I have always had it in my head that gold plated contacts were preferred.
I noticed today that the resistivity is as follows:
Silver 1.5x10-8
Gold 2.4x10-8
Is there another property that makes gold preferable to silver for use in electrical contacts?
Gold doesn't oxidize, which means that gold-to-gold contact has no extra resistance.
In other words, it isn't about the bulk resistance (the plating is very thin anyway), but about the contact resistance.
Silver is used in places where the resistance to current flow along a surface matters, such as the interior of a waveguide or the outside surface of a high-power RF coil. In these applications, a layer of oxidation doesn't affect performance.
Silver is also frequently used on larger RF connectors, where gold would be too expensive and not durable enough. The mechanical sliding of the contacts and threads when assembled is enough to break through any oxidation.
Gold doesn't oxidize, that is, its atoms don't lose electrons to an electron acceptor like oxygen or sulfur. There are trace amounts of hydrogen sulfide gas in the atmosphere, which is why silver tarnishes over time. The tarnish consists of a very thin layer of silver sulfide. Gold, however, is more inert than silver, and so does not combine with hydrogen sulfide, or, for that matter, oxygen. Gold's lesser ability to form non-conductive compounds, as opposed to silver's ability, trumps silver's higher conductivity.
Reference? Anyone with actual silverware or silver coins can attest to tarnish. I have a tie tack made from a silver 1949 (my year of birth) American 50-cent piece that I have to polish occasionally. My gold wedding ring doesn't tarnish.
