Mercury relay

[Kalee20]

"There would appear to be no advantage in mercury relays" 

That's quoting me out of context! 

No advantage in the particular application I described - switching 440V utility power (in fact the plunger of one used to stick sometimes, needing to switch on and off again to shock it free).

But I agree with the others - no contact wear; no bounce; predictable contact resistance (you didn't include no wetting voltage either, so they're suitable for microvolt level signals).