Artificial Ball lightning

A few years ago, a friend of mine and I decided to try and create artificial ball lightning in a microwave. All you need is a grape, a microwave and a knife.

WARNING: Attempting this could impair your health and that of your microwave! Approach with EXTREME CAUTION!
  1. Cut the grape in half so that there is only small piece of skin attaching the two halves together and they look like a pair of bongo drums.
  2. Take the tray out of your microwave.
  3. Put the grape in your microwave with the wet faces up.
  4. Start the microwave and have your finger on the stop button.

You should see a bright ball and maybe some licks of flame appearing within a second or two. I remember scortching the top of the microwave the last time I tried this so be sure to have a finger on the stop button.
I’m not totally sure the physics behind the whole thing. The explanation my friend had was that the microwave sets up a standing wave inside the microwave which has around the right wavelength to create a big potential difference between the two grape surfaces. Eventually, as some of the water in the grapes boils, steam is ejected. In the presence of such a large potential difference, this ionises and forms a plasma for a split second. I’m still not totally convinced that this is actually what’s going on, but I have no better explanation.

It turns out that it works even better when you have a flame in your microwave. The gas in the flame ionizes very easily and it’s even possible to contain the plasma in a vessel of some sort for some time. Here’s a video of someone trying it with a match:

Here’s what I think happens:

The microwave keeps giving electrons in the hot gas enough energy to escape their nuclei. When these electrons return, they give off a load of energy in the form of electromagnetic radiation, some of which is the light you see. Presumably different burning materials give different parts of the characteristic spectrum (and so different colours…)

Le Fox, I would guess that if your chimney is struck by lightning when you have a fire in the fireplace, much the same things happens…

I could well be wrong…

See here for a good explanation of a similar grape experiment (with diagrams!).

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