Physics/experiment i just did
hello, thank you for your time.
please tell me,
i have been attempting to find out theoretically which shape would better create negative air ions, needles or a sphere.
because the answer was very difficult to theoretically predict i set up an experiment to find out the answer. now i have some results and would love your opinion on the interpretation of the results.
so with a grounded (-)25kV multiplier i used two shapes as terminals: needles & a smooth metal hemisphere (cooking bowl) at EQUAL distances from the earthed amp-metered receiving terminal.
for the needles i tried a large sewing needle, a pin, a single carbon-fibre/needle (7Ám in diameter) & lots of long strands of the carbon-fibre spread out.
the amp-meter was set to 200.0Ám so it gives a reading into tens of nanoamps. this was on one end connected to ground and the other side of the amp-meter was a metal hemisphere set on non conducting supports. the needles were as i expected rated the thinner the better at ionising the air with the most efficiency of transferring the charge to the air with the spread out carbon-fibre bundle. the distance between the live terminal and the measuring terminal was 15cm; way too far for a streamer let alone a spark/arch. this left the ions of air as the only significant means of charge transfer.
the surprise was that the totally smooth hemisphere was by far the best, giving a reading many times that of the needles. and this was with the hemisphere facing upwards or downwards.
i am surprised at the results as the needle are as i expected but the bowl i expected because of its smoothness, specially when facing up (with rim facing down) to be the lowest but the opposite reading was found.
please tell me what you think.
We use needles to create coronal discharges for voltage control on Van de Graaf particle accelerators. This is well-understood, that sharp points maximize ionization due to the high electric fields around them. Smooth spheres (note, spheres and not hemispheres) do not ionize as well.
That said, you have opposite results for a reason. You're not at terribly high voltage, and your hemisphere has an edge, so the shape is not smooth. You also have to factor in where exactly your ions went. You say the distances were the same. Do you mean from the center of the bowl, or the closest part of the edge? How far off the ground was everything supported at? Were the supports the same? Ion drift to an alternate location seems to be a likely cause. You had a separation of 15 cm, were your supports very large compared to this distance (10x)? A picture would help explain this far better than a text description.