 = 92 Vpk, turns it on while 90 volts of direct current must be applied. All that need to be done to turn the bulbs on is to immerse them an electric field with an orientation sufficient to induce the trigger voltage.

Posted  11/23/2010