--- In chordite@yahoogroups.com, Tony Godshall <togo@...> wrote:

> Are you saying that if you switch AC this doesn't happen?

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I think the transfers tend to cancel somewhat.   Here's something  on the subject:

"Since the contact interface acts as a resistor, the heat produced by the closed contacts is proportional to the voltage across them and the current through them. Temperature rise of clean contacts is negligible, but when the contacts start to separate, the number of contacting asperities decreases. As the last contacting asperity decreases its contact area, contact resistance increases until contact voltage reaches melting potential (about 0.5 V for most metals). Molten metal bridges the separating surfaces and finally breaks. During this process some material may transfer between contacts, especially if direct current is interrupted."