Digging through a seldom-used parts bin, I made a cool discovery: a set of four mercury switches, still intact:
These are used as tilt and vibration sensors in mechanical systems. They’re heavy from the mercury inside, and it sloshes around with a very satisfying motion. I’m not sure I’ll ever need these for repairs, but they’re probably worth keeping in a drawer!
RETROVOLTAGE 
I once owned a Studebaker Avanti and the truck light was activated by a Mercury switch. I suspect this was standard for many old cars.
It is possible there are other cars out there with the same design. You could advertise them on Ebay. Dave
Maybe one of your readers will have a use for them.