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I am currently preparing to demonstrate the Hallwachs effect to my students. Specifically, I wanted to demonstrate that a Hg lamp will discharge a negatively charged zinc plate, but not a positively charged one. However, during my preparations I observed that the positively charged plate discharged under the influence of the lamp as well.

The experiment consists of a zinc plate connected to an electroscope. I charged the plate using the positive and negative poles of a kV generator, respectively. I also tried to do it using the two electrodes of a charged Wimshurst machine. In all cases, the electroscope showed a diminishing charge on the plate, regardless of the polarity. The electroscope and plate were sufficiently isolated: When I shut off the lamp, the electroscope showed a constant charge.

I suspect that in some way, the lamp frees electrons from some other source, and those are then drawn to the positively charged plate, discharging it.

My question is: What are possible sources of such electrons? Or if my suspicion is implausible, what are other possibilities to explain the discharging positive plate?

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    $\begingroup$ Related - Photoelectric effect for a positively charged plate in vacuum. Short wavelength UV can ionize air. $\endgroup$
    – Farcher
    Commented Nov 21 at 9:47
  • $\begingroup$ @Farcher In the video linked in that question, the demonstration only shows a discharging negative zinc plate. I think it's possible the asker misunderstood the video. However, I'm interested in the idea of ionized air. Is it plausible that the electrons won from the ionization of air are sufficient to discharge the positively charged plate? I am a bit sceptical because air mostly transmits UV light, while zinc absorbs it, which makes me think that the effect from ionized air should be significantly weaker than the normal photoelectric effect. $\endgroup$
    – Vercassivelaunos
    Commented Nov 21 at 12:58
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    $\begingroup$ From the Wikipedia article Ultaviolet, More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. $\endgroup$
    – Farcher
    Commented Nov 21 at 13:41

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