150223-2339 EST
Most ordinary relays will have either fine silver or silver-cadimium-oxide contacts (SCdO). Silver are better at low voltages, but do tranish, typically from sulfur. SCdO are usually rated at twice the current for 120 V circuits, but should never be used at 5 V logic levels. For example a P&B KUP relay with silver contacts is rated at 5 A while the same relay with SCdO contacts is rated 10 A. For low voltage signal circuits use gold over nickel, or sealed reed switches.
Gold at 50 millionths is porous, but has good life. Nickel is non-porous and serves as a good base under gold. I have gold selector switches that are 60 years old and no contact problems. All commericial instruments that I have that are that old with silver switches are a real problrem. No commerical instruments I have had gold contacts. Instruments I built did have gold contacts.
If your ever specify printed circuit board contacts require 50 millionths gold over nickel. I believe that 250 millionths of gold is fairly non-porous, but much more costly and not generally necessary. Gold over nickel is good for various connectors for low voltage signal circuits. Gold flash is not very good, wears too quickly and is porous.
Mercury contact relays or switches are very good for some applications. Old Honeywell thermostats used a solid contact, probably silver, somewhat newer ones used mercury switches and had very good life.
Some special applications may use other types of contact alloys. In the 1940s Ford experimented with many different contact alloys in the voltage regulator relay. The voltage regulator relay cycled at a relatively rapid rate with arcing. I will guess at 10 Hz or faster. In the 1950s Ford introduced ignition breaker points with a center hole. This reduced the problem of mounding on one contact from the uni-directional current flow. A cavity is created on the other contact.
A 8 cyclinder engine at 2000 RPM is 33.3 RPS. Eight cyclinders divide by 2 for a 4 cycle engine requires 4 * 33.3 = 133 cycles of the breaker points per second. At 6000 RPM this is 400 Hz.
.
