In What’s up with “broadcast power”?, we learn that “Tesla coils, in my experience, can illuminate fluorescent bulbs, but usually at a distance of only a few feet.”
I’m not contesting this, but it reminds me of an old science demo I saw about twenty years ago (so I may be misremembering). It was part of some show at the Baltimore Science Museum, so I’m sure it wasn’t intended as a magic trick.
The demonstrators had set up a big Tesla coil on the stage and were demonstrating broadcast power, particularly the fluorescent tube lighting up while unconnected to anything. Okay, fine. The weird thing was that the guy could grab the tube tightly at one end, slide his hand halfway along the tube, and turn off that half as his hand passed. As I remember it was the damnedest thing: a fluorescent tube, half on and half off.
So how does this work? I’m pretty sure he could take his hand away from the tube and leave it in that state, but I might be wrong. Is there some detail about what he did that I’m forgetting?
You can replicate this with any high-voltage power supply. Not the broadcast power part, but the operative principle here is the same. Touch one end of the fluorescent tube to the HV output of the supply, and grab the galls tube in the middle. Only the half between the end touching the supply and your hand will glow. This is because, counterintuitively enough, the electric field making the tube glow is basically grounded out through the glass and out through your body (this is somewhat oversimplified, but you get the idea, I trust). With the Tesla coil, it’s the same thing; the tube is immersed in the electric field produced by the coil. There exists a large voltage gradient radiating outward from the electrode on the top of the coil, which puts several hundred volts across the length of the tube. Putting your hand on the glass tube grounds out the field in a similar manner as with the HV supply. Those toy “lightning globes” work by a similar principle. Even glass conducts some electricity.
You don’t get electrocuted by this field, even though a few hundred volts exist across your body because it’s very high impedance so that very little current actually flows through you.