[Keri] selects them not with his golden ears, but by wiring both pots together as a Wheatstone bridge. A meter between the two wipers would detect any current due to a mismatch. A Wheatstone ...

Note that points B and D were really electrically the same so we can make them the same point and redraw the schematic in the more common diamond shape for a Wheatstone bridge. When we turn on ...