The mercury switches that I have seen (and designed for 120V 15A, may or may
not be pretty dubious at 12V, 5A DC. Small gap, contacts close together-just
roll mercury to the other end of a tube (as used in many thermostats).
Possibly gas filled as Daestrom indicates but no "color as Duane said. The
switches used in automotive work are intended for low voltage DC use. At
one time (still so?) 120V 15A AC switches were derated to 5A at 15V and
these did have a good snap action and gap.
Older household switches (heavy spring and snap action as well as a wide gap
and allowing arc extension) were capable of handling DC but newer ones are
not- in the interest of quietness and ease of operation.
However, I never tried a mercury switch on DC at 12V, any current.
I know from experience that an arc more than sufficient to light a cigarette
is possible at 120V, 5A DC.
Back in the OLD days, there was a Boat Autopilot built by Metal Marine
Pilot of Tacoma, WA that used Mercury Switches to switch 12/24/32 Vdc
at better than 20 amps. They were controlling a large DC motor that
was driving a geartrain, that then drove a chain loop, that turned the
Helm Wheel. The system used two switches, setup in a mutually exclusive
state that reversed the motor rotation and keep the boat going in
a straight line. There were a number of models and versions, of this
system and they started out with Glass Switches with flexible leads
mounted on a Teeter/Tooter Board that was moved by a selenoid. (to
provide the mutual exclusivity) the last version had sealed Mercury
Switches that used electromagnetic coils to change the switch state.
Very reliable stuff. I still see some of the very old systems on
fishing boats. Some of these are 50 years old now and still going.
Bruce in alaska