http://www.youtube.com/watch?v=E185G_JBd7U
At 37:00 he charges a conductor with a dielectric inside, removes the conductor plates and discharges them, and in the end puts them back into position.
Then something "remarkable" happens - when he connects the conductor plates with each-other, a spark is produced. Why did this happen?
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Is this because the dielectric is still polarized when he puts back the plates, and thus the plates themselves become charged (by induction) in turn? If not, what is the answer to this riddle?