On Sat, 23 Feb 2013 22:01:09 +0000, Tim Watts
wrote:

On Saturday 23 February 2013 15:41 wrote in
alt.home.repair:

What contacts are used on your "non mercury" bimetallic
Honeywells??? Every one I've ever seen had a "wet contact" bulb -
which had mercury in it. The bulb was about an inch long and 1/4 to
5/16 diameter with 2 wires coming out - 3 on the odd one.

Dry contacts - some sort of alloy that was common in switches - possibly
silver/cadmium - there were a few variants...

The bimetal mechanism had a "snap" action engineered into it to cause a
clean make and break. These stats tended to have quite a large hysteresis -
probably due in part to the snap action. So it was quite common to have a
small heater (nothing more than a 5W ish resistor under the bimetal strip to
cancel some of the hysteresis. It was wired between the stat output and
neutral to cause a bit of local heating when the stat was on "call for
heat". This was optional and required a neutral present at the stat, which
some installations did not have.

Sounds like it was a Lucas thermostat - - - -.

Looking at Wonkypedia, I do see a lot of historic info on thermostats with
mercury switches. Curious - never seen one here - and we never used to be
shy of mercury until the last decade or so...