In relation to my answer on Quora
I have posted originally there this interesting experiment I came across recently in the book Physik, Band 2 from the author Hermann von Baravalle (pages 83–84). Look at the setup below.
When high DC voltage is connected to it, then sparks start to jump across the upper part of the circuit.
As soon as the poles of the voltage source are reversed, then sparks start to jump across the lower part of the circuit (drawing below).
The two different ends where the sparks jump are a thick sharpened wire and a metal plate.
This is the original post on Quora where there are already some comments: