Try it Yourself!
I am very interested to see what results other people can get - plus different people have different insights. We might find out something else altogether. Please send me your videos!
So, where do you put the voltmeter? There seem to be various possibilities. The dolmen which produced the most electricity, was one we visited in Krasnodar, Russia. There was a very obvious socket-type hole in a stone right next to the dolmen.
Most dolmen seem to have holes, indentations and various markings. Sometimes they are on the front, sometimes elsewhere. If you can't find a hole there are other options. The dolmen might be missing some stones, have been reconstructed, or just have weathered badly.
All is not lost! From my experiments to date, it seems that the best area to try, if you cannot find an obvious hole, is what I refer to as the armpit(!). Where the supporting stone meets the top slab, if you can push your voltmeter probes between the two, you are most likely to get a reading.
Interestingly, this is the same location as the holes on the T stones
at Gobekli Tepe in Turkey. Each has a "socket" on as shown below.
Sadly, if you actually visit Gobekli Tepe, your chances of getting anywhere
near a T stone with a voltmeter are nil. Most of it is behind barbed wire
and you will need to bring binoculars if you want to see anything much.
Some dolmen are better preserved than others. The ones I saw in Russia in the Krasnodar region are very well preserved indeed. However, one I visited in Turkey was more like those in the UK. It had no sound hole as such. I was still able to get a reading in the "armpit". LOL
It seems likely (to me) that gaps between stones, as we see in many dolmen in the British isles, were filled in with a sort of dry stone walling, as has been done in Table des Marchand, in France. This keeps the sound in the sound chamber. Many dolmen are part buried, too, so the soil around it also helps form the walls.