While visiting my local science museum recently I was asked by a
young person how the plasma ball worked. It seemed to be able to
sense the presence of his hand or finger and direct a glowing
streamer to any point on the sphere that he touched, and it could
also clearly anticipate where he was going to touch it. I replied
that, of course, he was being struck by a low current sort of
lightning coming from the ball. The glass envelope limits the current
density to a small but not quite imperceptible value over the area of
a fingertip. If one collects current over a larger area of the ball
it can be mildly painful, frying flesh with a tiny, intense spark. I
demonstrated this by placing my metal-rimmed glasses, temples open,
on top of the ball and drawing an arc to my knuckle. This mini
lightning bolt was accompanied by the distinct odor of burning flesh,
a small price to pay for a teachable moment.