Hi CB. A few years back I ran into something wierd on three T87's.
Somethng happened to the mercury in the bulb-- instead of being a shiny
silver, it was a grey that looked more llike molten lead, and flowed
like lead. Normally, as soon as the bulb moves just past level the
mercury rolls freely to the lower side. In these cases, the mercury
wouldn't move either way until the bulb was at damn near a 45 deg angle.
For the amount of temperature control that gave, they were more of a
toggle switch than a thermostat. The first one I saw was on a wall
heater. I had installed the stat the fall before, so it was replaced
under warranty. The other 2 were on central a/c-- age unknown, probably
5- 10 y/o. Both of those calls were in a/c season as I recall. Funny,
never seen anything like that on any other mercury bulb stat, and never
seen another one since the last of those three, about 7 years ago more
or less. Larry