The really old style cuffs were attached to the wall, and the mercury would climb a tube on the wall (think thermometer) as you increased the pressure. You could also see the mercury bounce with the heartbeat, as you watched it like a hawk, while lowering pressure.

We had them in the labor rooms when I worked in L and D, but the portable ones in the delivery rooms.

I guess it works the same way as a barometer. As pressure increases, the mercury goes up. Or something like that!

Oh man... I think I'll stop the research right here! That's definitely TMI! :-)
https://en.wikipedia.org/wiki/Press...t#Liquid_column