Original poster: "resonance" <resonance@xxxxxxxxxxxx> Neal:It's because the charge is not residing in the metal wire which is connected to ground --- the charge is residing in the dielectric (wire coating) between the grounded wires.
A coil does have a DC component which can momemtarily charge the dielectric, ie, plastic or enamel wire covering. The def of a cap is two plates separated by a dielectric.
Recall the old physics trick of the removable dielectric Leyden jar? A Leyden jar type cap is charged, and, then, while charged, with a plastic stick, the center conductor is carefully removed and the outer conductor is removed. They are shorted together, and also both shorted to a ground.
The cap is then reassembled and the cap is powerfully discharged!! It proves the dielectric stores the charge, not the plates. In a Tesla coil, under certain conditions, the plastic PVC coating on the wire, or enamel coating, or protective coating over the windings, will hold a DC charge that discharges to you when you touch the sec coil. Good idea to at least slowly run the back of your hand or a ground along the sec winding to discharge any of this "static electricity" that remains.
Many coils usually don't have this problem some but coils with a heavy coating or PVC insulated wire coils can exhibit this phenomonea under certain conditions. Caution is in order until you discover if your particular coil design exhibits this interesting process.
The atomic electron orbitals are stretched when the cap is charged, and as they return to their normal obits, they give up some electric charge to the plates again.
I've noted it also seems to be more prominent in coils that are suddenly shut off at high power as opposed to operation in which the variac is slowly diminished. Perhaps a gradual shutdown of your particular coil will prevent this event from occuring. You could do some experiments with a grounded wire in a very dark room and you may see some "St. Elmo's fire" as you drag the wire down the side of the secondary coil.
Dr. Resonance
I've gotten bit by a couple secondaries in the past, which I thought was rather odd since they were still connected to ground at the time. I'm thinking that perhaps it's an electrostatic charge built up on the enameled outside of the secondary that carries the "bite". (?) Now, before I decide to touch a secondary that's just been running, I take a grounded wire and go over the outside of it a bit. (I bend the end over so I'm not scraping the finish off) Seems to have done the trick for me. Neal.