I have some mercury switches. These used to be common 50 years ago but have disappeared. I have no idea what current and voltage they can safely carry or break.
The small ones are a cylindrical glass bulb, about 5 mm diameter and 15 mm long and have a drop of mercury inside. I am thinking of making a float switch with a wine bottle cork stopper. Drill a hole from one end, insert the mercury bulb with thin, flexible cables, well sealed and put the cork pivoting on a wire so when it floats the mercury closes the contacts. The electrical part seems quite straightforward but I need to think about the mechanical part . I need it to not be subject to corrosion and have a heavy base which will hold it in place. I am thinking of a piece of brick cut into a U shape with the float in the center. I need to think about this. Y think I would only use them as sensors with low voltage and current. Or maybe they could handle, say, 2 Amps at 12 V?
I also have a big mercury switch, (12 x 65 mm) which came from a mechanical timer designed to switch many amps at 220 V. I suppose relay contacts would not last as long due to sparking and corrosion. When this thing tilts over the break is quite fast and I suppose there is no oxygen inside the capsule. It has a central, common terminal and one at each end (NO & NC). When it is horizontal all three terminals are connected so it is "make before break". I cannot think of any practical use for it today but it would be fun to put it to some use where it could be seen and admired in action. I could design an "over center" mechanism so the mercury bulb would only be stable tilted to one side or the other.
"over center" mechanism: