An old thermostat contains a mercury switch which I need for projects to sense mechanical position, especially valuable for outdoor projects since the switches are sealed. I am intending to use these for low voltage control where the switched current won't be more than an amp.
Honeywell/34 that had been removed from a church. The date on the "new" manual is 1994. This thermostat is actually a small but powerful computer the manual tells me. So far it is working fine. The default temperature of 20C is a bit too warm but nice as I enjoy the heat while it is not too cool outside. The furnace is not working hard yet, the outside temp is about 5C. Replacing a thermostat is best done on a day when you don't actually need the heat.
One possible use of these type of switches is for limit switches in a solar tracking array.
Mercury switches are used in the box at the red arrow in the pic below to limit the rotation of the array.
Link here goes to the motor drive of the project. The main index is diy solar parabolic trough gen2 intro
DRAFT - may change over the next day or two as I add/change - comments welcome - comments are moderated
Lion's Head, Ontario