I built this little guy today.
It works pretty good considering it was built in less than 6 hrs, and being poorly tuned. I tossed it together with junk I had around. Nothing was calculated or measured. Arcs are about 5-6" so far
The secondary diameter is smaller than a quarter. I just wrapped ~33awg magnet wire around a piece of PVC pipe until I felt satisfied with its height. The solid copper primary sits upon some ceramic insulators salvaged from an old runway strobe. The box is some 3M marketing shwag. The transformer is a 10kv that I found in a box of junk from an auction long ago. Not sure on the current, probably about 10-20ma. The tank cap is a Sprague 4nf doorknob. An acrylic box from Ikea, with 2 carriage bolts makes up the sparkgap. I'm running it though a variac, and found the cap begins to fail around 80v input, so it needs a better cap. Topload for now is a tennis ball wrapped in aluminum foil. Not much else to it.