How not to make a long arc. I put a piece of masking tape from the toroid to ground and sprinkled with iron filings. When I hit the power, the initial curtain of sparks was pretty cool, but the tape quickly burned in two and one half fell against the coil. I think the end of the burnt half hit the concrete floor and resulted in the bright stripe leading from the ground to the coil. It was fun to try, but keep a fire extinguisher handy!