Hi Jagau!
I didn't see much point in the experiment but I did it out of respect for you! (and for others, of course!)
The voltage collected from the environment and rectified by the diode bridge can only supply a very small current, because such a "voltage source" has a very high internal resistance and the terminal voltage drops even at a small (microAmp) load. This can be seen from the charging time of the capacitor. Also from the difference in the values measured by the multimeter and Deprez.
The AC input to the diode bridge was connected to two grounds and got 10 volts (or more) on the DC output of the bridge (no buffer capacitor). If there are large voltage differences between the grounds,that is a problem in my opinion. Or more likely some sort of mains leakage that you have stumbled across. But I obviously have no way of knowing that from here.
My answer to your question is shown in the video.
Here you see two metals of the same quality (electroplated) in the first case. I then replaced one with a brass electrode. Electrode potential. No further explanation needed.
Standard UHF antenna. Also the value is visible. This also needs no explanation.
However, if I touch the cover of the reading lamp, I can clearly see the difference due to the mains voltage! The reading lamp is double insulated and not switched on.
In my opinion, if there is a potential difference (or a radiating source), this is the case.
This is why I find it misleading to use the mains earthing of the national electricity supplier for such measurements. Especially for multi-storey prefabricated flats.
(All countries have their own relevant installation safety regulations. Also for earthing. EMI and transient mains filters, etc.)
I understand that transmission lines, radiating transmitters, mobile signal repeaters, etc. are everywhere in our environment, especially in densely populated areas, and can create strong EM fields, but in the case of general grid "blackouts", no such energy collection occurs.
Just to avoid any misunderstanding.
Of course if I am wrong and properly presented example then I will accept it and correct my opinion.
Atti.