Microwave Oven Experiments

(source unknown)

Over the years I've indulged my fascination with microwave ovens by doing unwise experimentation.

Disclaimer: This information is presented for your information only. Anyone who tries to duplicate these demonstrations does so entirely at their own risk. There is a chance that you will damage your microwave oven. There is a chance that you will cause a fire.

They're Heeeeere!

Years ago I was living with roomates, and while working in the kitchen I noticed that the florescent light over the sink was about 8 inches long. A light went on in my brain ;) because I'd always wondered what would happen if a flourescent tube was placed in a microwave oven. In theory the RF energy should have enough voltage to ignite the mercury vapor into a plasma, and the lamp should light. But standard ovens put out at least 500 watts, so the tiny flourescent tube should light quite bright, to say the least. I'd never encountered a flourescent tube which was short enough to fit. So, I pulled out the tube, stuck it in the oven, said "THEY'RE HEEEEEERE!" , and punched the ON switch. Sure enough, the kitchen was lit up by a blue-white blaze of light coming from the front of the microwave oven. I only let it run for about 1 second, but this was enough to heat the flourescent tube so it was too hot to touch.

Snifter of Neon

While working on a microwave article for an encyclopedia, it crossed my mind that it might be possible to map the pattern of RF energy in the oven by filling it with low pressure gas. The gas would glow in proportion to the RF electric field in various parts of the oven's volume. This would be an involved bit of construction to pull off, so I did the next best thing. I grabbed a big bag of NE-2 neon pilot lights and stuck them into a wineglass, hoping that this small volume would show some patterns when the glass was rotated by the oven's turntable. I filled the glass with water, to give the oven something to heat so it wouldn't be damaged by the small load presented by the bulbs. I ran the oven, and the bulbs glowed REALLY BRIGHT. As the turntable turned, various bulbs extinguished and others lit up. However, I could see no coherent patterns. When I emptied the glass, I discovered that several of the bulbs were stuck together. The short metal leads of some bulbs had melted into the glass of adjacent ones. Also, several of the bulbs had small holes melted through their glass, and were full of water. Apparently the plasma temperature was so high that it heated the glass to melting. Or, possibly some corona discharges developed between the inside and outside of the bulbs and burned through the glass.

Foil-eating Plasma

I'd seen flames produced by microwave ovens before. In the strong RF field, even the tiniest flame will absorb a large percent of the many-hundred-watts oven output. Thousand watt candle? So, I decided to try initiating an electrical flame-discharge intentionally. I tore aluminum foil into 2" squares, crumpled it lightly so it didn't lay flat, then placed it on the oven turntable with the two pieces in light contact. Sure enough, when the oven was turned on there was a loud buzz and a bright light, and a flame erupted from the contact point between the two pieces of foil. When I looked in on them, I found that the brief flame had eaten a bite about the size of a dime out of both pieces.

Mapping the Energy Nodes

Microwave ovens cook unevenly because a pattern of standing waves forms inside the oven chamber, and the pattern creates an array of hotspots throughout the oven's volume. An operating frequency of around 2000 MHZ will produce a wavelength of around 10cm, and the hotspots should be at halfwave points, or every 5cm, but in a complex 3D pattern. I'd always wondered how this could be visualized. Perhaps fill the entire oven with raw eggwhites, then let the oven cook them into an interesting white, rubbery 3D sculpture? Or fill the oven with solid wax, and let the RF hotspots melt out a 3D structure of holes? Finally someone figured it out:

Alistair Steyn-Ross and Alister Riddell, STANDING WAVES IN A MICROWAVE OVEN, The Physics Teacher October 1990, Vol. 28 No. 7 pp474-476

Steyn-Ross and Riddell were stimulated to investigate the pattern of melted cheese on an oven-cooked pizza. They hit on the use of Cobalt Chloride soaked paper. When wet, CoCl solution is pink, but turns sky blue when dry. (It's sometimes sold as "weather indicator" paper) They discovered that this worked beautifully, and a large square of the paper would give varying patterns of pink and blue when supported at different heights on a tile of cork within the oven.

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