I got a 2 X 2 sheet of 1/4 inch thick low density polyethylene in the mail today from Enco. It's a little thick I guess because with my Leyden jars the sparks are long but have less pop or zest. So I tried charging two large aluminum plates separated by the thick sheet. In the past I had been using 1 foot squares of 1/8 inch thick sheets of teflon, polycarbonate, and a gray PVC square. My acrylic I had wider sheets. I find some of the things I buy don't work as planned. "ha"
So the plate results were much like my little pint Leyden jar with the 4 inch diameter brass sphere atop. And too, I don't know if it's just a coincidence but both the pint jar and the plate capacitor effect seem hard to capture on camera. I did two takes of it discharging tonight with a very loud but short spark and neither was caught by the camera. So when I went to look at the clips I just took, you hear it but don't see it, there's a faint hint of a flash in one. But I saw the bright fat sparks and heard the extra loud noise - I really wanted to capture it. No sense in posting a sound without the visual though - alas. As mentioned several of the pint jar takes were also void of the spark. I wonder if those designs make a faster spark; is there such a thing? Maybe they don't last as long. Or maybe I was just unlucky with those two guys. Funny how both refuse to make long sparks. I can't gap them out much.
It took about 50 strokes to get the aluminum discs to fire. Another thing, when I had the plates a little too far from the titanium tubing, I couldn't make the setup spark but when I decided to stop filming, the thing was still crackling like a egg in a hot skillet from being charged up. The interaction of the aluminum plates and LDPE dielectric is interesting in that respect, when it was as charged as it was going to get. I figured I had better not touch it just yet. ha
One other thing, there is a fair amount of surface area with the two plates, they're a little over 16.5 inches in diameter. Maybe I should have saved the crackling clip, the crackling capacitor effect. I thought it might decide to fire on it's own it was so alive. ha
r = 8.25 in
C = 51.836278784232 in
A = 213.82464998496 in2
Factors affecting capacitance (plate area, plate spacing, dielectric material)http://www.allaboutcircuits.com/vol_1/chpt_13/3.html
There's another plate under the LDPE and a long copper bar/rectangle that the titanium pipe is resting on. I didn't use a ball bearing for the plates to arc over to being lazy I guess. So they just had the round titanium tubing surface area to attract the spark. Bearings probably would have teased out a little longer spark. There's also a short ~3 inch long cylinder of aluminum bar stock inside the end of the titanium tubing if you are wondering what that is. I was planning to make a linear pulsejet and just left it in there after lathing it to fit the tubing.