Mercury-wetted contact relays: Basically, a mercury-wetted contact relay consists of one or more glass switch capsules surrounded by a coil. These relays maintain their original resistance to within 1 mOhm throughout their life.
When two contacts wetted with mercury are joined, the area of contact between the surfaces is somewhat large because a fillet of mercury surrounds the mated surfaces. When the two surfaces are separated, the mercury stretches into a thin filament and then breaks at two points that isolate a thin rod of mercury in the middle. The thin rod then snaps into a ball and drops to the bottom of the switch.
Mercury loss from the contacts disturbs the equilibrium of the capillary system, and more mercury is fed up the armature from the pool. Thus, in effect, the mercury-wetted contact relay provides a new contact surface for each closure.
We don't use mercury relays very often now days as the use of mercury is discouraged, since it is quite toxic and hard to dispose of. It also has problems when used with high voltages across the contacts. Works very well with low voltage circuits.