i have seen indoor outdoor thermometers that have a feature that
"remembers" the minimum and maximun temps...the material in the tube
stays at the top and bottom temps..and you re-adjsut it or put it back
to zero or neutral by running a magnet along the tube to bring the
material back down or up..

http://en.wikipedia.org/wiki/Six%27s_thermometer

mercury itself is not magnetic, but these had a small iron device that
marked the level-
http://www2a.cdc.gov/vaccines/ed/shtoolkit/resources/How_to_Read_a_MinMax_Thermometer.pdf

oh well it seems as if you have solved it..
i am sometimes just a bubble off...