I recently happened upon some americium. I wanted to measure its alpha emissions, so I built a spark alpha detector which consisted of some thin copper wire stretched about five millimeters above a polished aluminum plate. You can see a very similar one on youtube. My setup looked something like this, with the black rectangle representing the plate:
The detector, consisting of five thin copper wires stretched taught over an aluminum plate.
As a consequence of dubious soldering, one wire hung about half a millimeter lower than the others. I wanted to see what effect that difference in distance would have on the sensitivity of the detector. I noticed that as I slowly brought the americium source closer to wire #1 (the low-hanging wire), a spark occurred when it and the source were within about 2.5 centimeters of each other. To compare, I went over to the other side of the detector and gradually brought the source closer to wire #5. While I didn't measure a difference in detection distance between the two wires, I did notice something else. Whenever wire #5 sparked, wire #1 would as well, despite the source being much further than 2.5 centimeters away from it. Here's a diagram, in case my description sucks.
After playing around some more, I found that wire #1 would always spark at exactly the same time as any other wire, even when exposed to no alpha particles. So, if I exposed wires #4 and #5 to the source, with wires #3, #2, and #1 all lacking line of sight to the americium, wire #1 would still spark simultaneously with wires #4 and #5, with no sparking observed in wires #3 or #2.
I imagine that the electricity has the "easiest" time arcing from wire #1 to the plate. However, it is incapable of doing so under normal conditions. I haven't observed any arcs from #1 to the plate with the alpha source completely removed and stowed away. So, why does it seem that electricity can indeed arc from wire #1 to the plate, but only when another arc is happening elsewhere in the circuit? The only answer I can think of is that the air is somehow becoming ionized, but I don't see how that can happen with the alpha source completely isolated from wire #1 by virtue of being held well in excess of 2.5 centimeters away. Does the arcing electricity perhaps ionize the air around it to a degree so minute that it only causes the lowest hanging wire to arc? I'm certainly no physicist or electrical engineer, so I would appreciate some answers or guidance. I'm sure this is pretty simple, but I couldn't find anything on google.
Here's a video if anyone's interested. The low hanging wire is on the far left.