Re: Why is lightning in my pillowcase? And should I worry about sparks?

Hi Andrew!  Your pillowcase-lightning is very strange... but not
unexpected.  Cloth surfaces can easily become electrically charged if
rubbed against a different cloth surface.  This happens all the time, but
the effect becomes very strong when humidity is very low.

Here's one possible explanation:  the pillowcase and the pillow are two
different types of cloth, and they developed strong opposite electric
charges while in contact.  Then after you removed the pillowcase and shook
it, miniature lightning bolts leaped from your fingers and travelled along
the charged cloth, discharging it.   Also, chances are good that the
pillowcase was treated with "whiteners" which glow intensely blue-white
under ultraviolet light.  Electric sparks put out lots of UV light, so the
dimmest purple spark will become very bright if it takes a path along a
piece of white cloth.

If the above is correct, then the "pillowcase lightning" would become
weaker and weaker as you shook the cloth, and then finally it would stop. 
However, you could "recharge" by putting it back on the pillow, rubbing
them together awhile, then removing the pillowcase again.

Also, the phenomenon would only appear during a day when the humidity in
the room was quite low.  That's why you probably never noticed it before.

Such sparks are harmless to humans, but "static electricity" can easily
destroy electronics.  If I were you I wouldn't put a cellphone or PDA
against the highly charged pillowcase.

Next time you see the flashes, try this experiment: turn on a radio, set it
to the AM band, then tune it between stations.   When you make your
pillowcase flash, the radio might pick up the crackling sound of the
electromagnetic pulse from your miniature lightning.