Cocktail Party Physics: welding: a matter of life and death.

The arc works on the same principle as spark plugs: electrons build up on the tip of the electrode. Charges of the same type want to be as far as possible from each other, but there’s only so much space on the tip for the charges to sit. When the electrons reach a critical concentration, they are so repulsed by each other that they jump through the air gap to get away from each other. Once in the metal that you’re trying to join, the electrons are free to move around and maximize their distance from each other.

I learned a little about how to arc weld when I was young — my neighbor had a machine, and many odd bits of iron got welded together for practice.