Thanks for the physics, Kai. In the days of Strowger telephone exchanges, the British Post Office used special contacts on the 'A' relay - the one that carried the speech from subscriber's line to the exchange. I believe they were silver. They also carried a DC 'wetting' current - actually, the line signalling and carbon microphone excitation current. The PO3000 relay was a thing of beauty. It also 'wiped' its contacts on very closure, due to the design of the moving parts.

I hadn't realised that gold plated contacts were, effectively 'useless'. What would you recommend for very low wetting currents?

Dave