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The other day I experienced a discharge of static electricity. It was unusually strong, so strong that it almost hurt. I would like to hear your answers to how such a strong static charge could have built up. Was it just a coincidence, or was it something I did that enabeled an unusually strong electric charge to build up?

At the moment, I was sifting whole bran rye flour using a wooden framed sifter with a metal net. The flour fell on to an enameled metal baking plate on a metal kitchen counter top. As I touched the flour on the baking plate, there was quite a powerful discharge. As I sifted I noticed that the flour stuck to the metal net of the sifter, so it seems to have been a static charge involved there as well.

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Yes, you accidentally built a high voltage generator.

When you were sifting bran, you were creating a small amount of electric charge on the bran. As it fell to the enameled surface, it took that charge with it - leaving the mesh, frame and you with the opposite charge. The more you did this, the greater the charge buildup. Because of the enamel, the charge on the surface was not leaking away.

And then you touched...

I am going to guess that is was cold outside, meaning that the relative humidity of the air was low and the conductivity (which is mediated by humidity) was also low.

Dielectric particles that rub against a surface (probably the wood of the frame) often pick up charge. I used to work with a sand blaster in which sand was carried down a rubber hose - we had to put copper mesh around the tube to discharge the static electricity that built up. There was a constant noise of electric discharge through the rubber... Which didn't last long before needing replacement!

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