My son and I built a Wimshurst machine (our second try) over the last couple of weekends. It works -- not terribly well, as it only makes 2cm sparks with 20 cm disks (something like 60 kVolt when it should be capable of 5 cm or over 100 kVolt), but we were happy to get it going at all since our first machine failed completely.
With a source of high voltage to play with, we've done a few of the simpler electrostatic experiments. Paper strips that stand up when charged, electroscopes, etc. One of the things we did was to hold a fluorescent light bulb close to one of the discharge terminals on the machine. Since a fluorescent bulb will light up if you rub it on wool to generate static electricity, I figured it would light up while the disks were turning.
Well, sort of. It flashes when a spark jumps the spark gap on the machine -- that is not a spark from the machine to the bulb, but just a normal spark between the terminals on the machine itself. The bulb will flash when held anywhere close to the machine -- say, within 1 meter. It doesn't light up continuously as I expected it to. If I open the spark gap far enough to prevent sparks all together, then the bulb doesn't light at all.
Why does the bulb flash when the machine sparks?
Why doesn't the bulb light up continuously when the machine is running?
Why does the bulb flash even though there's no contact with the discharge terminals?
Update: I've investigated the suggestions from WetSavannaAnimal aka Rod Vance as well as I could. I couldn't put the generator in a screen cage - it simply isn't practical to build a screen cage large enough and solid enough and still be able to crank the machine through it.
Since the heart of WetSavannaAnimal aka Rod Vance answer was that the flashing was caused by x-rays, I tried to prove that it really is from x-rays. X-rays should cause other fluorescent stuff to flash as well, or so I would think. I dug up some of the glow in the dark stars that used to be on the walls in the kids room. Those contain either zinc sulfide or strontium aluminate (both types can glow the same faded green, so I can't tell which I've got,) and should also glow in the presence of x-rays. No matter how close they were to the spark, they would not flash or even glow brighter.
I then got hold of another fluorescent tube. This is a straight tube about 15cm long and less than 1cm in diameter. I hung this tube on some strings above the work bench, and cranked the generator. This tube flashes as well, but with it held stationary, I could move the machine and the spark terminals and see what effect various orientations and distances had.
It comes down to the distance from the discharge terminal of the machine to the contact on the end of the tube. If I keep the tube the same distance away from the terminal but arrange it so that the contact on the tube is further away, then it doesn't flash as brightly. It doesn't matter whether the terminal (a solid metal ball) is between the spark and the tube. Only the distance to the contact matters.
It seems to be an electrical effect. It also happens if I charge up the machine and tough the discharge terminal with my finger - crank the machine until it is just short of making a spark, touch the terminal with a finger. A spark jumps from the terminal to my finger, and the fluorescent tube flashes.
I am guessing there is some kind of capacitive coupling between the generator and the tube. Any suggestions on how to verify that?