Perhaps we should move over to the other thread rather than running two in parallel?
Let me get this clear:
You have the two bars. One end of each is on your tongue. Are the other ends touching too?
You say that you feel nothing until they touch together. Is this on the tongue end? and if so, what about if they were only 1mm apart? Could it be the resistance is too great to feel? Could saliva or something is making the connection whilst bypassing the nerves?
Hang on, why don’t I try it….[fiddles about a bit]
Some time later:
Okay, I found that if I hold the two parts, I get no feedback until they connect. At that time there would be a small battery running from the zinc tip, through the tongue to the copper then back to the zinc.
If I connect the other two ends with wire, then I can apply them to the tongue with a gap and still get the tingle. Just touching the other two ends doesn’t seem to be as effective as with a proper wire.