One interesting place mercury is used is in near-frictionless bearings. Gyrocompasses from World War 2 used a big oscillating mercury bearing at the base to hold the sensitive element, which can get sort of heavy. The bearings essentially are a pool of mercury in a special bowl, and the part to be supported (in the case of gyros, the sensitive element) neatly just fits in, so it barely floats. In an Arma gyro, about a pint of mercury is used, I think. The term "oscillating" comes from a little mixer that sits in the bottom of the bowl, always running to keep the mercury in constant motion. This is to overcome any static vicosity in the mercury, which, like friction, is always greater than the dynamic viscosity. The difference is tiny, but the forces that work on a gyrocompass are tiny as well. I have an old Mk 10 Arma gyro, and I suppose that someday I will need to get some mercury, but to be realistic, I don't have nearly the skill to balance the thing. An unbalanced gyrocompass quickly turns into a very broken gyrocompass. William Donzelli aw288(a)osfn.org